path: root/doc/rfc/rfc8923.txt

Internet Engineering Task Force (IETF)                          M. Welzl
Request for Comments: 8923                                   S. Gjessing
Category: Informational                               University of Oslo
ISSN: 2070-1721                                             October 2020


          A Minimal Set of Transport Services for End Systems

Abstract

   This document recommends a minimal set of Transport Services offered
   by end systems and gives guidance on choosing among the available
   mechanisms and protocols.  It is based on the set of transport
   features in RFC 8303.

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 candidates 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/rfc8923.

Copyright Notice

   Copyright (c) 2020 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Deriving the Minimal Set
   4.  The Reduced Set of Transport Features
     4.1.  CONNECTION-Related Transport Features
     4.2.  DATA-Transfer-Related Transport Features
       4.2.1.  Sending Data
       4.2.2.  Receiving Data
       4.2.3.  Errors
   5.  Discussion
     5.1.  Sending Messages, Receiving Bytes
     5.2.  Stream Schedulers without Streams
     5.3.  Early Data Transmission
     5.4.  Sender Running Dry
     5.5.  Capacity Profile
     5.6.  Security
     5.7.  Packet Size
   6.  The Minimal Set of Transport Features
     6.1.  ESTABLISHMENT, AVAILABILITY, and TERMINATION
     6.2.  MAINTENANCE
       6.2.1.  Connection Groups
       6.2.2.  Individual Connections
     6.3.  DATA Transfer
       6.3.1.  Sending Data
       6.3.2.  Receiving Data
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  The Superset of Transport Features
     A.1.  CONNECTION-Related Transport Features
     A.2.  DATA-Transfer-Related Transport Features
       A.2.1.  Sending Data
       A.2.2.  Receiving Data
       A.2.3.  Errors
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Currently, the set of Transport Services that most applications use
   is based on TCP and UDP (and protocols that are layered on top of
   them); this limits the ability for the network stack to make use of
   features of other transport protocols.  For example, if a protocol
   supports out-of-order message delivery but applications always assume
   that the network provides an ordered byte stream, then the network
   stack can not immediately deliver a message that arrives out of
   order; doing so would break a fundamental assumption of the
   application.  The net result is unnecessary head-of-line blocking
   delay.

   By exposing the Transport Services of multiple transport protocols, a
   transport system can make it possible for applications to use these
   services without being statically bound to a specific transport
   protocol.  The first step towards the design of such a system was
   taken by [RFC8095], which surveys a large number of transports, and
   [RFC8303] as well as [RFC8304], which identify the specific transport
   features that are exposed to applications by the protocols TCP,
   Multipath TCP (MPTCP), UDP(-Lite), and Stream Control Transmission
   Protocol (SCTP), as well as the Low Extra Delay Background Transport
   (LEDBAT) congestion control mechanism.  LEDBAT was included as the
   only congestion control mechanism in this list because the "low extra
   delay background transport" service that it offers is significantly
   different from the typical service provided by other congestion
   control mechanisms.  This memo is based on these documents and
   follows the same terminology (also listed below).  Because the
   considered transport protocols conjointly cover a wide range of
   transport features, there is reason to hope that the resulting set
   (and the reasoning that led to it) will also apply to many aspects of
   other transport protocols that may be in use today or may be designed
   in the future.

   By decoupling applications from transport protocols, a transport
   system provides a different abstraction level than the Berkeley
   sockets interface [POSIX].  As with high- vs. low-level programming
   languages, a higher abstraction level allows more freedom for
   automation below the interface, yet it takes some control away from
   the application programmer.  This is the design trade-off that a
   transport system developer is facing, and this document provides
   guidance on the design of this abstraction level.  Some transport
   features are currently rarely offered by APIs, yet they must be
   offered or they can never be used.  Other transport features are
   offered by the APIs of the protocols covered here, but not exposing
   them in an API would allow for more freedom to automate protocol
   usage in a transport system.  The minimal set presented here is an
   effort to find a middle ground that can be recommended for transport
   systems to implement, on the basis of the transport features
   discussed in [RFC8303].

   Applications use a wide variety of APIs today.  While this document
   was created to ensure the API developed in the Transport Services
   (TAPS) Working Group [TAPS-INTERFACE] includes the most important
   transport features, the minimal set presented here must be reflected
   in *all* network APIs in order for the underlying functionality to
   become usable everywhere.  For example, it does not help an
   application that talks to a library that offers its own communication
   interface if the underlying Berkeley Sockets API is extended to offer
   "unordered message delivery", but the library only exposes an ordered
   byte stream.  Both the Berkeley Sockets API and the library would
   have to expose the "unordered message delivery" transport feature
   (alternatively, there may be ways for certain types of libraries to
   use this transport feature without exposing it, based on knowledge
   about the applications, but this is not the general case).
   Similarly, transport protocols such as the Stream Control
   Transmission Protocol (SCTP) offer multi-streaming, which cannot be
   utilized, e.g., to prioritize messages between streams, unless
   applications communicate the priorities and the group of connections
   upon which these priorities should be applied.  In most situations,
   in the interest of being as flexible and efficient as possible, the
   best choice will be for a library to expose at least all of the
   transport features that are recommended as a "minimal set" here.

   This "minimal set" can be implemented "one-sided" over TCP.  This
   means that a sender-side transport system can talk to a standard TCP
   receiver, and a receiver-side transport system can talk to a standard
   TCP sender.  If certain limitations are put in place, the "minimal
   set" can also be implemented "one-sided" over UDP.  While the
   possibility of such "one-sided" implementation may help deployment,
   it comes at the cost of limiting the set to services that can also be
   provided by TCP (or, with further limitations, UDP).  Thus, the
   minimal set of transport features here is applicable for many, but
   not all, applications; some application protocols have requirements
   that are not met by this "minimal set".

   Note that, throughout this document, protocols are meant to be used
   natively.  For example, when transport features of TCP, or
   "implementation over" TCP is discussed, this refers to native usage
   of TCP rather than TCP being encapsulated in some other transport
   protocol such as UDP.

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, that provides a
      complete service to an application.

   Transport Protocol:  An implementation that provides one or more
      different Transport Services using a specific framing and header
      format on the wire.

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

   Application-specific knowledge:  Knowledge that only applications
      have.

   End system:  An entity that communicates with one or more other end
      systems using a transport protocol.  An end system provides a
      transport-layer interface to applications.

   Connection:  Shared state of two or more end systems that persists
      across messages that are transmitted between these end systems.

   Connection Group:  A set of connections that share the same
      configuration (configuring one of them causes all other
      connections in the same group to be configured in the same way).
      We call connections that belong to a connection group "grouped",
      while "ungrouped" connections are not a part of a connection
      group.

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

   Moreover, throughout the document, the protocol name "UDP(-Lite)" is
   used when discussing transport features that are equivalent for UDP
   and UDP-Lite; similarly, the protocol name "TCP" refers to both TCP
   and MPTCP.

3.  Deriving the Minimal Set

   We assume that applications have no specific requirements that need
   knowledge about the network, e.g., regarding the choice of network
   interface or the end-to-end path.  Even with these assumptions, there
   are certain requirements that are strictly kept by transport
   protocols today, and these must also be kept by a transport system.
   Some of these requirements relate to transport features that we call
   "Functional".

   Functional transport features provide functionality that cannot be
   used without the application knowing about them, or else they violate
   assumptions that might cause the application to fail.  For example,
   ordered message delivery is a functional transport feature: it cannot
   be configured without the application knowing about it because the
   application's assumption could be that messages always arrive in
   order.  Failure includes any change of the application behavior that
   is not performance oriented, e.g., security.

   "Change DSCP" and "Disable Nagle algorithm" are examples of transport
   features that we call "Optimizing"; if a transport system
   autonomously decides to enable or disable them, an application will
   not fail, but a transport system may be able to communicate more
   efficiently if the application is in control of this optimizing
   transport feature.  These transport features require application-
   specific knowledge (e.g., about delay/bandwidth requirements or the
   length of future data blocks that are to be transmitted).

   The transport features of IETF transport protocols that do not
   require application-specific knowledge and could therefore be
   utilized by a transport system on its own without involving the
   application are called "Automatable".

   We approach the construction of a minimal set of transport features
   in the following way:

   1.  Categorization (Appendix A): The superset of transport features
       from [RFC8303] is presented, and transport features are
       categorized as Functional, Optimizing, or Automatable for later
       reduction.

   2.  Reduction (Section 4): A shorter list of transport features is
       derived from the categorization in the first step.  This removes
       all transport features that do not require application-specific
       knowledge or would result in semantically incorrect behavior if
       they were implemented over TCP or UDP.

   3.  Discussion (Section 5): The resulting list shows a number of
       peculiarities that are discussed, to provide a basis for
       constructing the minimal set.

   4.  Construction (Section 6): Based on the reduced set and the
       discussion of the transport features therein, a minimal set is
       constructed.

   Following [RFC8303] and retaining its terminology, we divide the
   transport features into two main groups as follows:

   1.  CONNECTION-related transport features

       *  ESTABLISHMENT
       *  AVAILABILITY
       *  MAINTENANCE
       *  TERMINATION

   2.  DATA-Transfer-related transport features

       *  Sending Data
       *  Receiving Data
       *  Errors

4.  The Reduced Set of Transport Features

   By hiding automatable transport features from the application, a
   transport system can gain opportunities to automate the usage of
   network-related functionality.  This can facilitate using the
   transport system for the application programmer and it allows for
   optimizations that may not be possible for an application.  For
   instance, system-wide configurations regarding the usage of multiple
   interfaces can better be exploited if the choice of the interface is
   not entirely up to the application.  Therefore, since they are not
   strictly necessary to expose in a transport system, we do not include
   automatable transport features in the reduced set of transport
   features.  This leaves us with only the transport features that are
   either optimizing or functional.

