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rfc:rfc8303

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

                                                            N. Khademi
                                                    University of Oslo
                                                         February 2018
                 On the Usage of Transport Features
                Provided by IETF Transport Protocols

Abstract

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

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8303.

Welzl, et al. Informational [Page 1] RFC 8303 Transport Services February 2018

Copyright Notice

 Copyright (c) 2018 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

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

Welzl, et al. Informational [Page 2] RFC 8303 Transport Services February 2018

1. Introduction

 This specification describes how transport protocols offer transport
 services, such that applications using them are no longer directly
 tied to a specific protocol.  Breaking this strict connection can
 reduce the effort for an application programmer, yet attain greater
 transport flexibility by pushing complexity into an underlying
 transport services (TAPS) system.
 This design process has started with a survey of the services
 provided by IETF transport protocols and congestion control
 mechanisms [RFC8095].  The present document and [RFC8304] complement
 this survey with an in-depth look at the defined interactions between
 applications and the following unicast transport protocols:
 Transmission Control Protocol (TCP), MultiPath TCP (MPTCP), Stream
 Control Transmission Protocol (SCTP), User Datagram Protocol (UDP),
 and Lightweight User Datagram Protocol (UDP-Lite).  We also define a
 primitive to enable/disable and configure the Low Extra Delay
 Background Transport (LEDBAT) unicast congestion control mechanism.
 For UDP and UDP-Lite, the first step of the protocol analysis -- a
 discussion of relevant RFC text -- is documented in [RFC8304].
 This snapshot in time of the IETF transport protocols is published as
 an RFC to document the analysis by the authors and the TAPS Working
 Group; this generates a set of transport abstractions that can be
 exported in a TAPS API.  It provides the basis for the minimal set of
 transport services that end systems supporting TAPS should implement
 [TAPS-MINSET].
 The list of primitives, events, and transport features in this
 document is strictly based on the parts of protocol specifications
 that describe what the protocol provides to an application using it
 and how the application interacts with it.  Transport protocols
 provide communication between processes that operate on network
 endpoints, which means that they allow for multiplexing of
 communication between the same IP addresses, and this multiplexing is
 achieved using port numbers.  Port multiplexing is therefore assumed
 to be always provided and not discussed in this document.
 Parts of a protocol that are explicitly stated as optional to
 implement are not covered.  Interactions between the application and
 a transport protocol that are not directly related to the operation
 of the protocol are also not covered.  For example, there are various
 ways for an application to use socket options to indicate its
 interest in receiving certain notifications [RFC6458].  However, for
 the purpose of identifying primitives, events, and transport
 features, the ability to enable or disable the reception of
 notifications is irrelevant.  Similarly, "one-to-many style sockets"

Welzl, et al. Informational [Page 3] RFC 8303 Transport Services February 2018

 [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 (e.g., via
 "get" in get/set operations [RFC6458]).
 The document presents a three-pass process to arrive at a list of
 transport features.  In the first pass (pass 1), the relevant RFC
 text is discussed per protocol.  In the second pass (pass 2), this
 discussion is used to derive a list of primitives and events that are
 uniformly categorized across protocols.  Here, an attempt is made to
 present or -- where text describing primitives or events does not yet
 exist -- construct primitives or events in a slightly generalized
 form to highlight similarities.  This is, for example, achieved by
 renaming primitives or events of protocols or by avoiding a strict
 1:1 mapping between the primitives or events in the protocol
 specification and primitives or events in the list.  Finally, the
 third pass (pass 3) presents transport features based on pass 2,
 identifying which protocols implement them.
 In the list resulting from the second pass, some transport features
 are missing because they are implicit in some protocols, and they
 only become explicit when we consider the superset of all transport
 features offered by all protocols.  For example, TCP always carries
 out congestion control; we have to consider it together with a
 protocol like UDP (which does not have congestion control) before we
 can consider congestion control as a transport feature.  The complete
 list of transport features across all protocols is therefore only
 available after pass 3.
 Some protocols are connection oriented.  Connection-oriented
 protocols often use an initial call to a specific primitive to open a
 connection before communication can progress and require
 communication to be explicitly terminated by issuing another call to
 a primitive (usually called 'Close').  A "connection" is the common
 state that some transport primitives refer to, e.g., to adjust
 general configuration settings.  Connection establishment,
 maintenance, and termination are therefore used to categorize
 transport primitives of connection-oriented transport protocols in
 pass 2 and pass 3.  For this purpose, UDP is assumed to be used with
 "connected" sockets, i.e., sockets that are bound to a specific pair
 of addresses and ports [RFC8304].

Welzl, et al. Informational [Page 4] RFC 8303 Transport Services February 2018

2. 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
    transport services using a specific framing and header format on
    the wire.
 Transport Protocol Component:  an implementation of a transport
    feature within a protocol.
 Transport Service Instance:  an arrangement of transport protocols
    with a selected set of features and configuration parameters that
    implement a single transport service, e.g., a protocol stack (RTP
    over UDP).
 Application:  an entity that uses the transport layer for end-to-end
    delivery of data across the network (this may also be an upper-
    layer protocol or tunnel encapsulation).
 Endpoint:  an entity that communicates with one or more other
    endpoints using a transport protocol.
 Connection:  shared state of two or more endpoints that persists
    across messages that are transmitted between these endpoints.
 Primitive:  a function call that is used to locally communicate
    between an application and a transport endpoint.  A primitive is
    related to one or more transport features.
 Event:  a primitive that is invoked by a transport endpoint.
 Parameter:  a value passed between an application and a transport
    protocol by a primitive.
 Socket:  the combination of a destination IP address and a
    destination port number.
 Transport Address:  the combination of an IP address, transport
    protocol, and the port number used by the transport protocol.

Welzl, et al. Informational [Page 5] RFC 8303 Transport Services February 2018

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).  When presenting primitives,
 events, and parameters, the use of lower- and upper-case characters
 is made uniform for the sake of readability.

3.1. Primitives Provided by TCP

 The initial TCP specification [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 of [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).  A procedure
    for aborting the connection is used to avoid excessive
    retransmissions, and an application is able to control the
    threshold used to determine the condition for aborting; this
    threshold may be measured in time units or as a count of
    retransmission [RFC1122].  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

Welzl, et al. Informational [Page 6] RFC 8303 Transport Services February 2018

    connection request has arrived.  Finally, the 'options' parameter
    allows the application to specify IP options such as Source Route,
    Record Route, or Timestamp [RFC1122].  It is not stated on which
    segments of a connection these options should be applied, but
    probably on all segments, as this is also stated in a
    specification given for the usage of the Source Route IP option
    (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.
    Master Key Tuples (MKTs) for authentication can optionally be
    configured when calling 'Open' (Section 7.1 of [RFC5925]).  When
    authentication is in use, complete TCP segments are authenticated,
    including the TCP IPv4 pseudoheader, TCP header, and TCP data.
    TCP Fast Open (TFO) [RFC7413] allows applications to immediately
    hand over a message from the active open to the passive open side
    of a TCP connection together with the first message establishment
    packet (the SYN).  This can be useful for applications that are
    sensitive to TCP's connection setup delay.  [RFC7413] states that
    "TCP implementations MUST NOT use TFO by default, but only use TFO
    if requested explicitly by the application on a per-service-port
    basis."  The size of the message sent with TFO cannot be more than
    TCP's maximum segment size (minus options used in the SYN).  For
    the active open side, it is recommended to change or replace the
    connect() call in order to support a user data buffer argument
    [RFC7413].  For the passive open side, the application needs to
    enable the reception of Fast Open requests, e.g., via a new
    TCP_FASTOPEN setsockopt() socket option before listen().  The
    receiving application must be prepared to accept duplicates of the
    TFO message, as the first data written to a socket can be
    delivered more than once to the application on the remote host.
 Send:  This is the primitive that an application uses to give the
    local TCP transport endpoint a number of bytes that TCP should
    reliably send to the other side of the connection.  The 'urgent'
    flag, if set, states that the data handed over by this send call
    is urgent and this urgency should be indicated to the receiving
    process in case the receiving application has not yet consumed all
    non-urgent data preceding it.  An optional timeout parameter can
    be provided that updates the connection's timeout (see 'Open').
    Additionally, optional parameters allow the ability to indicate
    the preferred outgoing MKT (current_key) and/or the preferred
    incoming MKT (rnext_key) of a connection (Section 7.1 of
    [RFC5925]).

