TAPS                                                            M. Welzl
Internet-Draft                                               S. Gjessing
Intended status: Informational                        University of Oslo
Expires: March 9, 17, 2019                               September 5, 13, 2018

          A Minimal Set of Transport Services for End Systems
                       draft-ietf-taps-minset-08
                       draft-ietf-taps-minset-09

Abstract

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

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   This Internet-Draft will expire on March 9, 17, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Deriving the minimal set  . . . . . . . . . . . . . . . . . .   5
   4.  The Minimal Reduced Set of Transport Features . . . . . . . . . . . .   5
     3.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION   6
     4.1.  CONNECTION Related Transport Features . . . . . . .   5
     3.2.  MAINTENANCE . . .   7
     4.2.  DATA Transfer Related Transport Features  . . . . . . . .   8
       4.2.1.  Sending Data  . . . . . . . . . . . .   8
       3.2.1.  Connection groups . . . . . . . .   8
       4.2.2.  Receiving Data  . . . . . . . . . .   8
       3.2.2.  Individual connections . . . . . . . . .   9
       4.2.3.  Errors  . . . . . .  10
     3.3.  DATA Transfer . . . . . . . . . . . . . . . . .   9
   5.  Discussion  . . . . .  10
       3.3.1.  Sending Data . . . . . . . . . . . . . . . . . . . .  10
       3.3.2.   9
     5.1.  Sending Messages, Receiving Data  . . Bytes . . . . . . . . . . . .   9
     5.2.  Stream Schedulers Without Streams . . . . .  11
   4.  Acknowledgements . . . . . . .  10
     5.3.  Early Data Transmission . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . .  11
     5.4.  Sender Running Dry  . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations
     5.5.  Capacity Profile  . . . . . . . . . . . . . . . . . . . .  12
   7.  References
     5.6.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.7.  Packet Size . .  12
     7.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References . . .  13
   6.  The Minimal Set of Transport Features . . . . . . . . . . . .  14
     6.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION . .  13
   Appendix A.  Deriving the minimal set . . . . .  14
     6.2.  MAINTENANCE . . . . . . . . .  14
     A.1.  Step 1: Categorization -- The Superset of Transport
           Features . . . . . . . . . . . . . .  17
       6.2.1.  Connection groups . . . . . . . . . .  15
       A.1.1.  CONNECTION Related Transport Features . . . . . . . .  17
       A.1.2.  18
       6.2.2.  Individual connections  . . . . . . . . . . . . . . .  19
     6.3.  DATA Transfer Related Transport Features . . . . . .  33
     A.2.  Step 2: Reduction -- The Reduced Set of Transport
           Features . . . . . . . . . . . . . . . .  20
       6.3.1.  Sending Data  . . . . . . . .  38
       A.2.1.  CONNECTION Related Transport Features . . . . . . . .  39
       A.2.2.  DATA Transfer Related Transport Features . . . .  20
       6.3.2.  Receiving Data  . .  40
     A.3.  Step 3: Discussion . . . . . . . . . . . . . . . . .  21
   7.  Acknowledgements  . .  41
       A.3.1.  Sending Messages, Receiving Bytes . . . . . . . . . .  41
       A.3.2.  Stream Schedulers Without Streams . . . . . . . . . .  42
       A.3.3.  Early Data Transmission  21
   8.  IANA Considerations . . . . . . . . . . . . . . .  43
       A.3.4.  Sender Running Dry . . . . . .  21
   9.  Security Considerations . . . . . . . . . . .  44
       A.3.5.  Capacity Profile . . . . . . . .  21
   10. References  . . . . . . . . . .  44
       A.3.6.  Security . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . .  45
       A.3.7.  Packet Size . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . .  45
   Appendix B.  Revision information . . . . . . .  22
   Appendix A.  The Superset of Transport Features . . . . . . . . .  46
   Authors' Addresses  24
     A.1.  CONNECTION Related Transport Features . . . . . . . . . .  25
     A.2.  DATA Transfer Related Transport Features  . . . . . . . .  41
       A.2.1.  Sending Data  . . . . .  47 . . . . . . . . . . . . . . .  41
       A.2.2.  Receiving Data  . . . . . . . . . . . . . . . . . . .  45
       A.2.3.  Errors  . . . . . . . . . . . . . . . . . . . . . . .  46
   Appendix B.  Revision information . . . . . . . . . . . . . . . .  47
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  48

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 bytestream, 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,
   MPTCP, UDP(-Lite) and SCTP as well as the 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. 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 in this
   document 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.  The transport
   features in the minimal set in this document 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 which 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 bytestream.  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 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.

2.  Terminology

   Transport Feature:  a specific end-to-end feature that  While the transport
      layer provides to an application.  Examples include
      confidentiality, reliable delivery, ordered delivery, message-
      versus-stream orientation, etc.
   Transport Service:  a set
   possibility of Transport Features, without an
      association to any given framing protocol, which provides a
      complete service to an application.
   Transport Protocol:  an such "one-sided" implementation that provides one or more
      different transport services using a specific framing and header
      format on may help deployment,
   it comes at the wire.
   Transport Service Instance:  an arrangement cost of transport protocols
      with a selected limiting the set to services that can also be
   provided by TCP (or, with further limitations, UDP).  Thus, the
   minimal set of transport features and configuration parameters here is applicable for many, but
   not all, applications: some application protocols have requirements
   that
      implements 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 UDP, or
   "implementation over" UDP is discussed, this refers to native usage
   of UDP.