   A transport system should be able to communicate via TCP or UDP if
   alternative transport protocols are found not to work.  For many
   transport features, this is possible, often by simply not doing
   anything when a specific request is made.  For some transport
   features, however, it was identified that direct usage of neither TCP
   nor UDP is possible; in these cases, even not doing anything would
   incur semantically incorrect behavior.  Whenever an application would
   make use of one of these transport features, this would eliminate the
   possibility to use TCP or UDP.  Thus, we only keep the functional and
   optimizing transport features for which an implementation over either
   TCP or UDP is possible in our reduced set.

   The following list contains the transport features from Appendix A,
   reduced using these rules.  The "minimal set" derived in this
   document is meant to be implementable "one-sided" over TCP and, with
   limitations, UDP.  In the list, we therefore precede a transport
   feature with "T:" if an implementation over TCP is possible, "U:" if
   an implementation over UDP is possible, and "T,U:" if an
   implementation over either TCP or UDP is possible.

4.1.  CONNECTION-Related Transport Features

   ESTABLISHMENT:

   *  T,U: Connect
   *  T,U: Specify number of attempts and/or timeout for the first
      establishment message
   *  T,U: Disable MPTCP
   *  T: Configure authentication
   *  T: Hand over a message to reliably transfer (possibly multiple
      times) before connection establishment
   *  T: Hand over a message to reliably transfer during connection
      establishment

   AVAILABILITY:

   *  T,U: Listen
   *  T,U: Disable MPTCP
   *  T: Configure authentication

   MAINTENANCE:

   *  T: Change timeout for aborting connection (using retransmit limit
      or time value)
   *  T: Suggest timeout to the peer
   *  T,U: Disable Nagle algorithm
   *  T,U: Notification of Excessive Retransmissions (early warning
      below abortion threshold)
   *  T,U: Specify DSCP field
   *  T,U: Notification of ICMP error message arrival
   *  T: Change authentication parameters
   *  T: Obtain authentication information
   *  T,U: Set Cookie life value
   *  T,U: Choose a scheduler to operate between streams of an
      association
   *  T,U: Configure priority or weight for a scheduler
   *  T,U: Disable checksum when sending
   *  T,U: Disable checksum requirement when receiving
   *  T,U: Specify checksum coverage used by the sender
   *  T,U: Specify minimum checksum coverage required by receiver
   *  T,U: Specify DF field
   *  T,U: Get max. transport-message size that may be sent using a non-
      fragmented IP packet from the configured interface
   *  T,U: Get max. transport-message size that may be received from the
      configured interface
   *  T,U: Obtain ECN field
   *  T,U: Enable and configure a "Low Extra Delay Background Transfer"

   TERMINATION:

   *  T: Close after reliably delivering all remaining data, causing an
      event informing the application on the other side
   *  T: Abort without delivering remaining data, causing an event
      informing the application on the other side
   *  T,U: Abort without delivering remaining data, not causing an event
      informing the application on the other side
   *  T,U: Timeout event when data could not be delivered for too long

4.2.  DATA-Transfer-Related Transport Features

4.2.1.  Sending Data

   *  T: Reliably transfer data, with congestion control
   *  T: Reliably transfer a message, with congestion control
   *  T,U: Unreliably transfer a message
   *  T: Configurable Message Reliability
   *  T: Ordered message delivery (potentially slower than unordered)
   *  T,U: Unordered message delivery (potentially faster than ordered)
   *  T,U: Request not to bundle messages
   *  T: Specifying a key id to be used to authenticate a message
   *  T,U: Request not to delay the acknowledgement (SACK) of a message

4.2.2.  Receiving Data

   *  T,U: Receive data (with no message delimiting)
   *  U: Receive a message
   *  T,U: Information about partial message arrival

4.2.3.  Errors

   This section describes sending failures that are associated with a
   specific call to in the "Sending Data" category (Appendix A.2.1).

   *  T,U: Notification of send failures
   *  T,U: Notification that the stack has no more user data to send
   *  T,U: Notification to a receiver that a partial message delivery
      has been aborted

5.  Discussion

   The reduced set in the previous section exhibits a number of
   peculiarities, which we will discuss in the following.  This section
   focuses on TCP because, with the exception of one particular
   transport feature ("Receive a message"; we will discuss this in
   Section 5.1), the list shows that UDP is strictly a subset of TCP.
   We can first try to understand how to build a transport system that
   can run over TCP, and then narrow down the result further to allow
   that the system can always run over either TCP or UDP (which
   effectively means removing everything related to reliability,
   ordering, authentication, and closing/aborting with a notification to
   the peer).

   Note that, because the functional transport features of UDP are, with
   the exception of "Receive a message", a subset of TCP, TCP can be
   used as a replacement for UDP whenever an application does not need
   message delimiting (e.g., because the application-layer protocol
   already does it).  This has been recognized by many applications that
   already do this in practice, by trying to communicate with UDP at
   first and falling back to TCP in case of a connection failure.

5.1.  Sending Messages, Receiving Bytes

   For implementing a transport system over TCP, there are several
   transport features related to sending, but only a single transport
   feature related to receiving: "Receive data (with no message
   delimiting)" (and, strangely, "information about partial message
   arrival").  Notably, the transport feature "Receive a message" is
   also the only non-automatable transport feature of UDP(-Lite) for
   which no implementation over TCP is possible.

   To support these TCP receiver semantics, we define an "Application-
   Framed Byte Stream" (AFra Byte Stream).  AFra Byte Streams allow
   senders to operate on messages while minimizing changes to the TCP
   socket API.  In particular, nothing changes on the receiver side;
   data can be accepted via a normal TCP socket.

   In an AFra Byte Stream, the sending application can optionally inform
   the transport about message boundaries and required properties per
   message (configurable order and reliability, or embedding a request
   not to delay the acknowledgement of a message).  Whenever the sending
   application specifies per-message properties that relax the notion of
   reliable in-order delivery of bytes, it must assume that the
   receiving application is 1) able to determine message boundaries,
   provided that messages are always kept intact, and 2) able to accept
   these relaxed per-message properties.  Any signaling of such
   information to the peer is up to an application-layer protocol and
   considered out of scope of this document.

   For example, if an application requests to transfer fixed-size
   messages of 100 bytes with partial reliability, this needs the
   receiving application to be prepared to accept data in chunks of 100
   bytes.  Then, if some of these 100-byte messages are missing (e.g.,
   if SCTP with Configurable Reliability is used), this is the expected
   application behavior.  With TCP, no messages would be missing, but
   this is also correct for the application, and the possible
   retransmission delay is acceptable within the best-effort service
   model (see Section 3.5 of [RFC7305]).  Still, the receiving
   application would separate the byte stream into 100-byte chunks.

   Note that this usage of messages does not require all messages to be
   equal in size.  Many application protocols use some form of Type-
   Length-Value (TLV) encoding, e.g., by defining a header including
   length fields; another alternative is the use of byte stuffing
   methods such as Consistent Overhead Byte Stuffing (COBS) [COBS].  If
   an application needs message numbers, e.g., to restore the correct
   sequence of messages, these must also be encoded by the application
   itself, as SCTP's transport features that are related to the sequence
   number are not provided by the "minimum set" (in the interest of
   enabling usage of TCP).

5.2.  Stream Schedulers without Streams

   We have already stated that multi-streaming does not require
   application-specific knowledge.  Potential benefits or disadvantages
   of, e.g., using two streams of an SCTP association versus using two
   separate SCTP associations or TCP connections are related to
   knowledge about the network and the particular transport protocol in
   use, not the application.  However, the transport features "Choose a
   scheduler to operate between streams of an association" and
   "Configure priority or weight for a scheduler" operate on streams.
   Here, streams identify communication channels between which a
   scheduler operates, and they can be assigned a priority.  Moreover,
   the transport features in the MAINTENANCE category all operate on
   associations in case of SCTP, i.e., they apply to all streams in that
   association.

   With only these semantics necessary to represent, the interface to a
   transport system becomes easier if we assume that connections may be
   not only a transport protocol's connection or association, but could
   also be a stream of an existing SCTP association, for example.  We
   only need to allow for a way to define a possible grouping of
   connections.  Then, all MAINTENANCE transport features can be said to
   operate on connection groups, not connections, and a scheduler
   operates on the connections within a group.

   To be compatible with multiple transport protocols and uniformly
   allow access to both transport connections and streams of a multi-
   streaming protocol, the semantics of opening and closing need to be
   the most restrictive subset of all of the underlying options.  For
   example, TCP's support of half-closed connections can be seen as a
   feature on top of the more restrictive "ABORT"; this feature cannot
   be supported because not all protocols used by a transport system
   (including streams of an association) support half-closed
   connections.