Welzl, et al. Informational [Page 7] RFC 8303 Transport Services February 2018

 Receive:  This primitive allocates a receiving buffer for a provided
    number of bytes.  It returns the number of received bytes provided
    in the buffer when these bytes have been received and written into
    the buffer by TCP.  The application is informed of urgent data via
    an 'urgent' flag: if it is on, there is urgent data; if it is off,
    there is no urgent data or this call to 'Receive' has returned all
    the urgent data.  The application is also informed about the
    current_key and rnext_key information carried in a recently
    received segment via an optional parameter (Section 7.1 of
    [RFC5925]).
 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].
 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 is executed when the user timeout
    (Section 3.9 of [RFC0793]) expires (see the definition of 'Open'
    in this section).  All queues are flushed, and the application is
    informed that the connection had to be aborted due to user
    timeout.
 Error_Report event:  This event informs the application of "soft
    errors" that can be safely ignored [RFC5461], including the
    arrival of an ICMP error message or excessive retransmissions
    (reaching a threshold below the threshold where the connection is
    aborted).  See Section 4.2.4.1 of [RFC1122].
 Type-of-Service:  Section 4.2.4.2 of the requirements for Internet
    hosts [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

Welzl, et al. Informational [Page 8] RFC 8303 Transport Services February 2018

    to the IP layer unchanged.  Since then, the TOS field has been
    redefined.  The Differentiated Services (Diffserv) model [RFC2475]
    [RFC3260] replaces this field in the IP header, assigning the six
    most significant bits to carry the Differentiated Services Code
    Point (DSCP) field [RFC2474].
 Nagle:  The Nagle algorithm delays sending data for some time to
    increase the likelihood of sending a full-sized segment
    (Section 4.2.3.4 of [RFC1122]).  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'), there are three per-
    connection state variables that an application can adjust to
    control the operation of the UTO: 'adv_uto' is the value of the
    UTO advertised to the remote TCP peer (default: system-wide
    default user timeout); 'enabled' (default false) is a boolean-type
    flag that controls whether the UTO option is enabled for a
    connection.  This applies to both sending and receiving.
    'changeable' is a boolean-type flag (default true) that controls
    whether the user timeout may be changed based on a UTO option
    received from the other end of the connection. 'changeable'
    becomes false when an application explicitly sets the user timeout
    (see 'Send').
 Set/Get Authentication Parameters:  The preferred outgoing MKT
    (current_key) and/or the preferred incoming MKT (rnext_key) of a
    connection can be configured.  Information about current_key and
    rnext_key carried in a recently received segment can be retrieved
    (Section 7.1 of [RFC5925]).

3.1.1. Excluded Primitives or Parameters

 The 'Open' primitive can be handed optional precedence or security/
 compartment information [RFC0793], but this was not included here
 because it is mostly irrelevant today [RFC7414].
 The 'Status' primitive was not included because the initial TCP
 specification describes this primitive as "implementation dependent"
 and states that it "could be excluded without adverse effect"
 [RFC0793].  Moreover, while a data block containing specific
 information is described, it is also stated that not all of this
 information may always be available.  While [RFC5925] states that
 'Status' "SHOULD be augmented to allow the MKTs of a current or
 pending connection to be read (for confirmation)", the same

Welzl, et al. Informational [Page 9] RFC 8303 Transport Services February 2018

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

3.2. Primitives Provided by MPTCP

 MPTCP is an extension to TCP that allows the use of multiple paths
 for a single data stream.  It achieves this by creating different so-
 called TCP subflows for each of the interfaces and scheduling the
 traffic across these TCP subflows.  The service provided by MPTCP is
 described as follows in [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, there are some constraints on the API exposed by MPTCP, as
 stated in [RFC6182]:
    A multipath-capable equivalent of TCP MUST retain some level of
    backward compatibility with existing TCP APIs, so that existing
    applications can use the newer transport merely by upgrading the
    operating systems of the end hosts.
 As such, the primitives provided by MPTCP are equivalent to the ones
 provided by TCP.  Nevertheless, the MPTCP RFCs [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.

Welzl, et al. Informational [Page 10] RFC 8303 Transport Services February 2018

 Open:  "An application should be able to request to turn on or turn
    off the usage of MPTCP" [RFC6897].  This functionality can be
    provided through a socket option called 'tcp_multipath_enable'.
    Further, MPTCP must be disabled in case the application is binding
    to a specific address [RFC6897].
 Send/Receive:  The sending and receiving of data does not require any
    changes to the application when MPTCP is being used [RFC6824].
    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 [RFC6824], which
    states: "a TCP subflow MUST NOT use the Urgent Pointer to
    interrupt an existing mapping."
 Address and Subflow Management:  MPTCP uses different addresses and
    allows a host to announce these addresses as part of the protocol.
    The MPTCP API Considerations RFC [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.  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

 TCP has a number of limitations that SCTP removes (Section 1.1 of
 [RFC4960]).  The following three removed limitations directly
 translate into transport features that are visible to an application
 using SCTP: 1) it allows for preservation of message delimiters; 2)
 it does not provide in-order or reliable delivery unless the
 application wants that; 3) multihoming is supported.  In SCTP,
 connections are called "associations" and they can be between not
 only two (as in TCP) but multiple addresses at each endpoint.
 Section 10 of the SCTP base protocol specification [RFC4960]
 specifies the interaction with the application (which SCTP calls the
 "Upper-Layer Protocol (ULP)").  It is assumed that the Operating
 System provides a means for SCTP to asynchronously signal the
 application; the primitives representing such signals are called
 'events' in this section.  Here, we describe the relevant primitives.
 In addition to the abstract API described in Section 10 of [RFC4960],
 an extension to the sockets API is described in [RFC6458].  This
 covers the functionality of the base protocol [RFC4960] and some of
 its extensions [RFC3758] [RFC4895] [RFC5061].  For other protocol
 extensions [RFC6525] [RFC6951] [RFC7053] [RFC7496] [RFC7829]

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 [RFC8260], the corresponding extensions of the sockets API are
 specified in these protocol specifications.  The functionality
 exposed to the ULP through all these APIs is considered here.
 The abstract API contains a 'SetProtocolParameters' primitive that
 allows elements of a parameter list [RFC4960] to be adjusted; it is
 stated that SCTP implementations "may allow ULP to customize some of
 these protocol parameters", indicating that none of the elements of
 this parameter list are mandatory to make ULP configurable.  Thus, we
 only consider the parameters in the abstract API that are also
 covered in one of the other RFCs listed above, which leads us to
 exclude the parameters 'RTO.Alpha', 'RTO.Beta', and 'HB.Max.Burst'.
 For clarity, we also replace 'SetProtocolParameters' itself with
 primitives that adjust parameters or groups of parameters that fit
 together.
 Initialize:  Initialize creates a local SCTP instance that it binds
    to a set of local addresses (and, if provided, a local port
    number) [RFC4960].  Initialize needs to be called only once per
    set of local addresses.  A number of per-association
    initialization parameters can be used when an association is
    created, but before it is connected (via the primitive 'Associate'
    below): the maximum number of inbound streams the application is
    prepared to support, the maximum number of attempts to be made
    when sending the INIT (the first message of association
    establishment), and the maximum retransmission timeout (RTO) value
    to use when attempting an INIT [RFC6458].  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") sockets (Section 9.9 of
    [RFC6458]).  Most primitives are associated with a specific
    association, which is assumed to first have been created.
    Associate can return a list of destination transport addresses so
    that multiple paths can later be used.  One of the returned
    sockets will be selected by the local endpoint as the default
    primary path for sending SCTP packets to this peer, but this
    choice can be changed by the application using the list of
    destination addresses.  Associate is also given the number of
    outgoing streams to request and optionally returns the number of
    negotiated outgoing streams.  An optional parameter of 32 bits,
    the adaptation layer indication, can be provided [RFC5061].  If
    authenticated chunks are used, the chunk types required to be sent
    authenticated by the peer can be provided [RFC4895].  An
    'SCTP_Cant_Str_Assoc' notification is used to inform the

Welzl, et al. Informational [Page 12] RFC 8303 Transport Services February 2018

    application of a failure to create an association [RFC6458].  An
    application could use sendto() or sendmsg() to implicitly set up
    an association, thereby handing over a message that SCTP might
    send during the association setup phase [RFC6458].  Note that this
    mechanism is different from TCP's TFO mechanism: the message would
    arrive only once, after at least one RTT, as it is sent together
    with the third message exchanged during association setup, the
    COOKIE-ECHO chunk).
 Send:  This sends a message of a certain length in bytes over an
    association.  A number can be provided to later refer to the
    correct message when reporting an error, and a stream id is
    provided to specify the stream to be used inside an association
    (we consider this as a mandatory parameter here for simplicity: if
    not provided, the stream id defaults to 0).  A condition to
    abandon the message can be specified (for example limiting the
    number of retransmissions or the lifetime of the user message).
    This allows control of the partial reliability extension [RFC3758]
    [RFC7496].  An optional maximum lifetime can specify the time
    after which the message should be discarded rather than sent.  A
    choice (advisory, i.e., not guaranteed) of the preferred path can
    be made by providing a socket, and the message can be delivered
    out-of-order if the 'unordered' flag is set.  An advisory flag
    indicates that the peer should not delay the acknowledgement of
    the user message provided [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
    authenticated chunks are used, the key identifier for
    authenticating DATA chunks can be provided [RFC4895].
 Receive:  Messages are received from an association, and optionally a
    stream within the association, with their size returned.  The
    application is notified of the availability of data via a 'Data
    Arrive' notification.  If the sender has included a payload
    protocol-id, this value is also returned.  If the received message
    is only a partial delivery of a whole message, a 'partial' flag
    will indicate so, in which case the stream id and a stream
    sequence number are provided to the application.
 Shutdown:  This primitive gracefully closes an association, reliably
    delivering any data that has already been handed over to SCTP.  A
    parameter lets the application control whether further receive or
    send operations or both are disabled when the call is issued.  A
    return code informs about success or failure of this procedure.