2.  Terminology

   Transport Feature:  a single specific end-to-end feature that the transport service, e.g.,
      layer provides to an application.  Examples include
      confidentiality, reliable delivery, ordered delivery, message-
      versus-stream orientation, etc.
   Transport Service:  a protocol stack (RTP
      over UDP). set of Transport Features, without an
      association to any given framing protocol, which provides a
      complete service to an application.
   Transport Protocol:  an implementation that provides one or more
      different transport services using a specific framing and header
      format on the wire.
   Application:  an entity that uses the 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.
   Endpoint:
   End system:  an entity that communicates with one or more other
      endpoints end
      systems using a transport protocol.  An end system provides a
      transport layer interface to applications.
   Connection:  shared state of two or more endpoints end systems that persists
      across messages that are transmitted between these endpoints. end systems.
   Connection Group:  a set of connections which 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.  The Minimal Set of Transport Features

   Based on  Deriving the categorization, reduction, and discussion in Appendix A,
   this section describes a minimal set of transport features

   We assume that end
   systems should offer.  The described transport system can be
   implemented over TCP.  Elements applications have no specific requirements that need
   knowledge about the network, e.g. regarding the choice of network
   interface or the system end-to-end path.  Even with these assumptions, there
   are certain requirements that are not marked
   with "!UDP" can strictly kept by transport
   protocols today, and these must also be implemented over UDP.

   The arguments laid out in Appendix A.3 ("discussion") were used to
   make the final representation kept by a transport system.
   Some 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 requirements relate to
   expose multi-streaming as a visible transport features that we call
   "Functional".

   Functional transport features provide functionality to applications, that cannot be
   used without the application knowing about them, or else they violate
   assumptions that might cause the restrictive open / close semantics may application to fail.  For example,
   ordered message delivery is a functional transport feature: it cannot
   be problematic under some
   circumstances.  In such cases, configured without the representation in Appendix A.2
   ("reduction") should application knowing about it because the
   application's assumption could be considered.

   As that messages always arrive in Appendix A, Appendix A.2 and [RFC8303], we categorize
   order.  Failure includes any change of the
   minimal set application behavior that
   is not performance oriented, e.g. security.

   "Change DSCP" and "Disable Nagle algorithm" are examples 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 we call "Optimizing": if a transport system offers as
   autonomously decides to enable or disable them, an
   abstraction application will
   not fail, but a transport system may be able to communicate more
   efficiently if the application, as opposed 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 connections be transmitted).

   The transport features of IETF transport protocols that the transport system uses.

3.1.  ESTABLISHMENT, AVAILABILITY do not
   require application-specific knowledge and TERMINATION

   A connection must first be "created" to allow for some initial
   configuration to could therefore be carried out before the
   utilized by a transport system can
   actively or passively establish communication with a remote endpoint.
   All configuration parameters in Section 3.2 can be used initially,
   although some on its own without involving the
   application are called "Automatable".

   We approach the construction of them may only take effect when a connection has been
   established with a chosen transport protocol.  Configuring a
   connection early helps a minimal set of transport system make features
   in the right decisions.
   For example, grouping information can influence following way:

   1.  Categorization (Appendix A): the superset of transport system
   to implement a connection features
       from [RFC8303] is presented, and transport features are
       categorized as Functional, Optimizing or Automatable for later
       reduction.
   2.  Reduction (Section 4): a stream shorter list of a multi-streaming protocol's
   existing association or not.

   For ungrouped connections, early configuration transport features is necessary because
   it allows
       derived from the categorization in the first step.  This removes
       all 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 features that must be kept, do not require application-specific
       knowledge or it can end up would result in semantically incorrect behavior if
       they were implemented over TCP or UDP.
   3.  Discussion (Section 5): the resulting list shows a deadlock
   situation (e.g., having chosen UDP and later be asked to support
   reliable transfer).  As an example description number of how
       peculiarities that are discussed, to correctly
   handle these cases, we provide the following decision tree (this 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
   derived from Appendix A.2.1 excluding authentication,
       constructed.

   Following [RFC8303] and retaining its terminology, we divide the
   transport features into two main groups as explained in
   Section 6): follows:

   1.  CONNECTION related transport features
       - Will it ever be necessary 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 offer any automate the usage of
   network-related functionality.  This can facilitate using the following?
     *  Reliably transfer data
     *  Notify
   transport system for the peer application programmer and it allows for
   optimizations that may not be possible for an application.  For
   instance, system-wide configurations regarding the usage of closing/aborting
     *  Preserve data ordering

     Yes: SCTP or TCP multiple
   interfaces can better be used.
     - Is any exploited if the choice of the following useful interface is
   not entirely up to the application?
       * Choosing a scheduler application.  Therefore, since they are not
   strictly necessary to operate between connections expose in a group, transport system, we do not include
   automatable transport features in the reduced set of transport
   features.  This leaves us with only the possibility transport features that are
   either optimizing or functional.

   A transport system should be able to configure a priority communicate via TCP or weight per connection
       * Configurable message reliability
       * Unordered message delivery
       * Request UDP if
   alternative transport protocols are found not to delay the acknowledgement (SACK) of work.  For many
   transport features, this is possible -- often by simply not doing
   anything when a message

       Yes: SCTP specific request is preferred.
       No:
       - Is any 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 useful 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 application?
         * 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:

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

   AVAILABILITY:

   o  T,U: Listen
   o  T: Configure authentication

   MAINTENANCE:

   o  T: Change timeout for aborting connection (using retransmit limit
      or time value)

   o  T: Suggest timeout to the peer
         *
   o  T,U: Disable Nagle algorithm
   o  T,U: Notification of Excessive Retransmissions (early warning
      below abortion threshold)
         *
   o  T,U: Specify DSCP field
   o  T,U: Notification of ICMP error message arrival

         Yes: TCP is preferred.
         No: SCTP and TCP are equally preferable.