5.3.  Early Data Transmission

   There are two transport features related to transferring a message
   early: "Hand over a message to reliably transfer (possibly multiple
   times) before connection establishment", which relates to TCP Fast
   Open [RFC7413], and "Hand over a message to reliably transfer during
   connection establishment", which relates to SCTP's ability to
   transfer data together with the COOKIE-Echo chunk.  Also without TCP
   Fast Open, TCP can transfer data during the handshake, together with
   the SYN packet; however, the receiver of this data may not hand it
   over to the application until the handshake has completed.  Also,
   different from TCP Fast Open, this data is not delimited as a message
   by TCP (thus, not visible as a "message").  This functionality is
   commonly available in TCP and supported in several implementations,
   even though the TCP specification does not explain how to provide it
   to applications.

   A transport system could differentiate between the cases of
   transmitting data "before" (possibly multiple times) or "during" the
   handshake.  Alternatively, it could also assume that data that are
   handed over early will be transmitted as early as possible, and
   "before" the handshake would only be used for messages that are
   explicitly marked as "idempotent" (i.e., it would be acceptable to
   transfer them multiple times).

   The amount of data that can successfully be transmitted before or
   during the handshake depends on various factors: the transport
   protocol, the use of header options, the choice of IPv4 and IPv6, and
   the Path MTU.  A transport system should therefore allow a sending
   application to query the maximum amount of data it can possibly
   transmit before (or, if exposed, during) connection establishment.

5.4.  Sender Running Dry

   The transport feature "Notification that the stack has no more user
   data to send" relates to SCTP's "SENDER DRY" notification.  Such
   notifications can, in principle, be used to avoid having an
   unnecessarily large send buffer, yet ensure that the transport sender
   always has data available when it has an opportunity to transmit it.
   This has been found to be very beneficial for some applications
   [WWDC2015].  However, "SENDER DRY" truly means that the entire send
   buffer (including both unsent and unacknowledged data) has emptied,
   i.e., when it notifies the sender, it is already too late; the
   transport protocol already missed an opportunity to send data.  Some
   modern TCP implementations now include the unspecified
   "TCP_NOTSENT_LOWAT" socket option that was proposed in [WWDC2015],
   which limits the amount of unsent data that TCP can keep in the
   socket buffer; this allows specifying at which buffer filling level
   the socket becomes writable, rather than waiting for the buffer to
   run empty.

   SCTP allows configuring the sender-side buffer too; the automatable
   Transport Feature "Configure send buffer size" provides this
   functionality, but only for the complete buffer, which includes both
   unsent and unacknowledged data.  SCTP does not allow to control these
   two sizes separately.  It therefore makes sense for a transport
   system to allow for uniform access to "TCP_NOTSENT_LOWAT" as well as
   the "SENDER DRY" notification.

5.5.  Capacity Profile

   The transport features:

   *  Disable Nagle algorithm
   *  Enable and configure a "Low Extra Delay Background Transfer"
   *  Specify DSCP field

   All relate to a QoS-like application need such as "low latency" or
   "scavenger".  In the interest of flexibility of a transport system,
   they could therefore be offered in a uniform, more abstract way,
   where a transport system could, e.g., decide by itself how to use
   combinations of LEDBAT-like congestion control and certain DSCP
   values, and an application would only specify a general "capacity
   profile" (a description of how it wants to use the available
   capacity).  A need for "lowest possible latency at the expense of
   overhead" could then translate into automatically disabling the Nagle
   algorithm.

   In some cases, the Nagle algorithm is best controlled directly by the
   application because it is not only related to a general profile but
   also to knowledge about the size of future messages.  For fine-grain
   control over Nagle-like functionality, the "Request not to bundle
   messages" is available.

5.6.  Security

   Both TCP and SCTP offer authentication.  TCP authenticates complete
   segments.  SCTP allows configuring which of SCTP's chunk types must
   always be authenticated; if this is exposed as such, it creates an
   undesirable dependency on the transport protocol.  For compatibility
   with TCP, a transport system should only allow to configure complete
   transport layer packets, including headers, IP pseudo-header (if any)
   and payload.

   Security is discussed in a separate document [RFC8922].  The minimal
   set presented in the present document excludes all security-related
   transport features from Appendix A: "Configure authentication",
   "Change authentication parameters", "Obtain authentication
   information", and "Set Cookie life value", as well as "Specifying a
   key id to be used to authenticate a message".  It also excludes
   security transport features not listed in Appendix A, including
   content privacy to in-path devices.

5.7.  Packet Size

   UDP(-Lite) has a transport feature called "Specify DF field".  This
   yields an error message in the case of sending a message that exceeds
   the Path MTU, which is necessary for a UDP-based application to be
   able to implement Path MTU Discovery (a function that UDP-based
   applications must do by themselves).  The "Get max. transport-message
   size that may be sent using a non-fragmented IP packet from the
   configured interface" transport feature yields an upper limit for the
   Path MTU (minus headers) and can therefore help to implement Path MTU
   Discovery more efficiently.

6.  The Minimal Set of Transport Features

   Based on the categorization, reduction, and discussion in Section 3,
   this section describes a minimal set of transport features that end
   systems should offer.  Any configuration based on the described
   minimum set of transport feature can always be realized over TCP but
   also gives the transport system flexibility to choose another
   transport if implemented.  In the text of this section, "not UDP" is
   used to indicate elements of the system that cannot be implemented
   over UDP.  Conversely, all elements of the system that are not marked
   with "not UDP" can also be implemented over UDP.

   The arguments laid out in Section 5 ("discussion") were used to make
   the final representation of the minimal set as short, simple, and
   general as possible.  There may be situations where these arguments
   do not apply, e.g., implementers may have specific reasons to expose
   multi-streaming as a visible functionality to applications, or the
   restrictive open/close semantics may be problematic under some
   circumstances.  In such cases, the representation in Section 4
   ("reduction") should be considered.

   As in Section 3, Section 4, and [RFC8303], we categorize the minimal
   set of transport features as 1) CONNECTION related (ESTABLISHMENT,
   AVAILABILITY, MAINTENANCE, TERMINATION) and 2) DATA Transfer related
   (Sending Data, Receiving Data, Errors).  Here, the focus is on
   connections that the transport system offers as an abstraction to the
   application, as opposed to connections of transport protocols that
   the transport system uses.

6.1.  ESTABLISHMENT, AVAILABILITY, and TERMINATION

   A connection must first be "created" to allow for some initial
   configuration to be carried out before the transport system can
   actively or passively establish communication with a remote end
   system.  As a configuration of the newly created connection, an
   application can choose to disallow usage of MPTCP.  Furthermore, all
   configuration parameters in Section 6.2 can be used initially,
   although some of them may only take effect when a connection has been
   established with a chosen transport protocol.  Configuring a
   connection early helps a transport system make the right decisions.
   For example, grouping information can influence whether or not the
   transport system implements a connection as a stream of a multi-
   streaming protocol's existing association.

   For ungrouped connections, early configuration is necessary because
   it allows the transport system to know which protocols it should try
   to use.  In particular, a transport system that only makes a one-time
   choice for a particular protocol must know early about strict
   requirements that must be kept, or it can end up in a deadlock
   situation (e.g., having chosen UDP and later be asked to support
   reliable transfer).  As an example description of how to correctly
   handle these cases, we provide the following decision tree (this is
   derived from Section 4.1 excluding authentication, as explained in
   Section 8):

      +----------------------------------------------------------+
      | Will it ever be necessary to offer any of the following? |
      | * Reliably transfer data                                 |
      | * Notify the peer of closing/aborting                    |
      | * Preserve data ordering                                 |
      +----------------------------------------------------------+
                |                                    |
                |Yes                                 |No
                | (SCTP or TCP)                      | (All protocols
                |  can be used.)                     |  can be used.)
                V                                    V
+--------------------------------------+ +-----------------------------+
| Is any of the following useful to    | | Is any of the following     |
| the application?                     | | useful to the application?  |
| * Choosing a scheduler to operate    | | * Specify checksum coverage |
|   between connections in a group,    | |   used by the sender        |
|   with the possibility to configure  | | * Specify minimum checksum  |
|   a priority or weight per connection| |   coverage required by the  |
| * Configurable message reliability   | |   receiver                  |
| * Unordered message delivery         | +-----------------------------+
| * Request not to delay the           |         |             |
|   acknowledgement (SACK) of a message|         |Yes          |No
+--------------------------------------+         |             |
          |                |                     |             |
          |Yes             |No                   |             |
          V                |                     V             V
        SCTP is            |                UDP-Lite is    UDP is
        preferred.         |                preferred.     preferred.
                           V
+------------------------------------------------------+
| Is any of the following useful to the application?   |
| * Hand over a message to reliably transfer (possibly |
|   multiple times) before connection establishment    |
| * Suggest timeout to the peer                        |
| * Notification of Excessive Retransmissions (early   |
|   warning below abortion threshold)                  |
| * Notification of ICMP error message arrival         |
+------------------------------------------------------+
          |                            |
          |Yes                         |No
          V                            V
    TCP is preferred.             SCTP and TCP
                                  are equally preferable.

   Note that this decision tree is not optimal for all cases.  For
   example, if an application wants to use "Specify checksum coverage
   used by the sender", which is only offered by UDP-Lite, and
   "Configure priority or weight for a scheduler", which is only offered
   by SCTP, the above decision tree will always choose UDP-Lite, making
   it impossible to use SCTP's schedulers with priorities between
   grouped connections.  Also, several other factors may influence the
   decisions for or against a protocol, e.g., penetration rates, the
   ability to work through NATs, etc.  We caution implementers to be
   aware of the full set of trade-offs, for which we recommend
   consulting the list in Section 4.1 when deciding how to initialize a
   connection.