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 Abort:  This ungracefully closes an association, by discarding any
    locally queued data and informing the peer that the association
    was aborted.  Optionally, an abort reason to be passed to the peer
    may be provided by the application.  A return code informs about
    success or failure of this procedure.
 Change Heartbeat / Request Heartbeat:  This allows the application to
    enable/disable heartbeats and optionally specify a heartbeat
    frequency as well as requesting a single heartbeat to be carried
    out upon a function call, with a notification about success or
    failure of transmitting the HEARTBEAT chunk to the destination.
 Configure Max. Retransmissions of an Association:  The parameter
    'Association.Max.Retrans' [RFC4960] (called "sasoc_maxrxt" in the
    SCTP sockets API extensions [RFC6458]) allows the configuration of
    the number of unsuccessful retransmissions after which an entire
    association is considered as failed; this should invoke a
    'Communication Lost' notification.
 Set Primary:  This allows the ability to set a new primary default
    path for an association by providing a socket.  Optionally, a
    default source address to be used in IP datagrams can be provided.
 Change Local Address / Set Peer Primary:  This allows an endpoint to
    add/remove local addresses to/from an association.  In addition,
    the peer can be given a hint for which address to use as the
    primary address [RFC5061].
 Configure Path Switchover:  The abstract API contains a primitive
    called 'Set Failure Threshold' [RFC4960].  This configures the
    parameter 'Path.Max.Retrans', which determines after how many
    retransmissions a particular transport address is considered as
    unreachable.  If there are more transport addresses available in
    an association, reaching this limit will invoke a path switchover.
    An extension called "SCTP-PF" adds a concept of "Potentially
    Failed (PF)" paths to this method [RFC7829].  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 [RFC4895].

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 Add/Reset Streams, Reset Association:  This allows an endpoint to add
    streams to an existing association or to reset them individually.
    Additionally, the association can be reset [RFC6525].
 Status:  The 'Status' primitive returns a data block with information
    about a specified association, containing: an association
    connection state; a destination transport address list;
    destination transport address reachability states; current local
    and peer receiver window sizes; current local congestion window
    sizes; number of unacknowledged DATA chunks; number of DATA chunks
    pending receipt; a primary path; the most recent Smoothed Round-
    Trip Time (SRTT) on a primary path; RTO on a primary path; SRTT
    and RTO on other destination addresses [RFC4960]; and an MTU per
    path [RFC6458].
 Enable/Disable Interleaving:  This allows the negotiation of user
    message interleaving support for future associations to be enabled
    or disabled.  For existing associations, it is possible to query
    whether user message interleaving support was negotiated or not on
    a particular association [RFC8260].
 Set Stream Scheduler:  This allows the ability to select a stream
    scheduler per association, with a choice of: First-Come, First-
    Served; Round-Robin; Round-Robin per Packet; Priority-Based; Fair
    Bandwidth; and Weighted Fair Queuing [RFC8260].
 Configure Stream Scheduler:  This allows the ability to change a
    parameter per stream for the schedulers: a priority value for the
    Priority-Based scheduler and a weight for the Weighted Fair
    Queuing scheduler.
 Enable/Disable NoDelay:  This turns on/off any Nagle-like algorithm
    for an association [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 Path MTU (PMTU)
    size), then the message will be fragmented by SCTP.  Disabling
    message fragmentation will produce an error instead of fragmenting
    the message [RFC6458].

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 Configure Path MTU Discovery:  Path MTU Discovery (PMTUD) can be
    enabled or disabled per peer address of an association
    (Section 8.1.12 of [RFC6458]).  When it is enabled, the current
    Path MTU value can be obtained.  When it is disabled, the Path MTU
    to be used can be controlled by the application.
 Configure Delayed SACK Timer:  The time before sending a SACK can be
    adjusted; delaying SACKs can be disabled; and the number of
    packets that must be received before a SACK is sent without
    waiting for the delay timer to expire can be configured [RFC6458].
 Set Cookie Life Value:  The cookie life value can be adjusted
    (Section 8.1.2 of [RFC6458]).  'Valid.Cookie.Life' is also one of
    the parameters that is potentially adjustable with
    'SetProtocolParameters' [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 (Section 8.1.2 of
    [RFC6458]).  'Max.Burst' is also one of the parameters that is
    potentially adjustable with 'SetProtocolParameters' [RFC4960].
 Configure RTO Calculation:  The abstract API contains the following
    adjustable parameters: 'RTO.Initial'; 'RTO.Min'; 'RTO.Max';
    'RTO.Alpha'; and 'RTO.Beta'.  Only the initial, minimum and
    maximum RTOs are also described as configurable in the SCTP
    sockets API extensions [RFC6458].
 Set DSCP Value:  The DSCP value can be set per peer address of an
    association (Section 8.1.12 of [RFC6458]).
 Set IPv6 Flow Label:  The flow label field can be set per peer
    address of an association (Section 8.1.12 of [RFC6458]).
 Set Partial Delivery Point:  This allows the ability to specify the
    size of a message where partial delivery will be invoked.  Setting
    this to a lower value will cause partial deliveries to happen more
    often [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.

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 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.
 Send Failure Notification / Receive Unsent Message / Receive
    Unacknowledged Message: When a message cannot be delivered via an
    association, the sender can be informed about it and learn whether
    the message has just not been acknowledged or (e.g., in case of
    lifetime expiry) if it has not even been sent.  This can also
    inform the sender that a part of the message has been successfully
    delivered.
 Network Status Change Notification:  This informs the application
    about a socket becoming active/inactive [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 association.
 Authentication Notification:  When SCTP wants to notify the upper
    layer regarding the key management related to authenticated chunks
    [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 [RFC5061] [RFC6458].
 Stream Reset Notification:  When SCTP completes the procedure for
    resetting streams [RFC6525], this notification is passed to the
    upper layer, informing it about the result.
 Association Reset Notification:  When SCTP completes the association
    reset procedure [RFC6525], this notification is passed to the
    upper layer, informing it about the result.

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 Stream Change Notification:  When SCTP completes the procedure used
    to increase the number of streams [RFC6525], this notification is
    passed to the upper layer, informing it about the result.
 Sender Dry Notification:  When SCTP has no more user data to send or
    retransmit on a particular association, this notification is
    passed to the upper layer [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 informs an application that the peer has sent a
 SHUTDOWN, and hence no further data should be sent on this socket
 (Section 6.1 of [RFC6458]).  However, if an application would try to
 send data on the socket, it would get an error message anyway; thus,
 this event is classified as "just affecting the application
 programming style, not how the underlying protocol operates" and is
 not included here.

3.4. Primitives Provided by UDP and UDP-Lite

 The set of pass 1 primitives for UDP and UDP-Lite is documented in
 [RFC8304].

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3.5. The Service of LEDBAT

 The service of the LEDBAT congestion control mechanism is described
 as follows:
    LEDBAT is designed for use by background bulk-transfer
    applications to be no more aggressive than standard TCP congestion
    control (as specified in RFC 5681) and to yield in the presence of
    competing flows, thus limiting interference with the network
    performance of competing flows [RFC6817].
 LEDBAT does not have any primitives, as LEDBAT is not a transport
 protocol.  According to its specification [RFC6817]:
    LEDBAT can be used as part of a transport protocol or as part of
    an application, as long as the data transmission mechanisms are
    capable of carrying timestamps and acknowledging data frequently.
    LEDBAT can be used with TCP, Stream Control Transmission Protocol
    (SCTP), and Datagram Congestion Control Protocol (DCCP), with
    appropriate extensions where necessary; and it can be used with
    proprietary application protocols, such as those built on top of
    UDP for peer-to-peer (P2P) applications.
 At the time of writing, the appropriate extensions for TCP, SCTP, or
 DCCP do not exist.
 A number of configurable parameters exist in the LEDBAT
 specification: TARGET, which is the queuing delay target at which
 LEDBAT tries to operate, must be set to 100 ms or less.
 'allowed_increase' (should be 1, must be greater than 0) limits the
 speed at which LEDBAT increases its rate. 'gain', which according to
 [RFC6817] "MUST be set to 1 or less" to avoid a faster ramp-up than
 TCP Reno, determines how quickly the sender responds to changes in
 queueing delay.  Implementations may divide 'gain' into two
 parameters: one for increase and a possibly larger one for decrease.
 We call these parameters 'Gain_Inc' and 'Gain_Dec' here.
 'Base_History' is the size of the list of measured base delays, and,
 according to [RFC6817], "SHOULD be 10".  This list can be filtered
 using a 'Filter' function, which is not prescribed [RFC6817], that
 yields a list of size 'Current_Filter'.  The initial and minimum
 congestion windows, 'Init_CWND' and 'Min_CWND', should both be 2.
 Regarding which of these parameters should be under control of an
 application, the possible range goes from exposing nothing on the one
 hand to considering everything that is not prescribed with a "MUST"
 in the specification as a parameter on the other hand.  Function
 implementations are not provided as a parameter to any of the
 transport protocols discussed here; hence, we do not regard the