     No: all protocols can be used.
     - Is any of the following useful
   o  T: Change authentication parameters
   o  T: Obtain authentication information
   o  T,U: Set Cookie life value
   o  T,U: Choose a scheduler to the application?
       * operate between streams of an
      association
   o  T,U: Configure priority or weight for a scheduler
   o  T,U: Disable checksum when sending
   o  T,U: Disable checksum requirement when receiving
   o  T,U: Specify checksum coverage used by the sender
       *
   o  T,U: Specify minimum checksum coverage required by receiver

       Yes: UDP-Lite is preferred.
       No: UDP is preferred.

   Note
   o  T,U: Specify DF field
   o  T,U: Get max. transport-message size 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.  We caution implementers to may be aware of the full
   set of trade-offs, for which we recommend consulting the list in
   Appendix A.2.1 when deciding how to initialize sent using a connection.

   To summarize, non-
      fragmented IP packet from the following parameters serve as input for configured interface
   o  T,U: Get max. transport-message size that may be received from the
   transport system to help it choose
      configured interface
   o  T,U: Obtain ECN field
   o  T,U: Enable and configure a suitable protocol: "Low Extra Delay Background Transfer"

   TERMINATION:

   o  Reliability: a boolean that should be set to true when any of the
      following will be useful to the application:  T: Close after reliably transfer
      data; notify delivering all remaining data, causing an
      event informing the peer of closing/aborting; preserve data ordering. application on the other side
   o  Checksum coverage: a boolean to specify whether it will be useful
      to  T: Abort without delivering remaining data, causing an event
      informing the application to specify checksum coverage when sending or
      receiving. on the other side
   o  Configure message priority: a boolean that should be set to true
      when any of  T,U: Abort without delivering remaining data, not causing an event
      informing the following per-message configuration or
      prioritization mechanisms will application on the other side
   o  T,U: Timeout event when data could not be useful delivered for too long

4.2.  DATA Transfer Related Transport Features

4.2.1.  Sending Data

   o  T: Reliably transfer data, with congestion control
   o  T: Reliably transfer a message, with congestion control
   o  T,U: Unreliably transfer a message
   o  T: Configurable Message Reliability
   o  T: Ordered message delivery (potentially slower than unordered)
   o  T,U: Unordered message delivery (potentially faster than ordered)
   o  T,U: Request not to the application:
      choosing bundle messages
   o  T: Specifying a scheduler key id to operate between grouped connections, with
      the possibility be used to configure authenticate a priority or weight per connection;
      configurable message reliability; unordered message delivery;
      requesting
   o  T,U: Request not to delay the acknowledgement (SACK) of a message. message

4.2.2.  Receiving Data

   o  Early  T,U: Receive data (with no message timeout notifications: delimiting)
   o  U: Receive a boolean message
   o  T,U: Information about partial message arrival

4.2.3.  Errors

   This section describes sending failures that should be set are associated with a
   specific call to true when any of in the following will be useful to "Sending Data" category (Appendix A.2.1).

   o  T,U: Notification of send failures
   o  T,U: Notification that the
      application: hand over a message stack has no more user data to reliably transfer (possibly
      multiple times) before connection establishment; suggest timeout send
   o  T,U: Notification to the peer; notification of excessive retransmissions (early
      warning below abortion threshold); notification of ICMP error a receiver that a partial message arrival.

   Once delivery
      has been aborted

5.  Discussion

   The reduced set in the previous section exhibits a connection is created, it can be queried for number of
   peculiarities, which we will discuss in the maximum
   amount following.  This section
   focuses on TCP because, with the exception of data that an application can possibly expect to have
   reliably transmitted before or during one particular
   transport connection
   establishment (with zero being feature ("Receive a possible answer) (see message" -- we will discuss this in
   Section 3.2.1).  An application can also give 5.1), the connection list shows that UDP is strictly a
   message for reliable transmission before or during connection
   establishment (!UDP); the transport system will then subset of TCP.
   We can first try to transmit
   it as early as possible.  An application can facilitate sending understand how to build a
   message particularly early by marking it as "idempotent" (see
   Section 3.3.1); in this case, transport system that
   can run over TCP, and then narrow down the receiving application must be
   prepared result further to potentially receive multiple copies of the message
   (because idempotent messages are reliably transferred, asking for
   idempotence is not necessary for systems allow
   that support UDP).

   After creation, a transport the system can actively establish
   communication with a peer, always run over either TCP or it can passively listen for incoming
   connection requests.  Note that active establishment may or may not
   trigger UDP (which
   effectively means removing everything related to reliability,
   ordering, authentication and closing/aborting with a notification on the listening side.  It is possible that
   the first notification on to
   the listening side is peer).

   Note that, because the arrival functional transport features of UDP are --
   with the
   first data that the active side sends (a receiver-side transport
   system could handle this by continuing to block exception of "Receive a "Listen" call,
   immediately followed by issuing "Receive", message" -- a subset of TCP, TCP can
   be used as a replacement for example; callback-
   based implementations could simply skip UDP whenever an application does not
   need message delimiting (e.g., because the equivalent of "Listen"). application-layer protocol
   already does it).  This also means has been recognized by many applications that the active opening side is assumed
   already do this in practice, by trying to be the
   first side sending data.

   A 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 can actively close over TCP, there are several
   transport features related to sending, but only a connection, i.e. terminate it
   after reliably delivering all remaining data single transport
   feature related to the peer (if reliable receiving: "Receive data delivery was requested earlier (!UDP)), in which case (with no message
   delimiting)" (and, strangely, "information about partial message
   arrival").  Notably, the peer transport feature "Receive a message" is notified that
   also the connection only non-automatable transport feature of UDP(-Lite) for
   which no implementation over TCP is closed.  Alternatively, a
   connection can be aborted without delivering outstanding data possible.