   To summarize, the following parameters serve as input for the
   transport system to help it choose and configure a suitable protocol:

   Reliability:  a boolean that should be set to true when any of the
      following will be useful to the application: reliably transfer
      data; notify the peer of closing/aborting; or preserve data
      ordering.

   Checksum coverage:  a boolean to specify whether it will be useful to
      the application to specify checksum coverage when sending or
      receiving.

   Configure message priority:  a boolean that should be set to true
      when any of the following per-message configuration or
      prioritization mechanisms will be useful to the application:
      choosing a scheduler to operate between grouped connections, with
      the possibility to configure a priority or weight per connection;
      configurable message reliability; unordered message delivery; or
      requesting not to delay the acknowledgement (SACK) of a message.

   Early message timeout notifications:  a boolean that should be set to
      true when any of the following will be useful to the application:
      hand over a message to reliably transfer (possibly multiple times)
      before connection establishment; suggest timeout to the peer;
      notification of excessive retransmissions (early warning below
      abortion threshold); or notification of ICMP error message
      arrival.

   Once a connection is created, it can be queried for the maximum
   amount of data that an application can possibly expect to have
   reliably transmitted before or during transport connection
   establishment (with zero being a possible answer) (see
   Section 6.2.1).  An application can also give the connection a
   message for reliable transmission before or during connection
   establishment (not UDP); the transport system will then try to
   transmit it as early as possible.  An application can facilitate
   sending a message particularly early by marking it as "idempotent"
   (see Section 6.3.1); in this case, the receiving application must be
   prepared to potentially receive multiple copies of the message
   (because idempotent messages are reliably transferred, asking for
   idempotence is not necessary for systems that support UDP).

   After creation, a transport system can actively establish
   communication with a peer, or it can passively listen for incoming
   connection requests.  Note that active establishment may or may not
   trigger a notification on the listening side.  It is possible that
   the first notification on the listening side is the arrival of the
   first data that the active side sends (a receiver-side transport
   system could handle this by continuing to block a "Listen" call,
   immediately followed, for example, by issuing "Receive"; callback-
   based implementations could simply skip the equivalent of "Listen").
   This also means that the active opening side is assumed to be the
   first side sending data.

   A transport system can actively close a connection, i.e., terminate
   it after reliably delivering all remaining data to the peer (if
   reliable data delivery was requested earlier (not UDP)), in which
   case the peer is notified that the connection is closed.
   Alternatively, a connection can be aborted without delivering
   outstanding data to the peer.  In case reliable or partially reliable
   data delivery was requested earlier (not UDP), the peer is notified
   that the connection is aborted.  A timeout can be configured to abort
   a connection when data could not be delivered for too long (not UDP);
   however, timeout-based abortion does not notify the peer application
   that the connection has been aborted.  Because half-closed
   connections are not supported, when a host implementing a transport
   system receives a notification that the peer is closing or aborting
   the connection (not UDP), its peer may not be able to read
   outstanding data.  This means that unacknowledged data residing in a
   transport system's send buffer may have to be dropped from that
   buffer upon arrival of a "close" or "abort" notification from the
   peer.

6.2.  MAINTENANCE

   A transport system must offer means to group connections, but it
   cannot guarantee truly grouping them using the transport protocols
   that it uses (e.g., it cannot be guaranteed that connections become
   multiplexed as streams on a single SCTP association when SCTP may not
   be available).  The transport system must therefore ensure that
   group- versus non-group-configurations are handled correctly in some
   way (e.g., by applying the configuration to all grouped connections
   even when they are not multiplexed, or informing the application
   about grouping success or failure).

   As a general rule, any configuration described below should be
   carried out as early as possible to aid the transport system's
   decision making.

6.2.1.  Connection Groups

   The following transport features and notifications (some directly
   from Section 4; some new or changed, based on the discussion in
   Section 5) automatically apply to all grouped connections:

   Configure a timeout (not UDP)
   This can be done with the following parameters:

   *  A timeout value for aborting connections, in seconds.

   *  A timeout value to be suggested to the peer (if possible), in
      seconds.

   *  The number of retransmissions after which the application should
      be notified of "Excessive Retransmissions".

   Configure urgency
   This can be done with the following parameters:

   *  A number to identify the type of scheduler that should be used to
      operate between connections in the group (no guarantees given).
      Schedulers are defined in [RFC8260].

   *  A "capacity profile" number to identify how an application wants
      to use its available capacity.  Choices can be "lowest possible
      latency at the expense of overhead" (which would disable any
      Nagle-like algorithm), "scavenger", or values that help determine
      the DSCP value for a connection.

   *  A buffer limit (in bytes); when the sender has less than the
      provided limit of bytes in the buffer, the application may be
      notified.  Notifications are not guaranteed, and it is optional
      for a transport system to support buffer limit values greater than
      0.  Note that this limit and its notification should operate
      across the buffers of the whole transport system, i.e., also any
      potential buffers that the transport system itself may use on top
      of the transport's send buffer.

   Following Section 5.7, these properties can be queried:

   *  The maximum message size that may be sent without fragmentation
      via the configured interface.  This is optional for a transport
      system to offer and may return an error ("not available").  It can
      aid applications implementing Path MTU Discovery.

   *  The maximum transport message size that can be sent, in bytes.
      Irrespective of fragmentation, there is a size limit for the
      messages that can be handed over to SCTP or UDP(-Lite); because
      the service provided by a transport system is independent of the
      transport protocol, it must allow an application to query this
      value: the maximum size of a message in an Application-Framed Byte
      Stream (see Section 5.1).  This may also return an error when data
      is not delimited ("not available").

   *  The maximum transport message size that can be received from the
      configured interface, in bytes (or "not available").

   *  The maximum amount of data that can possibly be sent before or
      during connection establishment, in bytes.

   In addition to the already mentioned closing/aborting notifications
   and possible send errors, the following notifications can occur:

   Excessive Retransmissions:  The configured (or a default) number of
      retransmissions has been reached, yielding this early warning
      below an abortion threshold.

   ICMP Arrival (parameter: ICMP message):  An ICMP packet carrying the
      conveyed ICMP message has arrived.

   ECN Arrival (parameter: ECN value):  A packet carrying the conveyed
      Explicit Congestion Notification (ECN) value has arrived.  This
      can be useful for applications implementing congestion control.

   Timeout (parameter: s seconds):  Data could not be delivered for s
      seconds.

   Drain:  The send buffer has either drained below the configured
      buffer limit or it has become completely empty.  This is a generic
      notification that tries to enable uniform access to
      "TCP_NOTSENT_LOWAT" as well as the "SENDER DRY" notification (as
      discussed in Section 5.4; SCTP's "SENDER DRY" is a special case
      where the threshold (for unsent data) is 0 and there is also no
      more unacknowledged data in the send buffer).

6.2.2.  Individual Connections

   Configure priority or weight for a scheduler, as described in
   [RFC8260].

   Configure checksum usage: This can be done with the following
   parameters, but there is no guarantee that any checksum limitations
   will indeed be enforced (the default behavior is "full coverage,
   checksum enabled"):

   *  a boolean to enable/disable usage of a checksum when sending

   *  the desired coverage (in bytes) of the checksum used when sending

   *  a boolean to enable/disable requiring a checksum when receiving

   *  the required minimum coverage (in bytes) of the checksum when
      receiving

6.3.  DATA Transfer

6.3.1.  Sending Data

   When sending a message, no guarantees are given about the
   preservation of message boundaries to the peer; if message boundaries
   are needed, the receiving application at the peer must know about
   them beforehand (or the transport system cannot use TCP).  Note that
   an application should already be able to hand over data before the
   transport system establishes a connection with a chosen transport
   protocol.  Regarding the message that is being handed over, the
   following parameters can be used:

   Reliability:  This parameter is used to convey a choice of: fully
      reliable with congestion control (not UDP), unreliable without
      congestion control, unreliable with congestion control (not UDP),
      and partially reliable with congestion control (see [RFC3758] and
      [RFC7496] for details on how to specify partial reliability) (not
      UDP).  The latter two choices are optional for a transport system
      to offer and may result in full reliability.  Note that
      applications sending unreliable data without congestion control
      should themselves perform congestion control in accordance with
      [RFC8085].

   Ordered (not UDP):  This boolean lets an application choose between
      ordered message delivery (true) and possibly unordered,
      potentially faster message delivery (false).

   Bundle:  This boolean expresses a preference for allowing to bundle
      messages (true) or not (false).  No guarantees are given.

   DelAck:  This boolean, if false, lets an application request that the
      peer not delay the acknowledgement for this message.

   Fragment:  This boolean expresses a preference for allowing to
      fragment messages (true) or not (false), at the IP level.  No
      guarantees are given.

   Idempotent (not UDP):  This boolean expresses whether a message is
      idempotent (true) or not (false).  Idempotent messages may arrive
      multiple times at the receiver (but they will arrive at least
      once).  When data is idempotent, it can be used by the receiver
      immediately on a connection establishment attempt.  Thus, if data
      is handed over before the transport system establishes a
      connection with a chosen transport protocol, stating that a
      message is idempotent facilitates transmitting it to the peer
      application particularly early.

   An application can be notified of a failure to send a specific
   message.  There is no guarantee of such notifications, i.e., send
   failures can also silently occur.

