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TAPS                                                            M. Welzl
Internet-Draft                                        University of Oslo
Intended status: Informational                                 M. Tuexen
Expires: September 9, 2017              Muenster Univ. of Appl. Sciences
                                                              N. Khademi
                                                      University of Oslo
                                                           March 8, 2017


On the Usage of Transport Features Provided by IETF Transport Protocols
                  draft-ietf-taps-transports-usage-03

Abstract

   This document describes how TCP, MPTCP, SCTP, UDP and UDP-Lite expose
   services to applications and how an application can configure and use
   the transport features that make up these services.  It also
   discusses the service provided by the LEDBAT congestion control
   mechanism.

Status of this Memo

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

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

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

   This Internet-Draft will expire on September 9, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Pass 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Primitives Provided by TCP . . . . . . . . . . . . . . . .  5
       3.1.1.  Excluded Primitives or Parameters  . . . . . . . . . .  8
     3.2.  Primitives Provided by MPTCP . . . . . . . . . . . . . . .  9
     3.3.  Primitives Provided by SCTP  . . . . . . . . . . . . . . . 10
       3.3.1.  Excluded Primitives or Parameters  . . . . . . . . . . 17
     3.4.  Primitives Provided by UDP and UDP-Lite  . . . . . . . . . 17
     3.5.  The service of LEDBAT  . . . . . . . . . . . . . . . . . . 17
   4.  Pass 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.1.  CONNECTION Related Primitives  . . . . . . . . . . . . . . 19
     4.2.  DATA Transfer Related Primitives . . . . . . . . . . . . . 30
   5.  Pass 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     5.1.  CONNECTION Related Transport Features  . . . . . . . . . . 33
     5.2.  DATA Transfer Related Transport Features . . . . . . . . . 39
       5.2.1.  Sending Data . . . . . . . . . . . . . . . . . . . . . 39
       5.2.2.  Receiving Data . . . . . . . . . . . . . . . . . . . . 40
       5.2.3.  Errors . . . . . . . . . . . . . . . . . . . . . . . . 40
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 41
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 41
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 41
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 44
   Appendix A.  Overview of RFCs used as input for pass 1 . . . . . . 45
   Appendix B.  How this document was developed . . . . . . . . . . . 45
   Appendix C.  Revision information  . . . . . . . . . . . . . . . . 47
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47















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1.  Terminology

   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
      different 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
      implements 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 and is related to
      one or more Transport Features.
   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.


2.  Introduction

   This document presents defined interactions between applications and
   the transport protocols TCP, MPTCP, SCTP, UDP and UDP-Lite as well as
   the LEDBAT congestion control mechanism in the form of primitives and
   Transport Features.  Primitives can be invoked by an application or a
   transport protocol; the latter type is called an "event".  The list
   of primitives 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.




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   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, [RFC6458]
   explains how an application can use socket options to indicate its
   interest in receiving certain notifications.  However, for the
   purpose of identifying primitives and Transport Services, the ability
   to enable or disable the reception of notifications is irrelevant.
   Similarly, one-to-many style sockets described in [RFC6458] 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 (this is the case
   for the "get" in many get/set operations in [RFC6458]).

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

   This document discusses unicast transport protocols and a unicast
   congestion control mechanism.  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 normally this multiplexing is achieved
   using port numbers.  Port multiplexing is therefore assumed to be
   always provided and not discussed in this document.

   Some protocols are connection-oriented.  Connection-oriented
   protocols often use an initial call to a specific transport primitive



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   to open a connection before communication can progress, and require
   communication to be explicitly terminated by issuing another call to
   a transport 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 [FJ16].


3.  Pass 1

   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).

3.1.  Primitives Provided by TCP

   [RFC0793] 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".  Section 3.8 in [RFC0793]
   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).  [RFC1122]
      describes a procedure for aborting the connection that must be
      used to avoid excessive retransmissions, and states that an
      application must be able to control the threshold used to
      determine the condition for aborting -- and that this threshold
      may be measured in time units or as a count of retransmission.