Welzl, et al. Informational [Page 19] RFC 8303 Transport Services February 2018

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

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
 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 [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'
 parameter.  This parameter controls a mechanism that is required to
 implement the "synch signal" used by telnet [RFC0854], but [RFC6093]
 states that "new applications SHOULD NOT employ the TCP urgent
 mechanism."  Because pass 2 is meant as a basis for the creation of
 future systems, the "urgent" mechanism is excluded.  This also
 concerns the notification 'Urgent Pointer Advance' in the
 'Error_Report' (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

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 limiting future implementations, it was decided to place it under
 CONNECTION.MAINTENANCE, with all parameters that are described in the
 specification [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
    of the succeeding incoming connection request will be maintained);
    timeout (optional); options (optional); MKT configuration
    (optional); and user message (optional)
    Comments: if the local IP address is not provided, a default
    choice will automatically be made.  The timeout can also be a
    retransmission count.  The options are IP options to be used on
    all segments of the connection.  At least the Source Route option
    is mandatory for TCP to provide.  'MKT configuration' refers to
    the ability to configure MKTs for authentication.  The user
    message may be transmitted to the peer application immediately
    upon reception of the TCP SYN packet.  To benefit from the lower
    latency this provides as part of the experimental TFO mechanism,
    its length must be at most the TCP's maximum segment size (minus
    TCP options used in the SYN).  The message may also be delivered
    more than once to the application on the remote host.
 o  CONNECT.SCTP:
    Pass 1 primitive/event: 'Initialize', followed by 'Enable/Disable
    Interleaving' (optional), followed by 'Associate'
    Parameters: list of local SCTP port number / IP address pairs
    ('Initialize'); one or several sockets (identifying the peer);
    outbound stream count; maximum allowed inbound stream count;
    adaptation layer indication (optional); chunk types required to be
    authenticated (optional); request interleaving on/off; maximum

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    number of INIT attempts (optional); maximum init.  RTO for INIT
    (optional); user message (optional); and remote UDP port number
    (optional)
    Returns: socket list or failure
    Comments: 'Initialize' needs to be called only once per list of
    local SCTP port number / IP address pairs.  One socket will
    automatically be chosen; it can later be changed in MAINTENANCE.
    The user message may be transmitted to the peer application
    immediately upon reception of the packet containing the
    COOKIE-ECHO chunk.  To benefit from the lower latency this
    provides, its length must be limited such that it fits into the
    packet containing the COOKIE-ECHO chunk.  If a remote UDP port
    number is provided, SCTP packets will be encapsulated in UDP.
 o  CONNECT.MPTCP:
    This is similar to CONNECT.TCP except for one additional boolean
    parameter that allows the ability to enable or disable MPTCP for a
    particular connection or socket (default: enabled).
 o  CONNECT.UDP(-Lite):
    Pass 1 primitive/event: 'Connect' followed by 'Send'
    Parameters: 1 local IP address (default (ANY) or specified); 1
    destination transport address; 1 local port (default (OS chooses)
    or specified); and 1 destination port (default (OS chooses) or
    specified).
    Comments: associates a transport address creating a UDP(-Lite)
    socket connection.  This can be called again with a new transport
    address to create a new connection.  The CONNECT function allows
    an application to receive errors from messages sent to a transport
    address.
 AVAILABILITY:
 Preparing to receive incoming connection requests.
 o  LISTEN.TCP:
    Pass 1 primitive/event: 'Open' (passive)
    Parameters: 1 local IP address (optional); 1 socket (optional);
    timeout (optional); buffer to receive a user message (optional);
    and MKT configuration (optional)

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    Comments: if the socket and/or local IP address is provided, this
    waits for incoming connections from only and/or to only the
    provided address.  Else this waits for incoming connections
    without this/these constraint(s).  ESTABLISHMENT can later be
    performed with 'Send'.  If a buffer is provided to receive a user
    message, a user message can be received from a TFO-enabled sender
    before the TCP's connection handshake is completed.  This message
    may arrive multiple times.  'MKT configuration' refers to the
    ability to configure MKTs for authentication.
 o  LISTEN.SCTP:
    Pass 1 primitive/event: 'Initialize', followed by the
    'Communication Up' or 'Restart' notification and possibly the
    'Adaptation Layer' notification
    Parameters: list of local SCTP port number / IP address pairs
    (initialize)
    Returns: socket list; outbound stream count; inbound stream count;
    adaptation layer indication; chunks required to be authenticated;
    and interleaving supported on both sides yes/no
    Comments: 'Initialize' needs to be called only once per list of
    local SCTP port number / IP address pairs.  'Communication Up' can
    also follow a 'Communication Lost' notification, indicating that
    the lost communication is restored.  If the peer has provided an
    adaptation layer indication, an 'Adaptation Layer' notification is
    issued.
 o  LISTEN.MPTCP:
    This is similar to LISTEN.TCP except for one additional boolean
    parameter that allows the ability to enable or disable MPTCP for a
    particular connection or socket (default: enabled).
 o  LISTEN.UDP(-Lite):
    Pass 1 primitive/event: 'Receive'
    Parameters: 1 local IP address (default (ANY) or specified); 1
    destination transport address; local port (default (OS chooses) or
    specified); and destination port (default (OS chooses) or
    specified)
    Comments: the 'Receive' function registers the application to
    listen for incoming UDP(-Lite) datagrams at an endpoint.

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 MAINTENANCE:
 Adjustments made to an open connection, or notifications about it.
 These are out-of-band messages to the protocol that can be issued at
 any time, at least after a connection has been established and before
 it has been terminated (with one exception: CHANGE_TIMEOUT.TCP can
 only be issued for an open connection when DATA.SEND.TCP is called).
 In some cases, these primitives can also be immediately issued during
 ESTABLISHMENT or AVAILABILITY, without waiting for the connection to
 be opened (e.g., CHANGE_TIMEOUT.TCP can be done using TCP's 'Open'
 primitive).  For UDP and UDP-Lite, these functions may establish a
 setting per connection but may also be changed per datagram message.
 o  CHANGE_TIMEOUT.TCP:
    Pass 1 primitive/event: 'Open' or 'Send' combined with unspecified
    control of per-connection state variables
    Parameters: timeout value (optional); adv_uto (optional); boolean
    uto_enabled (optional, default false); and boolean changeable
    (optional, default true)
    Comments: when sending data, an application can adjust the
    connection's timeout value (the time after which the connection
    will be aborted if data could not be delivered).  If 'uto_enabled'
    is true, the 'timeout value' (or, if provided, the value
    'adv_uto') will be advertised for the TCP on the other side of the
    connection to adapt its own user timeout accordingly.
    'uto_enabled' controls whether the UTO option is enabled for a
    connection.  This applies to both sending and receiving.
    'changeable' controls whether the user timeout may be changed
    based on a UTO option received from the other end of the
    connection; it becomes false when the 'timeout value' is used.
 o  CHANGE_TIMEOUT.SCTP:
    Pass 1 primitive/event: 'Change Heartbeat' combined with
    'Configure Max. Retransmissions of an Association'
    Parameters: 'Change Heartbeat': heartbeat frequency and 'Configure
    Max. Retransmissions of an Association': Association.Max.Retrans
    Comments: 'Change Heartbeat' can enable/disable heartbeats in SCTP
    as well as change their frequency.  The parameter
    'Association.Max.Retrans' defines after how many unsuccessful
    transmissions of any packets (including heartbeats) the

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    association will be terminated; thus, these two primitives/
    parameters together can yield a similar behavior for SCTP
    associations as CHANGE_TIMEOUT.TCP does for TCP connections.
 o  DISABLE_NAGLE.TCP:
    Pass 1 primitive/event: not specified
    Parameters: one boolean value
    Comments: the Nagle algorithm delays data transmission to increase
    the chance of sending a full-sized segment.  An application must
    be able to disable this algorithm for a connection.
 o  DISABLE_NAGLE.SCTP:
    Pass 1 primitive/event: 'Enable/Disable NoDelay'
    Parameters: one boolean value
    Comments: Nagle-like algorithms delay data transmission to
    increase the chance of sending a full-sized packet.
 o  REQUEST_HEARTBEAT.SCTP:
    Pass 1 primitive/event: 'Request Heartbeat'
    Parameters: socket
    Returns: success or failure
    Comments: requests an immediate heartbeat on a path, returning
    success or failure.
 o  ADD_PATH.MPTCP:
    Pass 1 primitive/event: not specified
    Parameters: local IP address and optionally the local port number
    Comments: the application specifies the local IP address and port
    number that must be used for a new subflow.

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 o  ADD_PATH.SCTP:
    Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'
    Parameters: local IP address
 o  REM_PATH.MPTCP:
    Pass 1 primitive/event: not specified
    Parameters: local IP address; local port number; remote IP
    address; and remote port number
    Comments: the application removes the subflow specified by the IP/
    port-pair.  The MPTCP implementation must trigger a removal of the
    subflow that belongs to this IP/port-pair.
 o  REM_PATH.SCTP:
    Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'
    Parameters: local IP address
 o  SET_PRIMARY.SCTP:
    Pass 1 primitive/event: 'Set Primary'
    Parameters: socket
    Returns: result of attempting this operation
    Comments: update the current primary address to be used, based on
    the set of available sockets of the association.
 o  SET_PEER_PRIMARY.SCTP:
    Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'
    Parameters: local IP address
    Comments: this is only advisory for the peer.