   To support these TCP receiver semantics, we define an "Application-
   Framed Bytestream" (AFra-Bytestream).  AFra-Bytestreams allow senders
   to operate on messages while minimizing changes to the
   peer. TCP socket
   API.  In case reliable or partially reliable data delivery was
   requested earlier (!UDP), the peer is notified that particular, nothing changes on the connection is
   aborted.  A timeout receiver side - data can
   be configured to abort accepted via a connection when data
   could not be delivered for too long (!UDP); however, timeout-based
   abortion does not notify normal TCP socket.

   In an AFra-Bytestream, the peer sending application that can optionally inform
   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 about message boundaries and required properties per
   message (configurable order and reliability, or aborting the connection (!UDP), its peer
   may not be able to read outstanding data.  This means that
   unacknowledged data residing embedding a transport system's send buffer may
   have request
   not to be dropped from that buffer upon arrival delay the acknowledgement of a "close" or
   "abort" notification from the peer.

3.2.  MAINTENANCE

   A transport system must offer means to group connections, but it
   cannot guarantee truly grouping them using message).  Whenever the transport protocols sending
   application specifies per-message properties that relax the notion of
   reliable in-order delivery of bytes, 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 assume 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
   receiving application
   about grouping success or failure).

   As a general rule, any configuration described below should be
   carried out as early as possible is 1) able to aid the transport system's
   decision making.

3.2.1.  Connection groups

   The following transport features determine message boundaries,
   provided that messages are always kept intact, and notifications (some directly
   from Appendix A.2, some new or changed, based on 2) able to accept
   these relaxed per-message properties.  Any signaling of such
   information to the discussion in
   Appendix A.3) automatically apply peer is up to all grouped connections:

   (!UDP) Configure a timeout: an application-layer protocol and
   considered out of scope of this can be done document.

   For example, if an application requests to transfer fixed-size
   messages of 100 bytes with partial reliability, this needs the following
   parameters:

   o  A timeout value for aborting connections, in seconds
   o  A timeout value
   receiving application to be suggested prepared to the peer (if possible), accept data in
      seconds
   o  The number chunks of retransmissions after which 100
   bytes.  If, then, some of these 100-byte messages are missing (e.g.,
   if SCTP with Configurable Reliability is used), this is the expected
   application should behavior.  With TCP, no messages would be notifed of "Excessive Retransmissions"

   Configure urgency: missing, but
   this can be done with is also correct for the following parameters:

   o  A number to identify application, and the type of scheduler possible
   retransmission delay is acceptable within the best-effort service
   model (see [RFC7305], Section 3.5).  Still, the receiving application
   would separate the byte stream into 100-byte chunks.

   Note that should this usage of messages does not require all messages to be used to
      operate between connections
   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 group (no guarantees given).
      Schedulers are defined in [RFC8260].
   o  A "capacity profile" number to identify how use of byte stuffing
   methods such as COBS [COBS].  If an application wants needs message
   numbers, e.g. to use its available capacity.  Choices can be "lowest possible
      latency at restore the expense correct sequence of overhead" (which would disable any
      Nagle-like algorithm), "scavenger", or values that help determine
      the DSCP value for a connection (e.g.  similar to table 1 in
      [I-D.ietf-tsvwg-rtcweb-qos]).
   o  A buffer limit (in bytes); when messages, these must
   also be encoded by the sender has less than application itself, as the
      provided limit sequence number
   related transport features of bytes in SCTP are not provided by the buffer, "minimum
   set" (in the application may be
      notified.  Notifications are interest of enabling usage of TCP).

5.2.  Stream Schedulers Without Streams

   We have already stated that multi-streaming does not guaranteed, and it is optional
      for a transport system 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 support buffer limit values greater than
      0.  Note that this limit and its notification should operate
      across
   knowledge about the buffers of network and the whole particular transport system, i.e.  also any
      potential buffers that protocol in
   use, not the application.  However, the transport system itself may use on top features "Choose a
   scheduler to operate between streams of the transport's send buffer.

   Following Appendix A.3.7, these properties 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 queried:

   o  The maximum message size assigned a priority.  Moreover,
   the transport features in the MAINTENANCE category all operate on
   assocations in case of SCTP, i.e. they apply to all streams in that may be sent without fragmentation
      via
   assocation.

   With only these semantics necessary to represent, the configured interface.  This is optional for interface to a
   transport system to offer, and becomes easier if we assume that connections may return an error ("not available").  It
      can aid applications implementing Path MTU Discovery.
   o  The maximum be
   not only a transport message size that can protocol's connection or association, but could
   also be sent, in bytes.
      Irrespective of fragmentation, there is a size limit stream of an existing SCTP association, for the
      messages that example.  We
   only need to allow for a way to define a possible grouping of
   connections.  Then, all MAINTENANCE transport features can be handed over said to SCTP or UDP(-Lite); because
   operate on connection groups, not connections, and a scheduler
   operates on the service provided by connections within a group.

   To be compatible with multiple transport system is independent of the protocols and uniformly
   allow access to both transport connections and streams of a multi-
   streaming protocol, it must allow an application the semantics of opening and closing need to query this
      value -- be
   the maximum size most restrictive subset of all of the underlying options.  For
   example, TCP's support of half-closed connections can be seen as a message in an Application-Framed-
      Bytestream (see Appendix A.3.1).  This may also return an error
      when data is
   feature on top of the more restrictive "ABORT"; this feature cannot
   be supported because not delimited ("not available").
   o  The maximum all protocols used by a transport message size that can be received from the
      configured interface, in bytes (or "not available").
   o  The maximum amount system
   (including streams of data that can possibly be sent 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 or
      during connection establishment, in bytes.