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      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 [RFC1122].  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
      connection request has arrived.  Finally, the 'options' parameter
      is explained in [RFC1122] to allow the application to specify IP
      options such as source route, record route, or timestamp.  It is
      not stated on which segments of a connection these options should
      be applied, but probably all segments, as this is also stated in a
      specification given for the usage of source route (section 4.2.3.8
      of [RFC1122]).  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.

   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').

   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.

   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 [RFC0793].






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   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 [RFC0793], Section 3.5.

   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 [RFC0793], Section 3.4.

   USER TIMEOUT event:  This event, described in Section 3.9 of
      [RFC0793], is executed when the user timeout expires (see 'open').
      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, described in Section 4.2.4.1 of
      [RFC1122], informs the application of "soft errors" that can be
      safely ignored [RFC5461], including the arrival of an ICMP error
      message or excessive retransmissions (reaching a threshold below
      the threshold where the connection is aborted).

   Type-of-Service:  Section 4.2.4.2 of [RFC1122] 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 during the connection lifetime,
      and the TOS value should be passed to the IP layer unchanged.
      Since then the TOS field has been redefined.  A part of the field
      has been assigned to ECN [RFC3168] and the six most significant
      bits have been assigned to carry the DiffServ CodePoint, DSField
      [RFC3260].  Staying with the intention behind the application's
      ability to specify the "Type of Service", this should probably be
      interpreted to mean the value in the DSField, which is the
      Differentiated Services Codepoint (DSCP).

   Nagle:  The Nagle algorithm, described in Section 4.2.3.4 of
      [RFC1122], delays sending data for some time to increase the
      likelihood of sending a full-sized segment.  An application can
      disable the Nagle algorithm for an individual connection.

   User Timeout Option:  The User Timeout Option (UTO) [RFC5482] 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'), [RFC5482] introduces three
      per-connection state variables that an application can adjust to
      control the operation of the User Timeout Option (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



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      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').

   Fast Open:  TCP Fast Open (TFO) [RFC7413] allows 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.  TCP implementations
      MUST NOT use TFO by default, but only use TFO if requested
      explicitly by the application on a per-service-port basis.  To
      benefit from TFO, the first application data unit (e.g., an HTTP
      request) needs to be no more than TCP's maximum segment size
      (minus options used in the SYN).  For the active open side,
      [RFC7413] recommends changing or replacing the connect() call in
      order to support a user data buffer argument.  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.


3.1.1.  Excluded Primitives or Parameters

   The 'open' primitive specified in [RFC0793] can be handed optional
   Precedence or security/compartment information according to
   [RFC0793], but this was not included here because it is mostly
   irrelevant today, as explained in [RFC7414].

   The 'status' primitive was not included because [RFC0793] describes
   this primitive as "implementation dependent" and states that it
   "could be excluded without adverse effect".  Moreover, while a data
   block containing specific information is described, it is also stated
   that not all of this information may always be available.  The 'send'
   primitive described in [RFC0793] includes an optional PUSH flag
   which, if set, requires data to be promptly transmitted to the
   receiver without delay; the 'receive' primitive described in
   [RFC0793] can (under some conditions) yield the status of the PUSH
   flag.  Because PUSH functionality is made optional to implement for
   both the 'send' and 'receive' primitives in [RFC1122], this
   functionality is not included here.  [RFC1122] also introduces keep-
   alives to TCP, but these are optional to implement and hence not
   considered here.  [RFC1122] describes that "some TCP implementations



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   have included a FLUSH call", indicating that this call is also
   optional to implement.  It is therefore not considered here.

3.2.  Primitives Provided by MPTCP

   Multipath TCP (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 in [RFC6182] "Multipath TCP MUST
   follow the same service model as TCP [RFC0793]: 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, [RFC6182] states constraints on the API exposed by MPTCP: "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 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, [RFC6824]
   and [RFC6897] 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.

   open:  [RFC6897] states "An application should be able to request to
      turn on or turn off the usage of MPTCP.".  The RFC states that
      this functionality can be provided through a socket-option called
      TCP_MULTIPATH_ENABLE.  Further, [RFC6897] says that MPTCP must be
      disabled in case the application is binding to a specific address.

   send/receive:  [RFC6824] states that the sending and receiving of
      data does not require any changes to the application when MPTCP is
      being used.  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 and [RFC6824]
      says "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.
      [RFC6897] 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.