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 o  CONFIG_SWITCHOVER.SCTP:
    Pass 1 primitive/event: 'Configure Path Switchover'
    Parameters: primary max retrans (number of retransmissions after
    which a path is considered inactive) and PF max retrans (number of
    retransmissions after which a path is considered to be
    "Potentially Failed", and others will be preferably used)
    (optional)
 o  STATUS.SCTP:
    Pass 1 primitive/event: 'Status', 'Enable/Disable Interleaving',
    and 'Network Status Change' notification
    Returns: data block with information about a specified
    association, containing: association connection state; destination
    transport address list; destination transport address reachability
    states; current local and peer receiver window sizes; current
    local congestion window sizes; number of unacknowledged DATA
    chunks; number of DATA chunks pending receipt; primary path; most
    recent SRTT on primary path; RTO on primary path; SRTT and RTO on
    other destination addresses; MTU per path; and interleaving
    supported yes/no
    Comments: the 'Network Status Change' notification informs the
    application about a socket becoming active/inactive; this only
    affects the programming style, as the same information is also
    available via 'Status'.
 o  STATUS.MPTCP:
    Pass 1 primitive/event: not specified
    Returns: list of pairs of tuples of IP address and TCP port number
    of each subflow.  The first of the pair is the local IP and port
    number, while the second is the remote IP and port number.
 o  SET_DSCP.TCP:
    Pass 1 primitive/event: not specified
    Parameters: DSCP value
    Comments: this allows an application to change the DSCP value for
    outgoing segments.

Welzl, et al. Informational [Page 27] RFC 8303 Transport Services February 2018

 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) and subreason (encoding
    not specified)
    Comments: soft errors that can be ignored without harm by many
    applications; an application should be able to disable these
    notifications.  The reported conditions include at least: ICMP
    error message arrived and excessive retransmissions.
 o  ERROR.UDP(-Lite):
    Pass 1 primitive/event: 'Error_Report'
    Returns: Error report
    Comments: this returns soft errors that may be ignored without
    harm by many applications; an application must connect to be able
    receive these notifications.

Welzl, et al. Informational [Page 28] RFC 8303 Transport Services February 2018

 o  SET_AUTH.TCP:
    Pass 1 primitive/event: not specified
    Parameters: current_key and rnext_key
    Comments: current_key and rnext_key are the preferred outgoing MKT
    and the preferred incoming MKT, respectively, for a segment that
    is sent on the connection.
 o  SET_AUTH.SCTP:
    Pass 1 primitive/event: 'Set/Get Authentication Parameters'
    Parameters: key_id; key; and hmac_id
 o  GET_AUTH.TCP:
    Pass 1 primitive/event: not specified
    Parameters: current_key and rnext_key
    Comments: current_key and rnext_key are the preferred outgoing MKT
    and the preferred incoming MKT, respectively, that were carried on
    a recently received segment.
 o  GET_AUTH.SCTP:
    Pass 1 primitive/event: 'Set/Get Authentication Parameters'
    Parameters: key_id and chunk_list
 o  RESET_STREAM.SCTP:
    Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'
    Parameters: sid and direction
 o  RESET_STREAM-EVENT.SCTP:
    Pass 1 primitive/event: 'Stream Reset' notification
    Parameters: information about the result of RESET_STREAM.SCTP
    Comments: this is issued when the procedure for resetting streams
    has completed.

Welzl, et al. Informational [Page 29] RFC 8303 Transport Services February 2018

 o  RESET_ASSOC.SCTP:
    Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'
    Parameters: information related to the extension, as 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 of outgoing and incoming streams to be added
 o  ADD_STREAM-EVENT.SCTP:
    Pass 1 primitive/event: 'Stream Change' notification
    Parameters: information about the result of ADD_STREAM.SCTP
    Comments: this is issued when the procedure for adding a stream
    has completed.
 o  SET_STREAM_SCHEDULER.SCTP:
    Pass 1 primitive/event: 'Set Stream Scheduler'
    Parameters: scheduler identifier
    Comments: choice of First-Come, First-Served; Round-Robin; Round-
    Robin per Packet; Priority-Based; Fair Bandwidth; and Weighted
    Fair Queuing.

Welzl, et al. Informational [Page 30] RFC 8303 Transport Services February 2018

 o  CONFIGURE_STREAM_SCHEDULER.SCTP:
    Pass 1 primitive/event: 'Configure Stream Scheduler'
    Parameters: priority
    Comments: the priority value only applies when Priority-Based or
    Weighted Fair Queuing scheduling is chosen with
    SET_STREAM_SCHEDULER.SCTP.  The meaning of the parameter differs
    between these two schedulers, but in both cases, it realizes some
    form of prioritization regarding how bandwidth is divided among
    streams.
 o  SET_FLOWLABEL.SCTP:
    Pass 1 primitive/event: 'Set IPv6 Flow Label'
    Parameters: flow label
    Comments: this allows an application to change the IPv6 header's
    flow label field for outgoing packets on a path.
 o  AUTHENTICATION_NOTIFICATION-EVENT.SCTP:
    Pass 1 primitive/event: 'Authentication' notification
    Returns: information regarding key management
 o  CONFIG_SEND_BUFFER.SCTP:
    Pass 1 primitive/event: 'Configure Send Buffer Size'
    Parameters: size value in octets
 o  CONFIG_RECEIVE_BUFFER.SCTP:
    Pass 1 primitive/event: 'Configure Receive Buffer Size'
    Parameters: size value in octets
    Comments: this controls the receiver window.

Welzl, et al. Informational [Page 31] RFC 8303 Transport Services February 2018

 o  CONFIG_FRAGMENTATION.SCTP:
    Pass 1 primitive/event: 'Configure Message Fragmentation'
    Parameters: one boolean value (enable/disable) and maximum
    fragmentation size (optional; default: PMTU)
    Comments: if fragmentation is enabled, messages exceeding the
    maximum fragmentation size will be fragmented.  If fragmentation
    is disabled, trying to send a message that exceeds the maximum
    fragmentation size will produce an error.
 o  CONFIG_PMTUD.SCTP:
    Pass 1 primitive/event: 'Configure Path MTU Discovery'
    Parameters: one boolean value (PMTUD on/off) and PMTU value
    (optional)
    Returns: PMTU value
    Comments: this returns a meaningful PMTU value when PMTUD is
    enabled (the boolean is true), and the PMTU value can be set if
    PMTUD is disabled (the boolean is false).
 o  CONFIG_DELAYED_SACK.SCTP:
    Pass 1 primitive/event: 'Configure Delayed SACK Timer'
    Parameters: one boolean value (delayed SACK on/off); timer value
    (optional); and number of packets to wait for (default 2)
    Comments: if delayed SACK is enabled, SCTP will send a SACK either
    upon receiving the provided number of packets or when the timer
    expires, whatever occurs first.
 o  CONFIG_RTO.SCTP:
    Pass 1 primitive/event: 'Configure RTO Calculation'
    Parameters: init (optional); min (optional); and max (optional)
    Comments: this adjusts the initial, minimum, and maximum RTO
    values.

Welzl, et al. Informational [Page 32] RFC 8303 Transport Services February 2018

 o  SET_COOKIE_LIFE.SCTP:
    Pass 1 primitive/event: 'Set Cookie Life Value'
    Parameters: cookie life value
 o  SET_MAX_BURST.SCTP:
    Pass 1 primitive/event: 'Set Maximum Burst'
    Parameters: max burst value
    Comments: not all implementations allow values above the default
    of 4.
 o  SET_PARTIAL_DELIVERY_POINT.SCTP:
    Pass 1 primitive/event: 'Set Partial Delivery Point'
    Parameters: partial delivery point (integer)
    Comments: this parameter must be smaller or equal to the socket
    receive buffer size.
 o  SET_CHECKSUM_ENABLED.UDP:
    Pass 1 primitive/event: 'Checksum_Enabled'
    Parameters: 0 when zero checksum is used at sender, 1 for checksum
    at sender (default)
 o  SET_CHECKSUM_REQUIRED.UDP:
    Pass 1 primitive/event: 'Require_Checksum'
    Parameter: 0 to allow zero checksum, 1 when a non-zero checksum is
    required (default) at the receiver
 o  SET_CHECKSUM_COVERAGE.UDP-Lite:
    Pass 1 primitive/event: 'Set_Checksum_Coverage'
    Parameters: coverage length at sender (default maximum coverage)

Welzl, et al. Informational [Page 33] RFC 8303 Transport Services February 2018

 o  SET_MIN_CHECKSUM_COVERAGE.UDP-Lite:
    Pass 1 primitive/event: 'Set_Min_Coverage'
    Parameter: coverage length at receiver (default minimum coverage)
 o  SET_DF.UDP(-Lite):
    Pass 1 primitive event: 'Set_DF'
    Parameter: 0 when DF is not set (default) in the IPv4 header, 1
    when DF is set
 o  GET_MMS_S.UDP(-Lite):
    Pass 1 primitive event: 'Get_MM_S'
    Comments: this retrieves the maximum transport-message size that
    may be sent using a non-fragmented IP packet from the configured
    interface.
 o  GET_MMS_R.UDP(-Lite):
    Pass 1 primitive event: 'Get_MMS_R'
    Comments: this retrieves the maximum transport-message size that
    may be received from the configured interface.
 o  SET_TTL.UDP(-Lite) (IPV6_UNICAST_HOPS):
    Pass 1 primitive/event: 'Set_TTL' and 'Set_IPV6_Unicast_Hops'
    Parameters: IPv4 TTL value or IPv6 Hop Count value
    Comments: this allows an application to change the IPv4 TTL of
    IPv6 Hop Count value for outgoing UDP(-Lite) datagrams.
 o  GET_TTL.UDP(-Lite) (IPV6_UNICAST_HOPS):
    Pass 1 primitive/event: 'Get_TTL' and 'Get_IPV6_Unicast_Hops'
    Returns: IPv4 TTL value or IPv6 Hop Count value
    Comments: this allows an application to read the IPv4 TTL of the
    IPv6 Hop Count value from a received UDP(-Lite) datagram.