   In addition establishment", which relates to the already mentioned closing / aborting notifications TCP Fast
   Open [RFC7413], and possible send errors, "Hand over a message to reliably transfer during
   connection establishment", which relates to SCTP's ability to
   transfer data together with the following notifications COOKIE-Echo chunk.  Also without TCP
   Fast Open, TCP can occur:

   o  Excessive Retransmissions: transfer data during the configured (or a default) number of
      retransmissions has been reached, yielding this early warning
      below an abortion threshold.
   o  ICMP Arrival (parameter: ICMP message): an ICMP packet carrying handshake, together with
   the conveyed ICMP message has arrived.
   o  ECN Arrival (parameter: ECN value): a SYN packet carrying -- however, the conveyed
      ECN value has arrived.  This can be useful for applications
      implementing congestion control.
   o  Timeout (parameter: s seconds): receiver of this data could may not be delivered for s
      seconds.
   o  Drain: hand it
   over to the send buffer has either drained below application until the configured
      buffer limit or it handshake has become completely empty.  This is a generic
      notification that tries to enable uniform access to
      "TCP_NOTSENT_LOWAT" completed.  Also,
   different from TCP Fast Open, this data is not delimited as well a message
   by TCP (thus, not visible as the "SENDER DRY" notification (as
      discussed in Appendix A.3.4 -- SCTP's "SENDER DRY" is a special
      case where the threshold (for unsent data) ``message'').  This functionality is 0
   commonly available in TCP and there is also
      no more unacknowledged data supported in several implementations,
   even though the send buffer).

3.2.2.  Individual connections

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

   Configure checksum usage: this can be done with "during" the following
   parameters, but there is no guarantee
   handshake.  Alternatively, it could also assume that any checksum limitations data that are
   handed over early will indeed be enforced (the default behavior is "full coverage,
   checksum enabled"):

   o  A boolean to enable / disable usage of a checksum when sending
   o  The desired coverage (in bytes) of transmitted as early as possible, and
   "before" the checksum handshake would only be used when sending
   o  A boolean for messages that are
   explicitly marked as "idempotent" (i.e., it would be acceptable to enable / disable requiring a checksum when receiving
   o
   transfer them multiple times).

   The required minimum coverage (in bytes) amount of data that can successfully be transmitted before or
   during the checksum when
      receiving

3.3.  DATA Transfer

3.3.1.  Sending Data

   When sending a message, no guarantees are given about handshake depends on various factors: the
   preservation transport
   protocol, the use of message boundaries to the peer; if message boundaries
   are needed, the receiving application at header options, the peer must know about
   them beforehand (or choice of IPv4 and IPv6 and
   the Path MTU.  A transport system cannot use TCP).  Note that
   an application should already be able therefore allow a sending
   application to hand over query the maximum amount of data it can possibly
   transmit before the
   transport system establishes a (or, if exposed, during) connection with a chosen establishment.

5.4.  Sender Running Dry

   The transport
   protocol.  Regarding the message feature "Notification that is being handed over, the
   following parameters can stack has no more user
   data to send" relates to SCTP's "SENDER DRY" notification.  Such
   notifications can, in principle, be used:

   o  Reliability: this parameter is used to convey a choice of: fully
      reliable with congestion control (!UDP), unreliable without
      congestion control, unreliable with congestion control (!UDP),
      partially reliable with congestion control (see [RFC3758] and
      [RFC7496] for details on how avoid having an
   unnecessarily large send buffer, yet ensure that the transport sender
   always has data available when it has an opportunity to specify partial reliability)
      (!UDP).  The latter two choices are optional transmit it.
   This has been found to be very beneficial for a 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
      system protocol already missed an opportunity to offer and may result send data.  Some
   modern TCP implementations now include the unspecified
   "TCP_NOTSENT_LOWAT" socket option that was proposed in full reliability.  Note [WWDC2015],
   which limits the amount of unsent data that
      applications sending unreliable data without congestion control
      should themselves perform congestion control TCP can keep in accordance with
      [RFC2914].
   o  (!UDP) Ordered: the
   socket buffer; this boolean parameter lets an application choose
      between ordered message delivery (true) and possibly unordered,
      potentially faster message delivery (false).
   o  Bundle: a boolean that expresses a preference allows to specify at which buffer filling level
   the socket becomes writable, rather than waiting for allowing the buffer to
      bundle messages (true) or not (false).  No guarantees are given.
   o  DelAck: a boolean that, if false, lets an application request that
   run empty.

   SCTP allows to configure the peer would not delay sender-side buffer too: the acknowledgement for automatable
   Transport Feature "Configure send buffer size" provides this message.
   o  Fragment: a boolean that expresses
   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 preference transport
   system to allow for allowing uniform access to
      fragment messages (true) or not (false), at "TCP_NOTSENT_LOWAT" as well as
   the IP level.  No
      guarantees are given. "SENDER DRY" notification.

5.5.  Capacity Profile

   The transport features:

   o  (!UDP) Idempotent:  Disable Nagle algorithm
   o  Enable and configure a boolean that expresses whether "Low Extra Delay Background Transfer"
   o  Specify DSCP field

   all relate to a message is
      idempotent (true) QoS-like application need such as "low latency" or not (false).  Idempotent messages may arrive
      multiple times at
   "scavenger".  In the receiver (but interest of flexibility of a transport system,
   they will arrive at least
      once).  When data is idempotent it can could therefore be used by the receiver
      immediately on a connection establishment attempt.  Thus, if data
      is handed over before the transport system establishes offered in a
      connection with uniform, more abstract way,
   where a chosen transport protocol, stating that 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
      message is idempotent facilitates transmitting general "capacity
   profile" (a description of how it wants to use the peer
      application particularly early.