6.3.2.  Receiving Data

   A receiving application obtains an "Application-Framed Byte Stream"
   (AFra Byte Stream); this concept is further described in Section 5.1.
   In line with TCP's receiver semantics, an AFra Byte Stream is just a
   stream of bytes to the receiver.  If message boundaries were
   specified by the sender, a receiver-side transport system
   implementing only the minimum set of Transport Services defined here
   will still not inform the receiving application about them (this
   limitation is only needed for transport systems that are implemented
   to directly use TCP).

   Different from TCP's semantics, if the sending application has
   allowed that messages are not fully reliably transferred, or
   delivered out of order, then such reordering or unreliability may be
   reflected per message in the arriving data.  Messages will always
   stay intact, i.e., if an incomplete message is contained at the end
   of the arriving data block, this message is guaranteed to continue in
   the next arriving data block.

7.  IANA Considerations

   This document has no IANA actions.

8.  Security Considerations

   Authentication, confidentiality protection, and integrity protection
   are identified as transport features by [RFC8095].  Often, these
   features are provided by a protocol or layer on top of the transport
   protocol; none of the full-featured standards-track transport
   protocols in [RFC8303], which this document is based upon, provide
   all of these transport features on its own.  Therefore, they 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.  The minimum
   requirements for a secure transport system are discussed in a
   separate document [RFC8922].

9.  References

9.1.  Normative References

   [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>.

   [RFC8303]  Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
              Transport Features Provided by IETF Transport Protocols",
              RFC 8303, DOI 10.17487/RFC8303, February 2018,
              <https://www.rfc-editor.org/info/rfc8303>.

   [RFC8922]  Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
              Wood, "A Survey of the Interaction between Security
              Protocols and Transport Services", RFC 8922,
              DOI 10.17487/RFC8922, October 2020,
              <https://www.rfc-editor.org/info/rfc8922>.

9.2.  Informative References

   [COBS]     Cheshire, S. and M. Baker, "Consistent overhead byte
              stuffing", IEEE/ACM Transactions on Networking, Volume 7,
              Issue 2, DOI 10.1109/90.769765, April 1999,
              <https://doi.org/10.1109/90.769765>.

   [POSIX]    The Open Group, "IEEE Standard for Information
              Technology--Portable Operating System Interface (POSIX(R))
              Base Specifications, Issue 7", (Revision of IEEE Std
              1003.1-2008), IEEE Std 1003.1-2017, January 2018,
              <https://www.opengroup.org/onlinepubs/9699919799/
              functions/contents.html>.

   [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>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <https://www.rfc-editor.org/info/rfc4987>.

   [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>.

   [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>.

   [RFC7305]  Lear, E., Ed., "Report from the IAB Workshop on Internet
              Technology Adoption and Transition (ITAT)", RFC 7305,
              DOI 10.17487/RFC7305, July 2014,
              <https://www.rfc-editor.org/info/rfc7305>.

   [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>.

   [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>.

   [RFC8622]  Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
              Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
              June 2019, <https://www.rfc-editor.org/info/rfc8622>.

   [SCTP-STREAM-1]
              Weinrank, F. and M. Tuexen, "Transparent Flow Mapping for
              NEAT", IFIP Networking 2017, Workshop on Future of
              Internet Transport (FIT 2017), June 2017.

   [SCTP-STREAM-2]
              Welzl, M., Niederbacher, F., and S. Gjessing, "Beneficial
              Transparent Deployment of SCTP: The Missing Pieces", IEEE
              GlobeCom 2011, DOI 10.1109/GLOCOM.2011.6133554, December
              2011, <https://doi.org/10.1109/GLOCOM.2011.6133554>.

   [TAPS-INTERFACE]
              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P. S., Wood, C. A.,
              and T. Pauly, "An Abstract Application Layer Interface to
              Transport Services", Work in Progress, Internet-Draft,
              draft-ietf-taps-interface-09, 27 July 2020,
              <https://tools.ietf.org/html/draft-ietf-taps-interface-
              09>.

   [WWDC2015] Lakhera, P. and S. Cheshire, "Your App and Next Generation
              Networks", Apple Worldwide Developers Conference 2015, San
              Francisco, USA, June 2015,
              <https://developer.apple.com/videos/wwdc/2015/?id=719>.

Appendix A.  The Superset of Transport Features

   In this description, transport features are presented following the
   nomenclature "CATEGORY.[SUBCATEGORY].FEATURENAME.PROTOCOL",
   equivalent to "pass 2" in [RFC8303].  We also sketch how functional
   or optimizing transport features can be implemented by a transport
   system.  The "minimal set" derived in this document is meant to be
   implementable "one-sided" over TCP and, with limitations, UDP.
   Hence, for all transport features that are categorized as
   "functional" or "optimizing", and for which no matching TCP and/or
   UDP primitive exists in "pass 2" of [RFC8303], a brief discussion on
   how to implement them over TCP and/or UDP is included.

   We designate some transport features as "automatable" on the basis of
   a broader decision that affects multiple transport features:

   *  Most transport features that are related to multi-streaming were
      designated as "automatable".  This was done because the decision
      on whether or not to use multi-streaming does not depend on
      application-specific knowledge.  This means that a connection that
      is exhibited to an application could be implemented by using a
      single stream of an SCTP association instead of mapping it to a
      complete SCTP association or TCP connection.  This could be
      achieved by using more than one stream when an SCTP association is
      first established (CONNECT.SCTP parameter "outbound stream
      count"), maintaining an internal stream number, and using this
      stream number when sending data (SEND.SCTP parameter "stream
      number").  Closing or aborting a connection could then simply free
      the stream number for future use.  This is discussed further in
      Section 5.2.

   *  With the exception of "Disable MPTCP", all transport features that
      are related to using multiple paths or the choice of the network
      interface were designated as "automatable".  For example, "Listen"
      could always listen on all available interfaces and "Connect"
      could use the default interface for the destination IP address.

   Finally, in three cases, transport features are aggregated and/or
   slightly changed from [RFC8303] in the description below.  These
   transport features are marked as "CHANGED FROM RFC 8303".  These do
   not add any new functionality but just represent a simple refactoring
   step that helps to streamline the derivation process (e.g., by
   removing a choice of a parameter for the sake of applications that
   may not care about this choice).  The corresponding transport
   features are automatable, and they are listed immediately below the
   "CHANGED FROM RFC 8303" transport feature.

A.1.  CONNECTION-Related Transport Features

   ESTABLISHMENT:

   *  Connect

      Protocols: TCP, SCTP, UDP(-Lite)

      Functional because the notion of a connection is often reflected
      in applications as an expectation to be able to communicate after
      a "Connect" succeeded, with a communication sequence relating to
      this transport feature that is defined by the application
      protocol.

      Implementation: via CONNECT.TCP, CONNECT.SCTP or CONNECT.UDP(-
      Lite).

   *  Specify which IP Options must always be used

      Protocols: TCP, UDP(-Lite)

      Automatable because IP Options relate to knowledge about the
      network, not the application.

   *  Request multiple streams

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge (example implementations of using
      multi-streaming without involving the application are described in
      [SCTP-STREAM-1] and [SCTP-STREAM-2]).

      Implementation: see Section 5.2.

   *  Limit the number of inbound streams

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Specify number of attempts and/or timeout for the first
      establishment message

      Protocols: TCP, SCTP

      Functional because this is closely related to potentially assumed
      reliable data delivery for data that is sent before or during
      connection establishment.

      Implementation: using a parameter of CONNECT.TCP and CONNECT.SCTP.

      Implementation over UDP: do nothing (this is irrelevant in the
      case of UDP because there, reliable data delivery is not assumed).

   *  Obtain multiple sockets

      Protocols: SCTP

      Automatable because the non-parallel usage of multiple paths to
      communicate between the same end hosts relates to knowledge about
      the network, not the application.

   *  Disable MPTCP

      Protocols: MPTCP

      Optimizing because the parallel usage of multiple paths to
      communicate between the same end hosts can improve performance.
      Whether or not to use this feature depends on knowledge about the
      network as well as application-specific knowledge (see Section 3.1
      of [RFC6897]).

      Implementation: via a boolean parameter in CONNECT.MPTCP.

      Implementation over TCP: do nothing.

      Implementation over UDP: do nothing.

   *  Configure authentication

      Protocols: TCP, SCTP

      Functional because this has a direct influence on security.

      Implementation: via parameters in CONNECT.TCP and CONNECT.SCTP.
      With TCP, this allows configuring Master Key Tuples (MKTs) to
      authenticate complete segments (including the TCP IPv4
      pseudoheader, TCP header, and TCP data).  With SCTP, this allows
      specifying which chunk types must always be authenticated.
      Authenticating only certain chunk types creates a reduced level of
      security that is not supported by TCP; to be compatible, this
      should therefore only allow to authenticate all chunk types.  Key
      material must be provided in a way that is compatible with both
      [RFC4895] and [RFC5925].

      Implementation over UDP: not possible (UDP does not offer this
      functionality).

   *  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point

      Protocols: SCTP

      Functional because it allows sending extra data for the sake of
      identifying an adaptation layer, which by itself is application
      specific.

      Implementation: via a parameter in CONNECT.SCTP.

      Implementation over TCP: not possible.  (TCP does not offer this
      functionality.)

      Implementation over UDP: not possible.  (UDP does not offer this
      functionality.)

   *  Request to negotiate interleaving of user messages

      Protocols: SCTP

      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.