Welzl, et al. Informational [Page 34] RFC 8303 Transport Services February 2018

 o  SET_ECN.UDP(-Lite):
    Pass 1 primitive/event: 'Set_ECN'
    Parameters: ECN value
    Comments: this allows a UDP(-Lite) application to set the Explicit
    Congestion Notification (ECN) code point field for outgoing
    UDP(-Lite) datagrams.  It defaults to sending '00'.
 o  GET_ECN.UDP(-Lite):
    Pass 1 primitive/event: 'Get_ECN'
    Parameters: ECN value
    Comments: this allows a UDP(-Lite) application to read the ECN
    code point field from a received UDP(-Lite) datagram.
 o  SET_IP_OPTIONS.UDP(-Lite):
    Pass 1 primitive/event: 'Set_IP_Options'
    Parameters: options
    Comments: this allows a UDP(-Lite) application to set IP options
    for outgoing UDP(-Lite) datagrams.  These options can at least be
    the Source Route, Record Route, and Timestamp option.
 o  GET_IP_OPTIONS.UDP(-Lite):
    Pass 1 primitive/event: 'Get_IP_Options'
    Returns: options
    Comments: this allows a UDP(-Lite) application to receive any IP
    options that are contained in a received UDP(-Lite) datagram.
 o  CONFIGURE.LEDBAT:
    Pass 1 primitive/event: N/A
    Parameters: enable (boolean); target; allowed_increase; gain_inc;
    gain_dec; base_history; current_filter; init_cwnd; and min_cwnd
    Comments: 'enable' is a newly invented parameter that enables or
    disables the whole LEDBAT service.

Welzl, et al. Informational [Page 35] RFC 8303 Transport Services February 2018

 TERMINATION:
 Gracefully or forcefully closing a connection or being informed about
 this event happening.
 o  CLOSE.TCP:
    Pass 1 primitive/event: 'Close'
    Comments: this terminates the sending side of a connection after
    reliably delivering all remaining data.
 o  CLOSE.SCTP:
    Pass 1 primitive/event: 'Shutdown'
    Comments: this terminates a connection after reliably delivering
    all remaining data.
 o  ABORT.TCP:
    Pass 1 primitive/event: 'Abort'
    Comments: this terminates a connection without delivering
    remaining data and sends an error message to the other side.
 o  ABORT.SCTP:
    Pass 1 primitive/event: 'Abort'
    Parameters: abort reason to be given to the peer (optional)
    Comments: this terminates a connection without delivering
    remaining data and sends an error message to the other side.
 o  ABORT.UDP(-Lite):
    Pass 1 primitive event: 'Close'
    Comments: this terminates a connection without delivering
    remaining data.  No further UDP(-Lite) datagrams are sent/received
    for this transport service instance.

Welzl, et al. Informational [Page 36] RFC 8303 Transport Services February 2018

 o  TIMEOUT.TCP:
    Pass 1 primitive/event: 'User Timeout' event
    Comments: the application is informed that the connection is
    aborted.  This event is executed on expiration of the timeout set
    in CONNECTION.ESTABLISHMENT.CONNECT.TCP (possibly adjusted in
    CONNECTION.MAINTENANCE.CHANGE_TIMEOUT.TCP).
 o  TIMEOUT.SCTP:
    Pass 1 primitive/event: 'Communication Lost' event
    Comments: the application is informed that the connection is
    aborted.  This event is executed on expiration of the timeout that
    should be enabled by default (see the beginning of 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.

Welzl, et al. Informational [Page 37] RFC 8303 Transport Services February 2018

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
 receiving data (although this is optional for the primitives of
 UDP(-Lite)).  In addition to the listed parameters, all sending
 primitives contain a reference to a data block, and all receiving
 primitives contain a reference to available buffer space for the
 data.  Note that CONNECT.TCP and LISTEN.TCP in the ESTABLISHMENT and
 AVAILABILITY categories also allow to transfer data (an optional user
 message) before the connection is fully established.
 o  SEND.TCP:
    Pass 1 primitive/event: 'Send'
    Parameters: timeout (optional); current_key (optional); and
    rnext_key (optional)
    Comments: this gives TCP a data block for reliable transmission to
    the TCP on the other side of the connection.  The timeout can be
    configured with this call (see also
    CONNECTION.MAINTENANCE.CHANGE_TIMEOUT.TCP). 'current_key' and
    'rnext_key' are authentication parameters that can be configured
    with this call (see also CONNECTION.MAINTENANCE.SET_AUTH.TCP).
 o  SEND.SCTP:
    Pass 1 primitive/event: 'Send'
    Parameters: stream number; context (optional); socket (optional);
    unordered flag (optional); no-bundle flag (optional); payload
    protocol-id (optional); pr-policy (optional) pr-value (optional);
    sack-immediately flag (optional); and key-id (optional)
    Comments: this gives SCTP a data block for transmission to the
    SCTP on the other side of the connection (SCTP association).  The
    'stream number' denotes the stream to be used.  The 'context'
    number can later be used to refer to the correct message when an
    error is reported.  The 'socket' can be used to state which path
    should be preferred, if there are multiple paths available (see
    also CONNECTION.MAINTENANCE.SETPRIMARY.SCTP).  The data block can
    be delivered out of order if the 'unordered' flag is set.  The
    'no-bundle flag' can be set to indicate a preference to avoid
    bundling.  The 'payload protocol-id' is a number that will, if
    provided, be handed over to the receiving application.  Using
    pr-policy and pr-value, the level of reliability can be
    controlled.  The 'sack-immediately' flag can be used to indicate

Welzl, et al. Informational [Page 38] RFC 8303 Transport Services February 2018

    that the peer should not delay the sending of a SACK corresponding
    to the provided user message.  If specified, the provided key-id
    is used for authenticating the user message.
 o  SEND.UDP(-Lite):
    Pass 1 primitive/event: 'Send'
    Parameters: IP address and port number of the destination endpoint
    (optional if connected)
    Comments: this provides a message for unreliable transmission
    using UDP(-Lite) to the specified transport address.  The IP
    address and port number may be omitted for connected UDP(-Lite)
    sockets.  All CONNECTION.MAINTENANCE.SET_*.UDP(-Lite) primitives
    apply per message sent.
 o  RECEIVE.TCP:
    Pass 1 primitive/event: 'Receive'
    Parameters: current_key (optional) and rnext_key (optional)
    Comments: 'current_key' and 'rnext_key' are authentication
    parameters that can be read with this call (see also
    CONNECTION.MAINTENANCE.GET_AUTH.TCP).
 o  RECEIVE.SCTP:
    Pass 1 primitive/event: 'Data Arrive' notification, followed by
    'Receive'
    Parameters: stream number (optional)
    Returns: stream sequence number (optional) and partial flag
    (optional)
    Comments: if the 'stream number' is provided, the call to receive
    only receives data on one particular stream.  If a partial message
    arrives, this is indicated by the 'partial flag', and then the
    'stream sequence number' must be provided such that an application
    can restore the correct order of data blocks that comprise an
    entire message.

Welzl, et al. Informational [Page 39] RFC 8303 Transport Services February 2018

 o  RECEIVE.UDP(-Lite):
    Pass 1 primitive/event: 'Receive'
    Parameters: buffer for received datagram
    Comments: all CONNECTION.MAINTENANCE.GET_*.UDP(-Lite) primitives
    apply per message received.
 o  SENDFAILURE-EVENT.SCTP:
    Pass 1 primitive/event: 'Send Failure' notification, optionally
    followed by 'Receive Unsent Message' or 'Receive Unacknowledged
    Message'
    Returns: cause code; context; and unsent or unacknowledged message
    (optional)
    Comments: 'cause code' indicates the reason of the failure, and
    'context' is the context number if such a number has been provided
    in DATA.SEND.SCTP, for later use with 'Receive Unsent Message' or
    'Receive Unacknowledged Message', respectively.  These primitives
    can be used to retrieve the unsent or unacknowledged message (or
    part of the message, in case a part was delivered) if desired.
 o  SEND_FAILURE.UDP(-Lite):
    Pass 1 primitive/event: 'Send'
    Comments: this may be used to probe for the effective PMTU when
    using in combination with the 'MAINTENANCE.SET_DF' primitive.
 o  SENDER_DRY-EVENT.SCTP:
    Pass 1 primitive/event: 'Sender Dry' notification
    Comments: this informs the application that the stack has no more
    user data to send.
 o  PARTIAL_DELIVERY_ABORTED-EVENT.SCTP:
    Pass 1 primitive/event: 'Partial Delivery Aborted' notification
    Comments: this informs the receiver of a partial message that the
    further delivery of the message has been aborted.