   An application can be notified available
   capacity).  A need for "lowest possible latency at the expense of a failure
   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 send a specific
   message.  There 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 no guarantee available.

5.6.  Security

   Both TCP and SCTP offer authentication.  TCP authenticates complete
   segments.  SCTP allows to configure which of such notifications, i.e. send
   failures can also silently occur.

3.3.2.  Receiving Data

   A receiving application obtains an "Application-Framed Bytestream"
   (AFra-Bytestream); SCTP's chunk types must
   always be authenticated -- if this concept is further described in
   Appendix A.3.1).  In line with TCP's receiver semantics, exposed as such, it creates an AFra-
   Bytestream is just a stream of bytes to the receiver.  If message
   boundaries were specified by
   undesirable dependency on the sender, transport protocol.  For compatibility
   with TCP, a receiver-side transport system implementing should only the minimum set of allow to configure complete
   transport services
   defined here will still not inform the receiving application about
   them (this limitation layer packets, including headers, IP pseudo-header (if any)
   and payload.

   Security is only needed for discussed in a separate document
   [I-D.ietf-taps-transport-security].  The minimal set presented in the
   present document excludes all security related transport systems that are
   implemented to directly use TCP).

   Different features
   from TCP's semantics, if the sending application has
   allowed that messages are not fully reliably transferred, or
   delivered out of order, then such re-ordering or unreliability may Appendix A: "Configure authentication", "Change authentication
   parameters", "Obtain authentication information" and and "Set Cookie
   life value" as well as "Specifying a key id to be
   reflected per message in the arriving data.  Messages will always
   stay intact - i.e. if used to
   authenticate a message".

5.7.  Packet Size

   UDP(-Lite) has a transport feature called "Specify DF field".  This
   yields an incomplete error message is contained at the end in case of the arriving data block, this sending a message that exceeds the
   Path MTU, which is guaranteed necessary for a UDP-based application to continue in
   the next arriving data block.

4.  Acknowledgements

   The authors would like be able
   to thank all 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 participants of
   configured interface" transport feature yields an upper limit for the TAPS
   Working Group
   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 NEAT categorization, reduction, and MAMI research projects for valuable
   input to discussion in Section 3,
   this document.  We especially thank Michael Tuexen for help
   with connection 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 programme under grant agreement No. 644334
   (NEAT).

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   Authentication, confidentiality protection, and integrity protection
   are identified as section describes a minimal set of transport features by [RFC8095].  As currently
   deployed in that end
   systems should offer.  Any configuration based the Internet, these features are generally provided by a
   protocol or layer on top described minimum
   set of transport feature can always be realized over TCP but also
   gives the transport protocol; no current full-
   featured standards-track system flexibility to choose another transport protocol provides all if
   implemented.  In the text of these
   transport features on its own.  Therefore, these transport features
   are not considered in this document, with the exception section, "not UDP" is used to
   indicate elements of native
   authentication capabilities the system that cannot be implemented over UDP.
   Conversely, all elements of TCP and SCTP for which the security
   considerations in [RFC5925] and [RFC4895] apply.  The minimum
   requirements for a secure transport system that are discussed not marked with "not
   UDP" can also be implemented over UDP.

   The arguments laid out in a
   separate document (Section Section 5 on Security Features and Transport
   Dependencies ("discussion") were used to make
   the final representation of [I-D.ietf-taps-transport-security]).

7.  References

7.1.  Normative References

   [I-D.ietf-taps-transport-security]
              Pauly, T., Perkins, C., Rose, K., the minimal set as short, simple and C. Wood, "A Survey
              of Transport Security Protocols", draft-ietf-taps-
              transport-security-02 (work in progress), June 2018.

   [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>.

7.2.  Informative References

   [COBS]     Cheshire, S. and M. Baker, "Consistent Overhead Byte
              Stuffing", IEEE/ACM Transactions on Networking Vol. 7, No.
              2, April 1999.

   [I-D.ietf-tsvwg-rtcweb-qos]
              Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
              Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
              qos-18 (work in progress), August 2016.

   [LBE-draft]
              Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
              Internet-draft draft-tsvwg-le-phb-03, February 2018.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.

   [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>.

   [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>.

   [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>.

   [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.  Deriving the minimal set

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

   1.  Categorization (Appendix A.1): the superset of transport features
       from [RFC8303] is presented, and transport features are
       categorized for later reduction.
   2.  Reduction (Appendix A.2): 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 (Appendix A.3): the resulting list shows a number of
       peculiarities that are discussed, to provide a basis for
       constructing the minimal set.

   4.  Construction (Section 3): Based on the reduced set and the
       discussion of the transport features therein, a minimal set is
       constructed.

A.1.  Step 1: Categorization -- The Superset of Transport Features

   Following [RFC8303], 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

   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
   general as possible.  There 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".

   Finally, in three cases, transport features are aggregated and/or
   slightly changed from [RFC8303] in the description below.  These
   transport features are marked as "ADDED".  These situations where these arguments
   do not add any new
   functionality but just represent a simple refactoring step that helps apply -- e.g., implementers may have specific reasons to streamline the derivation process (e.g., by removing a choice of
   expose multi-streaming as a
   parameter for visible functionality to applications, or
   the sake of applications that restrictive open / close semantics may not care about this
   choice).  The corresponding transport features are automatable, and
   they are listed immediately below the "ADDED" transport feature. be problematic under some
   circumstances.  In this description, transport services are presented following such cases, the
   nomenclature "CATEGORY.[SUBCATEGORY].SERVICENAME.PROTOCOL",
   equivalent to "pass 2" representation in [RFC8303].  We also sketch how functional
   or optimizing transport features can Section 4
   ("reduction") should be implemented by a transport
   system.  The "minimal set" derived considered.