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      Further, "An application should be able to obtain information on
      the pairs of addresses used by the MPTCP subflows.".


3.3.  Primitives Provided by SCTP

   Section 1.1 of [RFC4960] lists limitations of TCP that SCTP removes.
   Three of the four mentioned limitations directly translate into
   Transport Features that are visible to an application using SCTP: 1)
   it allows for preservation of message delineations; 2) these
   messages, while reliably transferred, do not require to be in order
   unless the application wants it; 3) multi-homing 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.

   Section 10 of [RFC4960] further specifies the interaction with the
   application (which RFC [RFC4960] 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 Section 10 of [RFC4960], an extension to the socket API
   is described in [RFC6458], covering the functionality of the base
   protocol specified in [RFC4960] and its extensions specified in
   [RFC3758], [RFC4895], and [RFC5061].  For the protocol extensions
   specified in [RFC6525], [RFC6951], [RFC7053], [RFC7496], [RFC7829]
   and [I-D.ietf-tsvwg-sctp-ndata], the corresponding extensions of the
   socket API are specified in these protocol specifications.  The
   functionality exposed to the ULP through this socket API is
   considered here in addition to the abstract API specified in Section
   10 of [RFC4960].

   [RFC4960] contains a "SETPROTOCOLPARAMETERS" primitive that allows to
   adjust elements of a parameter list; 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 [RFC4960] 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 which fit together.

   Initialize:  Initialize, described in [RFC4960], creates a local SCTP
      instance that it binds to a set of local addresses (and, if
      provided, port number).  Initialize needs to be called only once
      per set of local addresses.  [RFC6458] also describes a number of
      per-association initialization parameters that can be used when an



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      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.  At this point, before
      connecting, an application can also enable UDP encapsulation by
      configuring the remote UDP encapsulation port number [RFC6951].

   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 as described in section 9.9 of
      [RFC6458]) sockets.  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 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, as specified in
      [RFC5061].  If the extension specified in [RFC4895] is used, the
      chunk types required to be sent authenticated by the peer can be
      provided.  [RFC6458] describes a 'SCTP_CANT_STR_ASSOC'
      notification that is used to inform the application of a failure
      to create an association.  [RFC6458] describes how an application
      could use sendto() or sendmsg() to implicitly setup an
      association, thereby handing over a message that SCTP might send
      during the association setup phase.  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 to control the partial reliability extension specified
      in [RFC3758] and [RFC7496].  An optional maximum life time can
      specify the time after which the message should be discarded



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      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 by making use of the
      I-bit specified in [RFC7053].  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 the extension
      specified in [RFC4895] is used, the key identifier used for
      authenticating the DATA chunks can be provided.

   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.  A delivery number lets
      the application detect reordering.

   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.

   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 in [RFC4960], called sasoc_maxrxt in
      [RFC6458], allows to configure the number of unsuccessful
      retransmissions after which an entire association is considered as
      failed (which should invoke a COMMUNICATION LOST notification).





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   Set Primary:  This allows 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 which address to use as the primary
      address.  This is provided by the protocol extension defined in
      [RFC5061].

   Configure Path Switchover:  [RFC4960] contains a primitive called SET
      FAILURE THRESHOLD.  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.
      [RFC7829] extends this method with a concept of "Potentially
      Failed" (PF) paths.  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.  This is provided
      by the protocol extension defined in [RFC4895].

   Add / Reset Streams, Reset Association:  This allows an endpoint to
      add streams to an existing association or or to reset them
      individually.  Additionally, the association can be reset.  This
      is provided by the protocol extension defined in [RFC6525].

   Status:  The 'Status' primitive returns a 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 [RFC4960] and
      MTU per path [RFC6458].






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   Enable / Disable Interleaving:  This allows to enable or disable the
      negotiation of user message interleaving support for future
      associations.  For existing associations it is possible to query
      whether user message interleaving support was negotiated or not on
      a particular association [I-D.ietf-tsvwg-sctp-ndata].