Welzl, et al. Informational [Page 40] RFC 8303 Transport Services February 2018

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
 transport features that can be configured in one protocol and are
 static properties in another (congestion control, for example).
 Again, some minor details are omitted for the sake of generalization
 -- e.g., TCP may provide various different IP options, but only
 source route is mandatory to implement, and this detail is not
 visible in the pass 3 transport feature "Specify IP options".  As
 before, "UDP(-Lite)" represents both UDP and UDP-Lite, and "TCP"
 refers to both TCP and MPTCP.

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, and UDP(-Lite)
 o  Specify which IP options must always be used
    Protocols: TCP and UDP(-Lite)
 o  Request multiple streams
    Protocols: SCTP
 o  Limit the number of inbound streams
    Protocols: SCTP
 o  Specify number of attempts and/or timeout for the first
    establishment message
    Protocols: TCP and SCTP
 o  Obtain multiple sockets
    Protocols: SCTP
 o  Disable MPTCP
    Protocols: MPTCP

Welzl, et al. Informational [Page 41] RFC 8303 Transport Services February 2018

 o  Configure authentication
    Protocols: TCP and SCTP
    Comments: with TCP, this allows the configuration of MKTs.  In
    SCTP, this allows the specification of which chunk types must
    always be authenticated.  DATA, ACK, etc., are different 'chunks'
    in SCTP; one or more chunks may be included in a single packet.
 o  Indicate an Adaptation Layer (via an adaptation code point)
    Protocols: SCTP
 o  Request to negotiate interleaving of user messages
    Protocols: SCTP
 o  Hand over a message to reliably transfer (possibly multiple times)
    before connection establishment
    Protocols: TCP
 o  Hand over a message to reliably transfer during connection
    establishment
    Protocols: SCTP
 o  Enable UDP encapsulation with a specified remote UDP port number
    Protocols: SCTP
 AVAILABILITY:
 Preparing to receive incoming connection requests.
 o  Listen, 1 specified local interface
    Protocols: TCP, SCTP, and UDP(-Lite)
 o  Listen, N specified local interfaces
    Protocols: SCTP
 o  Listen, all local interfaces
    Protocols: TCP, SCTP, and UDP(-Lite)
 o  Obtain requested number of streams
    Protocols: SCTP
 o  Limit the number of inbound streams
    Protocols: SCTP
 o  Specify which IP options must always be used
    Protocols: TCP and UDP(-Lite)

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 o  Disable MPTCP
    Protocols: MPTCP
 o  Configure authentication
    Protocols: TCP and SCTP
    Comments: with TCP, this allows the configuration of MKTs.  In
    SCTP, this allows the specification of which chunk types must
    always be authenticated.  DATA, ACK, etc., are different 'chunks'
    in SCTP; one or more chunks may be included in a single packet.
 o  Indicate an Adaptation Layer (via an adaptation code point)
    Protocols: SCTP
 MAINTENANCE:
 Adjustments made to an open connection, or notifications about it.
 o  Change timeout for aborting connection (using retransmit limit or
    time value)
    Protocols: TCP and SCTP
 o  Suggest timeout to the peer
    Protocols: TCP
 o  Disable Nagle algorithm
    Protocols: TCP and SCTP
 o  Request an immediate heartbeat, returning success/failure
    Protocols: SCTP
 o  Notification of excessive retransmissions (early warning below
    abortion threshold)
    Protocols: TCP
 o  Add path
    Protocols: MPTCP and SCTP
    MPTCP Parameters: source-IP; source-Port; destination-IP; and
    destination-Port
    SCTP Parameters: local IP address
 o  Remove path
    Protocols: MPTCP and SCTP
    MPTCP Parameters: source-IP; source-Port; destination-IP; and
    destination-Port
    SCTP Parameters: local IP address

Welzl, et al. Informational [Page 43] RFC 8303 Transport Services February 2018

 o  Set primary path
    Protocols: SCTP
 o  Suggest primary path to the peer
    Protocols: SCTP
 o  Configure Path Switchover
    Protocols: SCTP
 o  Obtain status (query or notification)
    Protocols: SCTP and MPTCP
    SCTP parameters: association connection state; destination
    transport address list; destination transport address reachability
    states; current local and peer receiver window sizes; current
    local congestion window sizes; number of unacknowledged DATA
    chunks; number of DATA chunks pending receipt; primary path; most
    recent SRTT on primary path; RTO on primary path; SRTT and RTO on
    other destination addresses; MTU per path; and interleaving
    supported yes/no
    MPTCP parameters: subflow-list (identified by source-IP;
    source-Port; destination-IP; and destination-Port)
 o  Specify DSCP field
    Protocols: TCP, SCTP, and UDP(-Lite)
 o  Notification of ICMP error message arrival
    Protocols: TCP and UDP(-Lite)
 o  Change authentication parameters
    Protocols: TCP and SCTP
 o  Obtain authentication information
    Protocols: TCP and SCTP
 o  Reset Stream
    Protocols: SCTP
 o  Notification of Stream Reset
    Protocols: STCP
 o  Reset Association
    Protocols: SCTP
 o  Notification of Association Reset
    Protocols: STCP
 o  Add Streams
    Protocols: SCTP

Welzl, et al. Informational [Page 44] RFC 8303 Transport Services February 2018

 o  Notification of Added Stream
    Protocols: STCP
 o  Choose a scheduler to operate between streams of an association
    Protocols: SCTP
 o  Configure priority or weight for a scheduler
    Protocols: SCTP
 o  Specify IPv6 flow label field
    Protocols: SCTP
 o  Configure send buffer size
    Protocols: SCTP
 o  Configure receive buffer (and rwnd) size
    Protocols: SCTP
 o  Configure message fragmentation
    Protocols: SCTP
 o  Configure PMTUD
    Protocols: SCTP
 o  Configure delayed SACK timer
    Protocols: SCTP
 o  Set Cookie life value
    Protocols: SCTP
 o  Set maximum burst
    Protocols: SCTP
 o  Configure size where messages are broken up for partial delivery
    Protocols: SCTP
 o  Disable checksum when sending
    Protocols: UDP
 o  Disable checksum requirement when receiving
    Protocols: UDP
 o  Specify checksum coverage used by the sender
    Protocols: UDP-Lite
 o  Specify minimum checksum coverage required by receiver
    Protocols: UDP-Lite

Welzl, et al. Informational [Page 45] RFC 8303 Transport Services February 2018

 o  Specify DF field
    Protocols: UDP(-Lite)
 o  Get max. transport-message size that may be sent using a non-
    fragmented IP packet from the configured interface
    Protocols: UDP(-Lite)
 o  Get max. transport-message size that may be received from the
    configured interface
    Protocols: UDP(-Lite)
 o  Specify TTL/Hop Count field
    Protocols: UDP(-Lite)
 o  Obtain TTL/Hop Count field
    Protocols: UDP(-Lite)
 o  Specify ECN field
    Protocols: UDP(-Lite)
 o  Obtain ECN field
    Protocols: UDP(-Lite)
 o  Specify IP options
    Protocols: UDP(-Lite)
 o  Obtain IP options
    Protocols: UDP(-Lite)
 o  Enable and configure "Low Extra Delay Background Transfer"
    Protocols: A protocol implementing the LEDBAT congestion control
    mechanism
 TERMINATION:
 Gracefully or forcefully closing a connection, or being informed
 about this event happening.
 o  Close after reliably delivering all remaining data, causing an
    event informing the application on the other side
    Protocols: TCP and SCTP
    Comments: a TCP endpoint locally only closes the connection for
    sending; it may still receive data afterwards.
 o  Abort without delivering remaining data, causing an event that
    informs the application on the other side
    Protocols: TCP and SCTP

Welzl, et al. Informational [Page 46] RFC 8303 Transport Services February 2018

    Comments: in SCTP, a reason can optionally be given by the
    application on the aborting side, which can then be received by
    the application on the other side.
 o  Abort without delivering remaining data, not causing an event that
    informs the application on the other side
    Protocols: UDP(-Lite)
 o  Timeout event when data could not be delivered for too long
    Protocols: TCP and SCTP
    Comments: the timeout is configured with CONNECTION.MAINTENANCE
    "Change timeout for aborting connection (using retransmit limit or
    time value)".