   As 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", Section 3, Section 4 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 we categorize the minimal
   set of transport features as "automatable" on 1) CONNECTION related (ESTABLISHMENT,
   AVAILABILITY, MAINTENANCE, TERMINATION) and 2) DATA Transfer related
   (Sending Data, Receiving Data, Errors).  Here, the basis of
   a broader decision focus is on
   connections that affects multiple transport features:

   o  Most the transport features that are related to multi-streaming were
      designated system offers as "automatable".  This was done because an abstraction to the decision
      on whether
   application, as opposed to use multi-streaming or not does not depend on
      application-specific knowledge.  This means connections of transport protocols that a
   the transport system uses.

6.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION

   A connection that
      is exhibited to an application could must first be implemented by using a
      single stream of an SCTP association instead of mapping it "created" to a
      complete SCTP association allow for some initial
   configuration to be carried out before the transport system can
   actively or TCP connection.  This could passively establish communication with a remote end
   system.  All configuration parameters in Section 6.2 can be
      achieved by using more than one stream used
   initially, although some of them may only take effect when an SCTP association is
      first a
   connection has been 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 with a chosen transport protocol.
   Configuring a connection could then simply free
      the stream number for future use.  This is discussed further in
      Appendix A.3.2.
   o  All early helps a transport features that are related to using multiple paths or system make the choice of
   right decisions.  For example, grouping information can influence the network interface were designated
   transport system to implement a connection as
      "automatable".  Choosing a path stream of a multi-
   streaming protocol's existing association or an interface does not depend on
      application-specific knowledge. not.

   For example, "Listen" could
      always listen on all available interfaces and "Connect" could use
      the default interface for the destination IP address.

A.1.1.  CONNECTION Related Transport Features

   ESTABLISHMENT:

   o  Connect
      Protocols: TCP, SCTP, UDP(-Lite)
      Functional ungrouped connections, early configuration is necessary because
   it allows the notion of a connection is often reflected
      in applications as an expectation transport system to be able know which protocols it should try
   to communicate after
      a "Connect" succeeded, with use.  In particular, a communication sequence relating to
      this transport feature 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 defined by
   derived from Section 4.1 excluding authentication, as explained in
   Section 9):

   - Will it ever be necessary to offer any of the application
      protocol.
      Implementation: via CONNECT.TCP, CONNECT.SCTP following?
     *  Reliably transfer data
     *  Notify the peer of closing/aborting
     *  Preserve data ordering

     Yes: SCTP or CONNECT.UDP(-
      Lite).

   o  Specify which IP Options must always TCP can be used
      Protocols: TCP, UDP(-Lite)
      Automatable because IP Options relate used.
     - Is any of the following useful to knowledge about the
      network, not application?
       * Choosing a scheduler to operate between connections
         in a group, with the application.

   o possibility to configure a priority
         or weight per connection
       * Configurable message reliability
       * Unordered message delivery
       * Request multiple streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Limit to delay the number acknowledgement (SACK) of inbound streams
      Protocols: a message

       Yes: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Specify number is preferred.
       No:
       - Is any of attempts and/or the following useful to the application?
         * Hand over a message to reliably transfer (possibly
           multiple times) before connection establishment
         * Suggest timeout for to the first
      establishment peer
         * Notification of Excessive Retransmissions (early
           warning below abortion threshold)
         * Notification of ICMP error message
      Protocols: TCP, arrival

         Yes: TCP is preferred.
         No: SCTP
      Functional because and TCP are equally preferable.

     No: all protocols can be used.
     - Is any of the following useful to the application?
       *  Specify checksum coverage used by the sender
       *  Specify minimum checksum coverage required by receiver

       Yes: UDP-Lite is preferred.
       No: UDP is preferred.

   Note that this decision tree is closely related to potentially assumed
      reliable data delivery not optimal for data that all cases.  For
   example, if an application wants to use "Specify checksum coverage
   used by the sender", which is sent before only offered by UDP-Lite, and
   "Configure priority or during
      connection establishment.
      Implementation: Using weight for a parameter of CONNECT.TCP and CONNECT.SCTP.
      Implementation over UDP: Do nothing (this is irrelevant in case of
      UDP because there, reliable data delivery scheduler", which is not assumed).

   o  Obtain multiple sockets
      Protocols: SCTP
      Automatable because only offered
   by SCTP, the usage of multiple paths to communicate 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 same end host relates
   decisions for or against a protocol -- e.g.  penetration rates, the
   ability to knowledge about work through NATs, etc.  We caution implementers to be
   aware of the network, not full set of trade-offs, for which we recommend
   consulting the
      application.

   o  Disable MPTCP
      Protocols: MPTCP
      Automatable because list in Section 4.1 when deciding how to initialize a
   connection.