   Set Stream Scheduler:  This allows to select a stream scheduler per
      association, with a choice of: First Come First Serve, Round
      Robin, Round Robin per Packet, Priority Based, Fair Bandwidth,
      Weighted Fair Queuing.  How these schedulers operate is described
      in detail in [I-D.ietf-tsvwg-sctp-ndata].

   Configure Stream Scheduler:  This allows 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 [RFC6458].

   Configure send buffer size:  This controls the amount of data SCTP
      may have waiting in internal buffers to be sent or retransmitted
      [RFC6458].

   Configure receive buffer size:  This sets the receive buffer size in
      octets, thereby controlling the receiver window for an association
      [RFC6458].

   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 PMTU size), then
      the message will be fragmented by SCTP.  Disabling message
      fragmentation will produce an error instead of fragmenting the
      message [RFC6458].

   Configure Path MTU Discovery:  Section 8.1.12 of [RFC6458] explains
      how Path MTU Discovery can be enabled or disabled per peer address
      of an association.  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; the number of packets
      that must be received before a SACK is sent without waiting for
      the delay timer to expire can be configured [RFC6458].







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   Set Cookie life value:  The Cookie life value can be adjusted as
      explained in Section 8.1.2 of [RFC6458].  "Valid.Cookie.Life" is
      also one of the parameters listed as potentially adjustable with
      SETPROTOCOLPARAMETERS in [RFC4960].

   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 as explained in Section
      8.1.2 of [RFC6458].  "Max.Burst" is also one of the parameters
      listed as potentially adjustable with SETPROTOCOLPARAMETERS in
      [RFC4960].

   Configure RTO calculation:  [RFC4960] lists the following adjustable
      parameters: RTO.Initial; RTO.Min; RTO.Max; RTO.Alpha; RTO.Beta.
      Only the initial, minimum and maximum RTO are also described as
      configurable [RFC6458].

   Set DSCP value:  Section 8.1.12 of [RFC6458] explains how to set the
      DSCP value per peer address of an association.

   Set IPv6 flow label:  Section 8.1.12 of [RFC6458] explains how to set
      the flow label field per peer address of an association.

   Set Partial Delivery Point:  This allows 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
      [RFC6458].

   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.

   RESTART notification:  When SCTP has detected that the peer has
      restarted, this notification is passed to the upper layer
      [RFC6458].

   DATA ARRIVE notification:  When a message is ready to be retrieved
      via the Receive primitive, the application is informed by this
      notification.






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   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:  The NETWORK STATUS CHANGE
      notification informs the application about a socket becoming
      active/inactive [RFC4960] or "Potentially Failed" [RFC7829].

   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 assocation.

   AUTHENTICATION notification:  When SCTP wants to notify the upper
      layer regarding the key management related to the extension
      defined in [RFC4895], 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.  This extension is
      defined in [RFC5061] and [RFC6458].

   STREAM RESET notification:  When SCTP completes the procedure for
      resetting streams as specified in [RFC6525], this notification is
      passed to the upper layer, informing it about the result.

   ASSOCIATION RESET notification:  When SCTP completes the association
      reset procedure as specified in [RFC6525], this notification is
      passed to the upper layer, informing it about the result.

   STREAM CHANGE notification:  When SCTP completes the procedure used
      to increase the number of streams as specified in [RFC6525], this
      notification is passed to the upper layer, informing it about the
      result.







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   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 [RFC6458].

   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
      (section 6.1.7 of [RFC6458]).


3.3.1.  Excluded Primitives or Parameters

   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 described in Section 6.1 of [RFC6458] informs an
   application that the peer has sent a SHUTDOWN, and hence no further
   data should be sent on this socket.  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 not
   included here.

3.4.  Primitives Provided by UDP and UDP-Lite

   The primitives provided by UDP and UDP-Lite are described in [FJ16].

3.5.  The service of LEDBAT

   The service of the Low Extra Delay Background Transport (LEDBAT)
   congestion control mechanism is described in the abstract of
   [RFC6817] 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 RFC 5681) and to yield in the
   presence of competing flows, thus limiting interference with the
   network performance of competing flows."