      Implementation: controlled via a parameter in CONNECT.SCTP.  One
      possible implementation is to always try to enable interleaving.

   *  Hand over a message to reliably transfer (possibly multiple times)
      before connection establishment

      Protocols: TCP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via a parameter in CONNECT.TCP.

      Implementation over UDP: not possible.  (UDP does not provide
      reliability.)

   *  Hand over a message to reliably transfer during connection
      establishment

      Protocols: SCTP

      Functional because this can only work if the message is limited in
      size, making it closely tied to properties of the data that an
      application sends or expects to receive.

      Implementation: via a parameter in CONNECT.SCTP.

      Implementation over TCP: transmit the message with the SYN packet,
      sacrificing the ability to identify message boundaries.

      Implementation over UDP: not possible.  (UDP is unreliable.)

   *  Enable UDP encapsulation with a specified remote UDP port number

      Protocols: SCTP

      Automatable because UDP encapsulation relates to knowledge about
      the network, not the application.

   AVAILABILITY:

   *  Listen

      Protocols: TCP, SCTP, UDP(-Lite)

      Functional because the notion of accepting connection requests is
      often reflected in applications as an expectation to be able to
      communicate after a "Listen" succeeded, with a communication
      sequence relating to this transport feature that is defined by the
      application protocol.

      CHANGED FROM RFC 8303.  This differs from the 3 automatable
      transport features below in that it leaves the choice of
      interfaces for listening open.

      Implementation: by listening on all interfaces via LISTEN.TCP (not
      providing a local IP address) or LISTEN.SCTP (providing SCTP port
      number / address pairs for all local IP addresses).  LISTEN.UDP(-
      Lite) supports both methods.

   *  Listen, 1 specified local interface

      Protocols: TCP, SCTP, UDP(-Lite)

      Automatable because decisions about local interfaces relate to
      knowledge about the network and the Operating System, not the
      application.

   *  Listen, N specified local interfaces

      Protocols: SCTP

      Automatable because decisions about local interfaces relate to
      knowledge about the network and the Operating System, not the
      application.

   *  Listen, all local interfaces

      Protocols: TCP, SCTP, UDP(-Lite)

      Automatable because decisions about local interfaces relate to
      knowledge about the network and the Operating System, not the
      application.

   *  Specify which IP Options must always be used

      Protocols: TCP, UDP(-Lite)

      Automatable because IP Options relate to knowledge about the
      network, not the application.

   *  Disable MPTCP

      Protocols: MPTCP

      Optimizing because the parallel usage of multiple paths to
      communicate between the same end hosts can improve performance.
      Whether or not to use this feature depends on knowledge about the
      network as well as application-specific knowledge (see Section 3.1
      of [RFC6897]).

      Implementation: via a boolean parameter in LISTEN.MPTCP.

      Implementation over TCP: do nothing.

      Implementation over UDP: do nothing.

   *  Configure authentication

      Protocols: TCP, SCTP

      Functional because this has a direct influence on security.

      Implementation: via parameters in LISTEN.TCP and LISTEN.SCTP.

      Implementation over TCP: with TCP, this allows configuring Master
      Key Tuples (MKTs) to authenticate complete segments (including the
      TCP IPv4 pseudoheader, TCP header, and TCP data).  With SCTP, this
      allows specifying which chunk types must always be authenticated.
      Authenticating only certain chunk types creates a reduced level of
      security that is not supported by TCP; to be compatible, this
      should therefore only allow to authenticate all chunk types.  Key
      material must be provided in a way that is compatible with both
      [RFC4895] and [RFC5925].

      Implementation over UDP: not possible.  (UDP does not offer
      authentication.)

   *  Obtain requested number of streams

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Limit the number of inbound streams

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point

      Protocols: SCTP

      Functional because it allows sending extra data for the sake of
      identifying an adaptation layer, which by itself is application
      specific.

      Implementation: via a parameter in LISTEN.SCTP.

      Implementation over TCP: not possible.  (TCP does not offer this
      functionality.)

      Implementation over UDP: not possible.  (UDP does not offer this
      functionality.)

   *  Request to negotiate interleaving of user messages

      Protocols: SCTP

      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.

      Implementation: via a parameter in LISTEN.SCTP.

   MAINTENANCE:

   *  Change timeout for aborting connection (using retransmit limit or
      time value)

      Protocols: TCP, SCTP

      Functional because this is closely related to potentially assumed
      reliable data delivery.

      Implementation: via CHANGE_TIMEOUT.TCP or CHANGE_TIMEOUT.SCTP.

      Implementation over UDP: not possible.  (UDP is unreliable and
      there is no connection timeout.)

   *  Suggest timeout to the peer

      Protocols: TCP

      Functional because this is closely related to potentially assumed
      reliable data delivery.

      Implementation: via CHANGE_TIMEOUT.TCP.

      Implementation over UDP: not possible.  (UDP is unreliable and
      there is no connection timeout.)

   *  Disable Nagle algorithm

      Protocols: TCP, SCTP

      Optimizing because this decision depends on knowledge about the
      size of future data blocks and the delay between them.

      Implementation: via DISABLE_NAGLE.TCP and DISABLE_NAGLE.SCTP.

      Implementation over UDP: do nothing (UDP does not implement the
      Nagle algorithm).

   *  Request an immediate heartbeat, returning success/failure

      Protocols: SCTP

      Automatable because this informs about network-specific knowledge.

   *  Notification of Excessive Retransmissions (early warning below
      abortion threshold)

      Protocols: TCP

      Optimizing because it is an early warning to the application,
      informing it of an impending functional event.

      Implementation: via ERROR.TCP.

      Implementation over UDP: do nothing (there is no abortion
      threshold).

   *  Add path

      Protocols: MPTCP, SCTP

      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port

      SCTP Parameters: local IP address

      Automatable because the choice of paths to communicate between the
      same end hosts relates to knowledge about the network, not the
      application.

   *  Remove path

      Protocols: MPTCP, SCTP

      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port

      SCTP Parameters: local IP address

      Automatable because the choice of paths to communicate between the
      same end host relates to knowledge about the network, not the
      application.

   *  Set primary path

      Protocols: SCTP

      Automatable because the choice of paths to communicate between the
      same end hosts relates to knowledge about the network, not the
      application.

   *  Suggest primary path to the peer

      Protocols: SCTP

      Automatable because the choice of paths to communicate between the
      same end hosts relates to knowledge about the network, not the
      application.

   *  Configure Path Switchover

      Protocols: SCTP

      Automatable because the choice of paths to communicate between the
      same end hosts relates to knowledge about the network, not the
      application.

   *  Obtain status (query or notification)

      Protocols: SCTP, MPTCP

      SCTP parameters: association connection state; destination
      transport address list; destination transport address reachability
      states; current local and peer receiver window size; current local
      congestion window sizes; number of unacknowledged DATA chunks;
      number of DATA chunks pending receipt; primary path; most recent
      SRTT on primary path; RTO on primary path; SRTT and RTO on other
      destination addresses; MTU per path; interleaving supported yes/no

      MPTCP parameters: subflow-list (identified by source-IP; source-
      Port; destination-IP; destination-Port)

      Automatable because these parameters relate to knowledge about the
      network, not the application.

   *  Specify DSCP field

      Protocols: TCP, SCTP, UDP(-Lite)

      Optimizing because choosing a suitable DSCP value requires
      application-specific knowledge.

      Implementation: via SET_DSCP.TCP / SET_DSCP.SCTP / SET_DSCP.UDP(-
      Lite).

   *  Notification of ICMP error message arrival

      Protocols: TCP, UDP(-Lite)

      Optimizing because these messages can inform about success or
      failure of functional transport features (e.g., host unreachable
      relates to "Connect").

      Implementation: via ERROR.TCP or ERROR.UDP(-Lite.)

   *  Obtain information about interleaving support

      Protocols: SCTP

      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.

      Implementation: via STATUS.SCTP.

   *  Change authentication parameters

      Protocols: TCP, SCTP

      Functional because this has a direct influence on security.

      Implementation: via SET_AUTH.TCP and SET_AUTH.SCTP.

      Implementation over TCP: with SCTP, this allows adjusting key_id,
      key, and hmac_id.  With TCP, this allows changing the preferred
      outgoing MKT (current_key) and the preferred incoming MKT
      (rnext_key), respectively, for a segment that is sent on the
      connection.  Key material must be provided in a way that is
      compatible with both [RFC4895] and [RFC5925].

      Implementation over UDP: not possible.  (UDP does not offer
      authentication.)

   *  Obtain authentication information

      Protocols: SCTP

      Functional because authentication decisions may have been made by
      the peer, and this has an influence on the necessary application-
      level measures to provide a certain level of security.

      Implementation: via GET_AUTH.SCTP.

      Implementation over TCP: with SCTP, this allows obtaining key_id
      and a chunk list.  With TCP, this allows obtaining current_key and
      rnext_key from a previously received segment.  Key material must
      be provided in a way that is compatible with both [RFC4895] and
      [RFC5925].

      Implementation over UDP: not possible.  (UDP does not offer
      authentication.)

   *  Reset Stream

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Notification of Stream Reset

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Reset Association

      Protocols: SCTP

      Automatable because deciding to reset an association does not
      require application-specific knowledge.

      Implementation: via RESET_ASSOC.SCTP.

   *  Notification of Association Reset

      Protocols: SCTP

      Automatable because this notification does not relate to
      application-specific knowledge.