5.2. DATA-Transfer-Related Transport Features

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

5.2.1. Sending Data

 All transport features in this section are provided by DATA.SEND from
 pass 2.  DATA.SEND is given a data block from the application, which
 here we call a "message" if the beginning and end of the data block
 can be identified at the receiver, and "data" otherwise.
 o  Reliably transfer data, with congestion control
    Protocols: TCP
 o  Reliably transfer a message, with congestion control
    Protocols: SCTP
 o  Unreliably transfer a message, with congestion control
    Protocols: SCTP
 o  Unreliably transfer a message, without congestion control
    Protocols: UDP(-Lite)
 o  Configurable Message Reliability
    Protocols: SCTP

Welzl, et al. Informational [Page 47] RFC 8303 Transport Services February 2018

 o  Choice of stream
    Protocols: SCTP
 o  Choice of path (destination address)
    Protocols: SCTP
 o  Ordered message delivery (potentially slower than unordered)
    Protocols: SCTP
 o  Unordered message delivery (potentially faster than ordered)
    Protocols: SCTP and UDP(-Lite)
 o  Request not to bundle messages
    Protocols: SCTP
 o  Specifying a 'payload protocol-id' (handed over as such by the
    receiver)
    Protocols: SCTP
 o  Specifying a key identifier to be used to authenticate a message
    Protocols: SCTP
 o  Request not to delay the acknowledgement (SACK) of a message
    Protocols: SCTP

5.2.2. Receiving Data

 All transport features in this section are provided by DATA.RECEIVE
 from pass 2.  DATA.RECEIVE fills a buffer provided by the
 application, with what here we call a "message" if the beginning and
 end of the data block can be identified at the receiver, and "data"
 otherwise.
 o  Receive data (with no message delimiting)
    Protocols: TCP
 o  Receive a message
    Protocols: SCTP and UDP(-Lite)
 o  Choice of stream to receive from
    Protocols: SCTP
 o  Information about partial message arrival
    Protocols: SCTP
    Comments: in SCTP, partial messages are combined with a stream
    sequence number so that the application can restore the correct
    order of data blocks an entire message consists of.

Welzl, et al. Informational [Page 48] RFC 8303 Transport Services February 2018

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 and UDP(-Lite)
 o  Notification of an unacknowledged (part of a) message
    Protocols: SCTP
 o  Notification that the stack has no more user data to send
    Protocols: SCTP
 o  Notification to a receiver that a partial message delivery has
    been aborted
    Protocols: SCTP

6. IANA Considerations

 This document does not require any IANA actions.

7. Security Considerations

 Authentication, confidentiality protection, and integrity protection
 are identified as transport features [RFC8095].  These transport
 features are generally provided by a protocol or layer on top of the
 transport protocol; none of the transport protocols considered in
 this document provides these transport features on its own.
 Therefore, these transport features are not considered in this
 document, with the exception of native authentication capabilities of
 TCP and SCTP for which the security considerations in [RFC5925] and
 [RFC4895] apply.
 Security considerations for the use of UDP and UDP-Lite are provided
 in the referenced RFCs.  Security guidance for application usage is
 provided in the UDP Guidelines [RFC8085].

Welzl, et al. Informational [Page 49] RFC 8303 Transport Services February 2018

8. References

8.1. Normative References

 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, DOI 10.17487/RFC0793, September 1981,
            <https://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,
            <https://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,
            <https://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, <https://www.rfc-editor.org/info/rfc4895>.
 [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
            RFC 4960, DOI 10.17487/RFC4960, September 2007,
            <https://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,
            <https://www.rfc-editor.org/info/rfc5061>.
 [RFC5482]  Eggert, L. and F. Gont, "TCP User Timeout Option",
            RFC 5482, DOI 10.17487/RFC5482, March 2009,
            <https://www.rfc-editor.org/info/rfc5482>.
 [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
            Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
            June 2010, <https://www.rfc-editor.org/info/rfc5925>.
 [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,
            <https://www.rfc-editor.org/info/rfc6182>.

Welzl, et al. Informational [Page 50] RFC 8303 Transport Services February 2018

 [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
            Yasevich, "Sockets API Extensions for the Stream Control
            Transmission Protocol (SCTP)", RFC 6458,
            DOI 10.17487/RFC6458, December 2011,
            <https://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,
            <https://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,
            <https://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,
            <https://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, <https://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,
            <https://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,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc7496>.

Welzl, et al. Informational [Page 51] RFC 8303 Transport Services February 2018

 [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,
            <https://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, <https://www.rfc-editor.org/info/rfc8085>.
 [RFC8260]  Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
            "Stream Schedulers and User Message Interleaving for the
            Stream Control Transmission Protocol", RFC 8260,
            DOI 10.17487/RFC8260, November 2017,
            <https://www.rfc-editor.org/info/rfc8260>.
 [RFC8304]  Fairhurst, G. and T. Jones, "Transport Features of the
            User Datagram Protocol (UDP) and Lightweight UDP (UDP-
            Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018,
            <https://www.rfc-editor.org/info/rfc8304>.

8.2. Informative References

 [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
            Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
            1983, <https://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,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS
            Field) in the IPv4 and IPv6 Headers", RFC 2474,
            DOI 10.17487/RFC2474, December 1998,
            <https://www.rfc-editor.org/info/rfc2474>.
 [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
            and W. Weiss, "An Architecture for Differentiated
            Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
            <https://www.rfc-editor.org/info/rfc2475>.
 [RFC3260]  Grossman, D., "New Terminology and Clarifications for
            Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
            <https://www.rfc-editor.org/info/rfc3260>.

Welzl, et al. Informational [Page 52] RFC 8303 Transport Services February 2018

 [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
            DOI 10.17487/RFC5461, February 2009,
            <https://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, <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc7657>.
 [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
            Ed., "Services Provided by IETF Transport Protocols and
            Congestion Control Mechanisms", RFC 8095,
            DOI 10.17487/RFC8095, March 2017,
            <https://www.rfc-editor.org/info/rfc8095>.
 [TAPS-MINSET]
            Welzl, M. and S. Gjessing, "A Minimal Set of Transport
            Services for TAPS Systems", Work in Progress, draft-ietf-
            taps-minset-01, February 2018.

Welzl, et al. Informational [Page 53] RFC 8303 Transport Services February 2018

Appendix A. Overview of RFCs Used as Input for Pass 1

 TCP:        [RFC0793], [RFC1122], [RFC5482], [RFC5925], and
             [RFC7413].
 MPTCP:      [RFC6182], [RFC6824], and [RFC6897].
 SCTP:       RFCs without a sockets API specification:
             [RFC3758], [RFC4895], [RFC4960], and [RFC5061].
             RFCs that include a sockets API specification:
             [RFC6458], [RFC6525], [RFC6951], [RFC7053], [RFC7496],
             and [RFC7829].
 UDP(-Lite): See [RFC8304].
 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 transport feature is truly relevant for an
 application, the RFCs should say so, and they should describe how to
 use and configure it.  Thus, the approach followed for developing
 this document was to identify the right RFCs, then analyze and
 process their text.
 Primitives that "MAY" be implemented by a transport protocol were
 excluded.  To be included, the minimum requirement level for a
 primitive to be implemented by a protocol was "SHOULD".  Where style
 requirement levels as described in [RFC2119] 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],

Welzl, et al. Informational [Page 54] RFC 8303 Transport Services February 2018

 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 interconnected
    systems of such networks.
 Some text for the pass 1 subsections was developed by copying and
 pasting all the relevant text parts from the relevant RFCs then
 adjusting the terminology to match that in Section 2 and shortening
 phrasing to match the general style of the document.  An effort was
 made to formulate everything as a primitive description such that the
 primitive descriptions became as complete as possible (e.g., the
 'SEND.TCP' primitive in pass 2 is explicitly described as reliably
 transferring data); text that is relevant for the primitives
 presented in this pass but still does not fit directly under any
 primitive was used in a subsection's introduction.
 Pass 2: The main goal of this pass is unification of primitives.  As
 input, only text from pass 1 was used (no exterior sources).  The
 list in pass 2 is not arranged by protocol (i.e., "first protocol X,
 here are all the primitives; then protocol Y, here are all the
 primitives, ...") but by primitive (i.e., "primitive A, implemented
 this way in protocol X, this way in protocol Y, ...").  It was a goal
 to obtain as many similar pass 2 primitives as possible.  For
 instance, this was sometimes achieved by not always maintaining a 1:1
 mapping between pass 1 and pass 2 primitives, renaming primitives,
 etc.  For every new primitive, the already-existing primitives were
 considered to try to make them as coherent as possible.
 For each primitive, the following style was used:
 o  PRIMITIVENAME.PROTOCOL:
    Pass 1 primitive/event:
    Parameters:
    Returns:
    Comments:
 The entries "Parameters", "Returns", and "Comments" were skipped when
 a primitive had no parameters, no described return value, or no
 comments seemed necessary, respectively.  Optional parameters are
 followed by "(optional)".  When known, default values were provided.
 Pass 3: The main point of this pass is to identify transport features
 that are the result of static properties of protocols, for which all
 protocols have to be listed together; this is then the final list of

Welzl, et al. Informational [Page 55] RFC 8303 Transport Services February 2018

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

Acknowledgements

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

Authors' Addresses

 Michael Welzl
 University of Oslo
 PO Box 1080 Blindern
 Oslo  N-0316
 Norway
 Email: michawe@ifi.uio.no
 Michael Tuexen
 Muenster University of Applied Sciences
 Stegerwaldstrasse 39
 Steinfurt  48565
 Germany
 Email: tuexen@fh-muenster.de
 Naeem Khademi
 University of Oslo
 PO Box 1080 Blindern
 Oslo  N-0316
 Norway
 Email: naeemk@ifi.uio.no

Welzl, et al. Informational [Page 56]

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