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   LEDBAT does not have any primitives, as LEDBAT is not a transport
   protocol.  [RFC6817] states: "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 numer 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 100ms or less.  ALLOWED_INCREASE (should be
   1, must be greater than 0) limits the speed at which LEDBAT increases
   its rate.  GAIN, which 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 SHOULD be 10.  This
   list can be filtered using a FILTER() function which is not
   prescribed in [RFC6817], yielding 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 fully prescribed with a
   MUST in [RFC6817] as a parameter on the other hand.  Function
   implementations are not provided as a parameter to any of the
   transport protocols discussed here, and hence we do not regard the
   FILTER() function as a parameter.  However, to avoid unnecessarily
   limiting future implementations, we consider all other parameters
   above as tunable parameters that a TAPS system should expose.


4.  Pass 2

   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



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   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' [RFC4960] returns success or failure, and lets
   the application control whether further receive or send operations or
   both are disabled [RFC6458].  This is not described in the same way
   for TCP in [RFC0793], 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"
   mechanism.  This mechanism is required to implement the "synch
   signal" used by telnet [RFC0854], but SHOULD NOT be used by new
   applications [RFC6093].  Because pass 2 is meant as a basis for the
   creation of TAPS systems, the "URGENT" mechanism is excluded.  This
   also concerns the notification "Urgent pointer advance" in the
   ERROR_REPORT described in Section 4.2.4.1 of [RFC1122].

   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
   limiting future implementations, it was decided to place it under
   CONNECTION.MAINTENANCE, with all parameters that are described in
   [RFC6817] made configurable.

4.1.  CONNECTION Related Primitives

   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 . [RFC8085]

   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



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      of the succeeding incoming connection request will be maintained);
      timeout (optional); options (optional); 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.  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
      number of INIT attemps (optional); maximum init.  RTO for INIT
      (optional); user message (optional); 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 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); 1 destination port (default (OS chooses), or
      specified).
      Comments: Associates a transport address creating a UDP(-Lite)



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      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)
      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 TCP's connection handshake is completed.  This message may
      arrive multiple times.

   o  LISTEN.SCTP:
      Pass 1 primitive / event: 'initialize', followed by 'COMMUNICATION
      UP' or 'RESTART' notification and possibly '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;
      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 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); destination port (default (OS chooses), or



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


   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); boolean CHANGEABLE
      (optional, default true)
      Comments: when sending data, an application can adjust the
      connection's timeout value (time after which the connection will
      be aborted if data could not be delivered).  If UTO_ENABLED is
      true, the user 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 '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; '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
      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.




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   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 to send 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 to send 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.

   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, 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



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      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.

   o  CONFIG_SWITCHOVER.SCTP:
      Pass 1 primitive / event: 'Configure Path Switchover'
      Parameters: primary max retrans (no. of retransmissions after
      which a path is considered inactive), PF max retrans (no. 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; 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.  For TCP this was originally specified for the
      TOS field [RFC1122], which is here interpreted to refer to the
      DSField [RFC3260].





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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.  [RFC7657] and [RFC8085] provide
      current guidance on using this value with UDP.

   o  ERROR.TCP:
      Pass 1 primitive / event: 'ERROR_REPORT'
      Returns: reason (encoding not specified); 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; 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.

   o  SET_AUTH.SCTP:
      Pass 1 primitive / event: 'Set / Get Authentication Parameters'
      Parameters: key_id, key, hmac_id

   o  GET_AUTH.SCTP:
      Pass 1 primitive / event: 'Set / Get Authentication Parameters'
      Parameters: key_id, chunk_list

   o  RESET_STREAM.SCTP:
      Pass 1 primitive / event: 'Add / Reset Streams, Reset Association'
      Parameters: sid, 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 defined in
      [RFC3260].

   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 if 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 Serve, Round Robin, Round
      Robin per Packet, Priority Based, Fair Bandwidth, Weighted Fair
      Queuing.

   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.




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   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.

   o  CONFIG_FRAGMENTATION.SCTP:
      Pass 1 primitive / event: 'Configure message fragmentation'
      Parameters: one boolean value (enable/disable), 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), 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), number of packets to wait for (default 2)
      Comments: If delayed SACK is enabled, SCTP will send a SACK upon
      either 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), max (optional)
      Comments: This adjusts the initial, minimum and maximum RTO
      values.