   *  Add Streams

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Notification of Added Stream

      Protocols: SCTP

      Automatable because using multi-streaming does not require
      application-specific knowledge.

      Implementation: see Section 5.2.

   *  Choose a scheduler to operate between streams of an association

      Protocols: SCTP

      Optimizing because the scheduling decision requires application-
      specific knowledge.  However, if a transport system would not use
      this, or wrongly configure it on its own, this would only affect
      the performance of data transfers; the outcome would still be
      correct within the "best effort" service model.

      Implementation: using SET_STREAM_SCHEDULER.SCTP.

      Implementation over TCP: do nothing (streams are not available in
      TCP, but no guarantee is given that this transport feature has any
      effect).

      Implementation over UDP: do nothing (streams are not available in
      UDP, but no guarantee is given that this transport feature has any
      effect).

   *  Configure priority or weight for a scheduler

      Protocols: SCTP

      Optimizing because the priority or weight requires application-
      specific knowledge.  However, if a transport system would not use
      this, or wrongly configure it on its own, this would only affect
      the performance of data transfers; the outcome would still be
      correct within the "best effort" service model.

      Implementation: using CONFIGURE_STREAM_SCHEDULER.SCTP.

      Implementation over TCP: do nothing (streams are not available in
      TCP, but no guarantee is given that this transport feature has any
      effect).

      Implementation over UDP: do nothing (streams are not available in
      UDP, but no guarantee is given that this transport feature has any
      effect).

   *  Configure send buffer size

      Protocols: SCTP

      Automatable because this decision relates to knowledge about the
      network and the Operating System, not the application (see also
      the discussion in Section 5.4).

   *  Configure receive buffer (and rwnd) size

      Protocols: SCTP

      Automatable because this decision relates to knowledge about the
      network and the Operating System, not the application.

   *  Configure message fragmentation

      Protocols: SCTP

      Automatable because this relates to knowledge about the network
      and the Operating System, not the application.  Note that this
      SCTP feature does not control IP-level fragmentation, but decides
      on fragmentation of messages by SCTP, in the end system.

      Implementation: done by always enabling it with
      CONFIG_FRAGMENTATION.SCTP and auto-setting the fragmentation size
      based on network or Operating System conditions.

   *  Configure PMTUD

      Protocols: SCTP

      Automatable because Path MTU Discovery relates to knowledge about
      the network, not the application.

   *  Configure delayed SACK timer

      Protocols: SCTP

      Automatable because the receiver-side decision to delay sending
      SACKs relates to knowledge about the network, not the application
      (it can be relevant for a sending application to request not to
      delay the SACK of a message, but this is a different transport
      feature).

   *  Set Cookie life value

      Protocols: SCTP

      Functional because it relates to security (possibly weakened by
      keeping a cookie very long) versus the time between connection
      establishment attempts.  Knowledge about both issues can be
      application specific.

      Implementation over TCP: the closest specified TCP functionality
      is the cookie in TCP Fast Open; for this, [RFC7413] states that
      the server "can expire the cookie at any time to enhance
      security", and Section 4.1.2 of [RFC7413] describes an example
      implementation where updating the key on the server side causes
      the cookie to expire.  Alternatively, for implementations that do
      not support TCP Fast Open, this transport feature could also
      affect the validity of SYN cookies (see Section 3.6 of [RFC4987]).

      Implementation over UDP: not possible.  (UDP does not offer this
      functionality.)

   *  Set maximum burst

      Protocols: SCTP

      Automatable because it relates to knowledge about the network, not
      the application.

   *  Configure size where messages are broken up for partial delivery

      Protocols: SCTP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation over TCP: not possible.  (TCP does not offer
      identification of message boundaries.)

      Implementation over UDP: not possible.  (UDP does not fragment
      messages.)

   *  Disable checksum when sending

      Protocols: UDP

      Functional because application-specific knowledge is necessary to
      decide whether it can be acceptable to lose data integrity with
      respect to random corruption.

      Implementation: via SET_CHECKSUM_ENABLED.UDP.

      Implementation over TCP: do nothing (TCP does not offer to disable
      the checksum, but transmitting data with an intact checksum will
      not yield a semantically wrong result).

   *  Disable checksum requirement when receiving

      Protocols: UDP

      Functional because application-specific knowledge is necessary to
      decide whether it can be acceptable to lose data integrity with
      respect to random corruption.

      Implementation: via SET_CHECKSUM_REQUIRED.UDP.

      Implementation over TCP: do nothing (TCP does not offer to disable
      the checksum, but transmitting data with an intact checksum will
      not yield a semantically wrong result).

   *  Specify checksum coverage used by the sender

      Protocols: UDP-Lite

      Functional because application-specific knowledge is necessary to
      decide for which parts of the data it can be acceptable to lose
      data integrity with respect to random corruption.

      Implementation: via SET_CHECKSUM_COVERAGE.UDP-Lite.

      Implementation over TCP: do nothing (TCP does not offer to limit
      the checksum length, but transmitting data with an intact checksum
      will not yield a semantically wrong result).

      Implementation over UDP: if checksum coverage is set to cover
      payload data, do nothing.  Else, either do nothing (transmitting
      data with an intact checksum will not yield a semantically wrong
      result), or use the transport feature "Disable checksum when
      sending".

   *  Specify minimum checksum coverage required by receiver

      Protocols: UDP-Lite

      Functional because application-specific knowledge is necessary to
      decide for which parts of the data it can be acceptable to lose
      data integrity with respect to random corruption.

      Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite.

      Implementation over TCP: do nothing (TCP does not offer to limit
      the checksum length, but transmitting data with an intact checksum
      will not yield a semantically wrong result).

      Implementation over UDP: if checksum coverage is set to cover
      payload data, do nothing.  Else, either do nothing (transmitting
      data with an intact checksum will not yield a semantically wrong
      result), or use the transport feature "Disable checksum
      requirement when receiving".

   *  Specify DF field

      Protocols: UDP(-Lite)

      Optimizing because the DF field can be used to carry out Path MTU
      Discovery, which can lead an application to choose message sizes
      that can be transmitted more efficiently.

      Implementation: via MAINTENANCE.SET_DF.UDP(-Lite) and
      SEND_FAILURE.UDP(-Lite).

      Implementation over TCP: do nothing (with TCP, the sending
      application is not in control of transport message sizes, making
      this functionality irrelevant).

   *  Get max. transport-message size that may be sent using a non-
      fragmented IP packet from the configured interface

      Protocols: UDP(-Lite)

      Optimizing because this can lead an application to choose message
      sizes that can be transmitted more efficiently.

      Implementation over TCP: do nothing (this information is not
      available with TCP).

   *  Get max. transport-message size that may be received from the
      configured interface

      Protocols: UDP(-Lite)

      Optimizing because this can, for example, influence an
      application's memory management.

      Implementation over TCP: do nothing (this information is not
      available with TCP).

   *  Specify TTL/Hop count field

      Protocols: UDP(-Lite)

      Automatable because a transport system can use a large enough
      system default to avoid communication failures.  Allowing an
      application to configure it differently can produce notifications
      of ICMP error message arrivals that yield information that only
      relates to knowledge about the network, not the application.

   *  Obtain TTL/Hop count field

      Protocols: UDP(-Lite)

      Automatable because the TTL/Hop count field relates to knowledge
      about the network, not the application.

   *  Specify ECN field

      Protocols: UDP(-Lite)

      Automatable because the ECN field relates to knowledge about the
      network, not the application.

   *  Obtain ECN field

      Protocols: UDP(-Lite)

      Optimizing because this information can be used by an application
      to better carry out congestion control (this is relevant when
      choosing a data transmission Transport Service that does not
      already do congestion control).

      Implementation over TCP: do nothing (this information is not
      available with TCP).

   *  Specify IP Options

      Protocols: UDP(-Lite)

      Automatable because IP Options relate to knowledge about the
      network, not the application.

   *  Obtain IP Options

      Protocols: UDP(-Lite)

      Automatable because IP Options relate to knowledge about the
      network, not the application.

   *  Enable and configure a "Low Extra Delay Background Transfer"

      Protocols: a protocol implementing the LEDBAT congestion control
      mechanism

      Optimizing because whether this feature is appropriate or not
      depends on application-specific knowledge.  However, wrongly using
      this will only affect the speed of data transfers (albeit
      including other transfers that may compete with the transport
      system's transfer in the network), so it is still correct within
      the "best effort" service model.

      Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP /
      SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [RFC8622].

      Implementation over TCP: do nothing (TCP does not support LEDBAT
      congestion control, but not implementing this functionality will
      not yield a semantically wrong behavior).

      Implementation over UDP: do nothing (UDP does not offer congestion
      control).

   TERMINATION:

   *  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side

      Protocols: TCP, SCTP

      Functional because the notion of a connection is often reflected
      in applications as an expectation to have all outstanding data
      delivered and no longer be able to communicate after a "Close"
      succeeded, with a communication sequence relating to this
      transport feature that is defined by the application protocol.

      Implementation: via CLOSE.TCP and CLOSE.SCTP.

      Implementation over UDP: not possible.  (UDP is unreliable and
      hence does not know when all remaining data is delivered; it does
      also not offer to cause an event related to closing at the peer.)