   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



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      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  CHECKSUM.UDP:
      Pass 1 primitive / event: 'DISABLE_CHECKSUM'.
      Parameters: 0 when no checksum is used at sender, 1 for checksum
      at sender (default)

   o  CHECKSUM_REQUIRED.UDP:
      Pass 1 primitive / event: 'REQUIRE_CHECKSUM'.
      Parameter: 0 when checksum is required at receiver, 1 to allow
      zero checksum at receiver (default)

   o  SET_CHECKSUM_COVERAGE.UDP-Lite:
      Pass 1 primitive / event: 'SET_CHECKSUM_COVERAGE'
      Parameters: Coverage length at sender (default maximum coverage)

   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), 1 when DF is set

   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 the IPv4 TTL of
      IPv6 Hop count value from a received UDP(-Lite) datagram.

   o  SET_ECN.UDP(-Lite):
      Pass 1 primitive / event: 'SET_ECN'
      Parameters: ECN value
      Comments: This allows a UDP(-Lite) application to set the ECN
      codepoint field for outgoing UDP(-Lite) datagrams.



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   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
      codepoint 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 Time Stamp 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, MIN_CWND
      Comments: enable is a newly invented parameter that enables or
      disables the whole LEDBAT service.


   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  CLOSE.UDP(-Lite):
      Pass 1 primitive event: 'CLOSE'
      Comments: No further UDP(-Lite) datagrams are sent/received on
      this connection.






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   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  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 beginning of section 8.3 in
      [RFC4960]) 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.


4.2.  DATA Transfer Related Primitives

   All primitives in this section refer to an existing connection, i.e.
   a connection that was either established or made available for



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   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
   category 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)
      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 whenever data are sent (see also
      CONNECTION.MAINTENANCE.CHANGE-TIMEOUT.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); 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 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.  IP address
      and Port may be omitted for connected UDP(-Lite) sockets.  All
      CONNECTION.MAINTENANCE.SET_*.UDP(-Lite) primitives apply per
      message sent.



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   o  RECEIVE.TCP:
      Pass 1 primitive / event: 'receive'.

   o  RECEIVE.SCTP:
      Pass 1 primitive / event: 'DATA ARRIVE' notification, followed by
      'Receive'
      Parameters: stream number (optional)
      Returns: stream sequence number (optional), 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.  Additionally, a delivery number lets the
      application detect reordering.

   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; 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.






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   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.



5.  Pass 3

   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
   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
   feature "Specify IP Options".

5.1.  CONNECTION Related Transport Features

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

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

   o  Specify which IP Options must always be used
      Protocols: TCP

   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, SCTP

   o  Obtain multiple sockets
      Protocols: SCTP

   o  Disable MPTCP
      Protocols: MPTCP





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   o  Specify which chunk types must always be authenticated
      Protocols: SCTP
      Comments: 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 transfer (possibly multiple times) before
      connection establishment
      Protocols: TCP

   o  Hand over a message to 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, UDP(-Lite)

   o  Listen, N specified local interfaces
      Protocols: SCTP, UDP(-Lite)

   o  Listen, all local interfaces
      Protocols: TCP, SCTP, 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

   o  Disable MPTCP
      Protocols: MPTCP






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   o  Specify which chunk types must always be authenticated
      Protocols: SCTP
      Comments: 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, SCTP

   o  Suggest timeout to the peer
      Protocols: TCP

   o  Disable Nagle algorithm
      Protocols: TCP, 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, SCTP
      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port
      SCTP Parameters: local IP address

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

   o  Set primary path
      Protocols: SCTP

   o  Suggest primary path to the peer
      Protocols: SCTP





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   o  Configure Path Switchover
      Protocols: SCTP

   o  Obtain status (query or notification)
      Protocols: SCTP, 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; interleaving supported
      yes/no
      MPTCP parameters: subflow-list (identified by source-IP; source-
      Port; destination-IP; destination-Port)

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

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

   o  Change authentication parameters
      Protocols: SCTP

   o  Obtain authentication information
      Protocols: 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

   o  Notification of Added Stream
      Protocols: STCP






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   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

   o  Specify DF field
      Protocols: UDP(-Lite)




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






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5.2.  DATA Transfer Related Transport Features

   All 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 to transfer 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.

5.2.1.  Sending Data

   All features in this section are provided by DATA.SEND from pass 2.
   DATA.SEND is given a data block from the application, which we here
   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

   o  Choice of stream
      Protocols: SCTP

   o  Choice of path (destination address)
      Protocols: SCTP

   o  Choice between unordered (potentially faster) or ordered delivery
      of messages
      Protocols: SCTP

   o  Request not to bundle messages
      Protocols: SCTP

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



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

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


5.2.2.  Receiving Data

   All features in this section are provided by DATA.RECEIVE from pass
   2.  DATA.RECEIVE fills a buffer provided by the application, with
   what we here 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 delineation)
      Protocols: TCP

   o  Receive a message
      Protocols: SCTP, 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.

   o  Obtain a message delivery number
      Protocols: SCTP
      Comments: This number can let applications detect and, if desired,
      correct reordering.


5.2.3.  Errors

   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, UDP(-Lite)

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






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   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



6.  Acknowledgements

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


7.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.


8.  Security Considerations

   Authentication, confidentiality protection, and integrity protection
   are identified as Transport Features by [RFC8095].  As currently
   deployed in the Internet, these features are generally provided by a
   protocol or layer on top of the transport protocol; no current full-
   featured standards-track transport protocol provides these features
   on its own.  Therefore, these features are not considered in this
   document, with the exception of native authentication capabilities of
   SCTP for which the security considerations in [RFC4895] apply.


9.  References

9.1.  Normative References

   [FJ16]     Fairhurst, G. and T. Jones, "Features of the User Datagram
              Protocol (UDP) and Lightweight UDP (UDP-Lite) Transport
              Protocols", draft-ietf-taps-transports-usage-udp-00 (work
              in progress), November 2016.



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   [I-D.ietf-tsvwg-sctp-ndata]
              Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol",
              draft-ietf-tsvwg-sctp-ndata-08 (work in progress),
              October 2016.

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

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, DOI 10.17487/
              RFC1122, October 1989,
              <http://www.rfc-editor.org/info/rfc1122>.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758, DOI 10.17487/
              RFC3758, May 2004,
              <http://www.rfc-editor.org/info/rfc3758>.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895,
              August 2007, <http://www.rfc-editor.org/info/rfc4895>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <http://www.rfc-editor.org/info/rfc4960>.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061, DOI 10.17487/
              RFC5061, September 2007,
              <http://www.rfc-editor.org/info/rfc5061>.

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

   [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
              <http://www.rfc-editor.org/info/rfc6182>.

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control



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              Transmission Protocol (SCTP)", RFC 6458, DOI 10.17487/
              RFC6458, December 2011,
              <http://www.rfc-editor.org/info/rfc6458>.

   [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              RFC 6525, DOI 10.17487/RFC6525, February 2012,
              <http://www.rfc-editor.org/info/rfc6525>.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,
              <http://www.rfc-editor.org/info/rfc6817>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
              <http://www.rfc-editor.org/info/rfc6824>.

   [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", RFC 6897, DOI 10.17487/RFC6897,
              March 2013, <http://www.rfc-editor.org/info/rfc6897>.

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

   [RFC7053]  Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
              IMMEDIATELY Extension for the Stream Control Transmission
              Protocol", RFC 7053, DOI 10.17487/RFC7053, November 2013,
              <http://www.rfc-editor.org/info/rfc7053>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <http://www.rfc-editor.org/info/rfc7413>.

   [RFC7496]  Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
              "Additional Policies for the Partially Reliable Stream
              Control Transmission Protocol Extension", RFC 7496,
              DOI 10.17487/RFC7496, April 2015,
              <http://www.rfc-editor.org/info/rfc7496>.