   *  Abort without delivering remaining data, causing an event
      informing the application on the other side

      Protocols: TCP, SCTP

      Functional because the notion of a connection is often reflected
      in applications as an expectation to potentially not have all
      outstanding data delivered and no longer be able to communicate
      after an "Abort" succeeded.  On both sides of a connection, an
      application protocol may define a communication sequence relating
      to this transport feature.

      Implementation: via ABORT.TCP and ABORT.SCTP.

      Implementation over UDP: not possible.  (UDP does not offer to
      cause an event related to aborting at the peer.)

   *  Abort without delivering remaining data, not causing an event
      informing the application on the other side

      Protocols: UDP(-Lite)

      Functional because the notion of a connection is often reflected
      in applications as an expectation to potentially not have all
      outstanding data delivered and no longer be able to communicate
      after an "Abort" succeeded.  On both sides of a connection, an
      application protocol may define a communication sequence relating
      to this transport feature.

      Implementation: via ABORT.UDP(-Lite).

      Implementation over TCP: stop using the connection, wait for a
      timeout.

   *  Timeout event when data could not be delivered for too long

      Protocols: TCP, SCTP

      Functional because this notifies that potentially assumed reliable
      data delivery is no longer provided.

      Implementation: via TIMEOUT.TCP and TIMEOUT.SCTP.

      Implementation over UDP: do nothing (this event will not occur
      with UDP).

A.2.  DATA-Transfer-Related Transport Features

A.2.1.  Sending Data

   *  Reliably transfer data, with congestion control

      Protocols: TCP, SCTP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via SEND.TCP and SEND.SCTP.

      Implementation over UDP: not possible.  (UDP is unreliable.)

   *  Reliably transfer a message, with congestion control

      Protocols: SCTP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via SEND.SCTP.

      Implementation over TCP: via SEND.TCP.  With SEND.TCP, message
      boundaries will not be identifiable by the receiver, because TCP
      provides a byte-stream service.

      Implementation over UDP: not possible.  (UDP is unreliable.)

   *  Unreliably transfer a message

      Protocols: SCTP, UDP(-Lite)

      Optimizing because only applications know about the time
      criticality of their communication, and reliably transferring a
      message is never incorrect for the receiver of a potentially
      unreliable data transfer, it is just slower.

      CHANGED FROM RFC 8303.  This differs from the 2 automatable
      transport features below in that it leaves the choice of
      congestion control open.

      Implementation: via SEND.SCTP or SEND.UDP(-Lite).

      Implementation over TCP: use SEND.TCP.  With SEND.TCP, messages
      will be sent reliably, and message boundaries will not be
      identifiable by the receiver.

   *  Unreliably transfer a message, with congestion control

      Protocols: SCTP

      Automatable because congestion control relates to knowledge about
      the network, not the application.

   *  Unreliably transfer a message, without congestion control

      Protocols: UDP(-Lite)

      Automatable because congestion control relates to knowledge about
      the network, not the application.

   *  Configurable Message Reliability

      Protocols: SCTP

      Optimizing because only applications know about the time
      criticality of their communication, and reliably transferring a
      message is never incorrect for the receiver of a potentially
      unreliable data transfer, it is just slower.

      Implementation: via SEND.SCTP.

      Implementation over TCP: done by using SEND.TCP and ignoring this
      configuration.  Based on the assumption of the best-effort service
      model, unnecessarily delivering data does not violate application
      expectations.  Moreover, it is not possible to associate the
      requested reliability to a "message" in TCP anyway.

      Implementation over UDP: not possible.  (UDP is unreliable.)

   *  Choice of stream

      Protocols: SCTP

      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.

      Implementation: see Section 5.2.

   *  Choice of path (destination address)

      Protocols: SCTP

      Automatable because it requires using multiple sockets, but
      obtaining multiple sockets in the CONNECTION.ESTABLISHMENT
      category is automatable.

   *  Ordered message delivery (potentially slower than unordered)

      Protocols: SCTP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via SEND.SCTP.

      Implementation over TCP: done by using SEND.TCP.  With SEND.TCP,
      messages will not be identifiable by the receiver.

      Implementation over UDP: not possible.  (UDP does not offer any
      guarantees regarding ordering.)

   *  Unordered message delivery (potentially faster than ordered)

      Protocols: SCTP, UDP(-Lite)

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via SEND.SCTP.

      Implementation over TCP: done by using SEND.TCP and always sending
      data ordered.  Based on the assumption of the best-effort service
      model, ordered delivery may just be slower and does not violate
      application expectations.  Moreover, it is not possible to
      associate the requested delivery order to a "message" in TCP
      anyway.

   *  Request not to bundle messages

      Protocols: SCTP

      Optimizing because this decision depends on knowledge about the
      size of future data blocks and the delay between them.

      Implementation: via SEND.SCTP.

      Implementation over TCP: done by using SEND.TCP and
      DISABLE_NAGLE.TCP to disable the Nagle algorithm when the request
      is made and enable it again when the request is no longer made.
      Note that this is not fully equivalent because it relates to the
      time of issuing the request rather than a specific message.

      Implementation over UDP: do nothing (UDP never bundles messages).

   *  Specifying a "payload protocol-id" (handed over as such by the
      receiver)

      Protocols: SCTP

      Functional because it allows sending extra application data with
      every message, for the sake of identification of data, which by
      itself is application specific.

      Implementation: SEND.SCTP.

      Implementation over TCP: not possible.  (This functionality is not
      available in TCP.)

      Implementation over UDP: not possible.  (This functionality is not
      available in UDP.)

   *  Specifying a key id to be used to authenticate a message

      Protocols: SCTP

      Functional because this has a direct influence on security.

      Implementation: via a parameter in SEND.SCTP.

      Implementation over TCP: this could be emulated by using
      SET_AUTH.TCP before and after the message is sent.  Note that this
      is not fully equivalent because it relates to the time of issuing
      the request rather than a specific message.

      Implementation over UDP: not possible.  (UDP does not offer
      authentication.)

   *  Request not to delay the acknowledgement (SACK) of a message

      Protocols: SCTP

      Optimizing because only an application knows for which message it
      wants to quickly be informed about success/failure of its
      delivery.

      Implementation over TCP: do nothing (TCP does not offer this
      functionality, but ignoring this request from the application will
      not yield a semantically wrong behavior).

      Implementation over UDP: do nothing (UDP does not offer this
      functionality, but ignoring this request from the application will
      not yield a semantically wrong behavior).

A.2.2.  Receiving Data

   *  Receive data (with no message delimiting)

      Protocols: TCP

      Functional because a transport system must be able to send and
      receive data.

      Implementation: via RECEIVE.TCP.

      Implementation over UDP: do nothing (UDP only works on messages;
      these can be handed over, the application can still ignore the
      message boundaries).

   *  Receive a message

      Protocols: SCTP, UDP(-Lite)

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite).

      Implementation over TCP: not possible.  (TCP does not support
      identification of message boundaries.)

   *  Choice of stream to receive from

      Protocols: SCTP

      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.

      Implementation: see Section 5.2.

   *  Information about partial message arrival

      Protocols: SCTP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation: via RECEIVE.SCTP.

      Implementation over TCP: do nothing (this information is not
      available with TCP).

      Implementation over UDP: do nothing (this information is not
      available with UDP).

A.2.3.  Errors

   This section describes sending failures that are associated with a
   specific call to in the "Sending Data" category (Appendix A.2.1).

   *  Notification of send failures

      Protocols: SCTP, UDP(-Lite)

      Functional because this notifies that potentially assumed reliable
      data delivery is no longer provided.

      CHANGED FROM RFC 8303.  This differs from the 2 automatable
      transport features below in that it does not distinguish between
      unsent and unacknowledged messages.

      Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(-
      Lite).

      Implementation over TCP: do nothing (this notification is not
      available and will therefore not occur with TCP).

   *  Notification of an unsent (part of a) message

      Protocols: SCTP, UDP(-Lite)

      Automatable because the distinction between unsent and
      unacknowledged does not relate to application-specific knowledge.

   *  Notification of an unacknowledged (part of a) message

      Protocols: SCTP

      Automatable because the distinction between unsent and
      unacknowledged does not relate to application-specific knowledge.

   *  Notification that the stack has no more user data to send

      Protocols: SCTP

      Optimizing because reacting to this notification requires the
      application to be involved, and ensuring that the stack does not
      run dry of data (for too long) can improve performance.

      Implementation over TCP: do nothing (see the discussion in
      Section 5.4).

      Implementation over UDP: do nothing (this notification is not
      available and will therefore not occur with UDP).

   *  Notification to a receiver that a partial message delivery has
      been aborted

      Protocols: SCTP

      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.

      Implementation over TCP: do nothing (this notification is not
      available and will therefore not occur with TCP).

      Implementation over UDP: do nothing (this notification is not
      available and will therefore not occur with UDP).

Acknowledgements

   The authors would like to thank all the participants of the TAPS
   Working Group and the NEAT and MAMI research projects for valuable
   input to this document.  We especially thank Michael Tüxen for help
   with connection establishment/teardown, Gorry Fairhurst for his
   suggestions regarding fragmentation and packet sizes, and Spencer
   Dawkins for his extremely detailed and constructive review.  This
   work has received funding from the European Union's Horizon 2020
   research and innovation program under grant agreement No. 644334
   (NEAT).

Authors' Addresses

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

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


   Stein Gjessing
   University of Oslo
   PO Box 1080 Blindern
   N-0316 Oslo
   Norway

   Phone: +47 22 85 24 44
   Email: steing@ifi.uio.no