   [RFC7829]  Nishida, Y., Natarajan, P., Caro, A., Amer, P., and K.
              Nielsen, "SCTP-PF: A Quick Failover Algorithm for the
              Stream Control Transmission Protocol", RFC 7829,
              DOI 10.17487/RFC7829, April 2016,



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              <http://www.rfc-editor.org/info/rfc7829>.

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

9.2.  Informative References

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, DOI 10.17487/RFC0854,
              May 1983, <http://www.rfc-editor.org/info/rfc854>.

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
              <http://www.rfc-editor.org/info/rfc3260>.

   [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
              DOI 10.17487/RFC5461, February 2009,
              <http://www.rfc-editor.org/info/rfc5461>.

   [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
              January 2011, <http://www.rfc-editor.org/info/rfc6093>.

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

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

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", RFC 8095, DOI 10.17487/



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              RFC8095, March 2017,
              <http://www.rfc-editor.org/info/rfc8095>.


Appendix A.  Overview of RFCs used as input for pass 1

   TCP:  [RFC0793], [RFC1122], [RFC5482], [RFC7413]
   MPTCP:  [RFC6182], [RFC6824], [RFC6897]
   SCTP:  RFCs without a socket API specification: [RFC3758], [RFC4895],
      [RFC4960], [RFC5061].  RFCs that include a socket API
      specification: [RFC6458], [RFC6525], [RFC6951], [RFC7053],
      [RFC7496] [RFC7829].
   UDP(-Lite):  See [FJ16]
   LEDBAT:  [RFC6817].


Appendix B.  How this document was developed

   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 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
   [RFC2119]-style requirements levels 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 [RFC0793],
   for instance, does not say that data transfer is reliable.  TCP's
   reliability is clear, however, from this text in Section 1 of
   [RFC0793]: "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



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   interconnected systems of such networks."

   Some text for pass 1 subsections was developed copy+pasting all the
   relevant text parts from the relevant RFCs, then adjusting
   terminology to match the terminology in Section 1 and adjusting
   (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 ("first protocol X, here
   are all the primitives; then protocol Y, here are all the primitives,
   ..") but by primitive ("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 a default value is known, this was
   also provided.

   Pass 3: the main point of this pass is to identify transport protocol
   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 all available Transport Features.  This list was
   primarily based on text from pass 2, with additional input from pass
   1 (but no external sources).






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Appendix C.  Revision information

   XXX RFC-Ed please remove this section prior to publication.

   -00 (from draft-welzl-taps-transports): this now covers TCP based on
   all TCP RFCs (this means: if you know of something in any TCP RFC
   that you think should be addressed, please speak up!) as well as
   SCTP, exclusively based on [RFC4960].  We decided to also incorporate
   [RFC6458] for SCTP, but this hasn't happened yet.  Terminology made
   in line with [RFC8095].  Addressed comments by Karen Nielsen and
   Gorry Fairhurst; various other fixes.  Appendices (TCP overview and
   how-to-contribute) added.

   -01: this now also covers MPTCP based on [RFC6182], [RFC6824] and
   [RFC6897].

   -02: included UDP, UDP-Lite, and all extensions of SCTPs.  This
   includes fixing the [RFC6458] omission from -00.

   -03: wrote security considerations.  The "how to contribute" section
   was updated to reflect how the document WAS created, not how it
   SHOULD BE created; it also no longer wrongly says that Experimental
   RFCs are excluded.  Included LEDBAT.  Changed abstract and intro to
   reflect which protocols/mechanisms are covered (TCP, MPTCP, SCTP,
   UDP, UDP-Lite, LEDBAT) instead of talking about "transport
   protocols".  Interleaving and stream scheduling added
   (draft-ietf-tsvwg-sctp-ndata).  TFO added.  "Set protocol parameters"
   in SCTP replaced with per-parameter (or parameter group) primitives.
   More primitives added, mostly previously overlooked ones from
   [RFC6458].  Updated terminology (s/transport service feature/
   transport feature) in line with an update of [RFC8095].  Made
   sequence of transport features / primitives more logical.  Combined
   MPTCP's add/rem subflow with SCTP's add/remove local address.


Authors' Addresses

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

   Phone: +47 22 85 24 20
   Email: michawe@ifi.uio.no






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   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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