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

   o  Reliability: a boolean that should be set to true when any of the same end host relates
      following will be useful to knowledge about the network, not application: reliably transfer
      data; notify the
      application.
      Implementation: via a boolean parameter in CONNECT.MPTCP. peer of closing/aborting; preserve data ordering.
   o  Configure authentication
      Protocols: TCP, SCTP
      Functional because this has  Checksum coverage: a direct influence on security.
      Implementation: via parameters in CONNECT.TCP and CONNECT.SCTP.
      With TCP, this allows boolean to configure Master Key Tuples (MKTs) specify whether it will be useful
      to
      authenticate complete segments (including the TCP IPv4
      pseudoheader, TCP header, and TCP data).  With SCTP, this allows application to specify which chunk types must always be authenticated.
      Authenticating only certain chunk types creates checksum coverage when sending or
      receiving.
   o  Configure message priority: a reduced level of
      security boolean that is not supported by TCP; to be compatible, this should therefore only allow be set to authenticate all chunk types.  Key
      material must true
      when any of the following per-message configuration or
      prioritization mechanisms will be provided in useful to the application:
      choosing a way that is compatible scheduler to operate between grouped connections, with both
      [RFC4895] and [RFC5925].
      Implementation over UDP: Not possible (UDP does
      the possibility to configure a priority or weight per connection;
      configurable message reliability; unordered message delivery;
      requesting not offer this
      functionality).

   o  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point
      Protocols: SCTP
      Functional because it allows to send extra data for delay the sake acknowledgement (SACK) 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). message.
   o  Request  Early message timeout notifications: a boolean that should be set
      to negotiate interleaving true when any 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 CONNECT.SCTP.

   o  Hand 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); 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
      Protocols: TCP
      Functional because (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 is closely tied case, the receiving application must be
   prepared to properties potentially receive multiple copies of the data 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 an application sends active establishment may or expects to receive.
      Implementation: via a parameter in CONNECT.TCP.
      Implementation over UDP: not possible (UDP does may not provide
      reliability).

   o  Hand over
   trigger a message to reliably transfer during connection
      establishment
      Protocols: SCTP
      Functional because this can only work if notification on the message listening side.  It is limited in
      size, making it closely tied to properties possible that
   the first notification on the listening side is the arrival of the
   first data that an
      application the active side sends or expects (a receiver-side transport
   system could handle this by continuing to receive.
      Implementation: via block a parameter in CONNECT.SCTP.
      Implementation over TCP: not possible (TCP does not allow
      identification "Listen" call,
   immediately followed by issuing "Receive", for example; callback-
   based implementations could simply skip the equivalent of message boundaries because it provides "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 byte
      stream service)
      Implementation over UDP: not possible (UDP 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 unreliable).

   o  Enable UDP encapsulation with notified that the connection is closed.  Alternatively, a specified remote UDP port number
      Protocols: SCTP
      Automatable because UDP encapsulation relates
   connection can be aborted without delivering outstanding data to knowledge about the network, not
   peer.  In case reliable or partially reliable data delivery was
   requested earlier (not UDP), the application.

   AVAILABILITY:

   o  Listen
      Protocols: TCP, SCTP, UDP(-Lite)
      Functional because peer is notified that the notion of accepting connection requests
   is
      often reflected in applications as an expectation to aborted.  A timeout can be able configured to
      communicate after a "Listen" succeeded, with abort a communication
      sequence relating to this transport feature that is defined by connection when
   data could not be delivered for too long (not UDP); however, timeout-
   based abortion does not notify the peer application protocol.
      ADDED.  This differs from that the 3 automatable
   connection has been aborted.  Because half-closed connections are not
   supported, when a host implementing a transport features
      below in system receives a
   notification 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) peer is closing or LISTEN.SCTP (providing SCTP port
      number / address pairs for all local IP addresses).  LISTEN.UDP(-
      Lite) supports both methods.

   o  Listen, 1 specified local interface
      Protocols: TCP, SCTP, UDP(-Lite)
      Automatable because decisions about local interfaces relate to
      knowledge about the network and aborting the Operating System, connection (not
   UDP), its peer may not the
      application.

   o  Listen, N specified local interfaces
      Protocols: SCTP
      Automatable because decisions about local interfaces relate be able to
      knowledge about the network and the Operating System, not the
      application.

   o  Listen, all local interfaces
      Protocols: TCP, SCTP, UDP(-Lite)
      Automatable because decisions about local interfaces relate read outstanding data.  This means
   that unacknowledged data residing in a transport system's send buffer
   may have to
      knowledge about the network and the Operating System, not the
      application.

   o  Specify which IP Options must always be used
      Protocols: TCP, UDP(-Lite)
      Automatable because IP Options relate 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 knowledge about group connections, but it
   cannot guarantee truly grouping them using the
      network, 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 application.

   o  Disable MPTCP
      Protocols: MPTCP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates configuration to knowledge about the network, all grouped connections
   even when they are not multiplexed, or informing the
      application.

   o  Configure authentication
      Protocols: TCP, SCTP
      Functional because this has application
   about grouping success or failure).

   As a direct influence on security.
      Implementation: via parameters in LISTEN.TCP and LISTEN.SCTP.
      Implementation over TCP: With TCP, this allows to configure Master
      Key Tuples (MKTs) general rule, any configuration described below should be
   carried out as early as possible to authenticate complete segments (including aid the
      TCP IPv4 pseudoheader, TCP header, transport system's
   decision making.

6.2.1.  Connection groups

   The following transport features and TCP data).  With SCTP, this
      allows notifications (some directly
   from Section 4, some new or changed, based on the discussion in
   Section 5) automatically apply to specify which chunk types must always be authenticated.
      Authenticating only certain chunk types creates all grouped connections:

   (not UDP) Configure a reduced level of
      security that is not supported by TCP; to be compatible, timeout: this
      should therefore only allow can be done with the following
   parameters:

   o  A timeout value for aborting connections, in seconds
   o  A timeout value to authenticate all chunk types.  Key
      material must be provided suggested to the peer (if possible), in a way that is compatible with both
      [RFC4895] and [RFC5925].
      Implementation over UDP: not possible (UDP does not offer
      authentication).
      seconds
   o  Obtain requested  The number of streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Limit retransmissions after which the number application should
      be notifed of inbound streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2. "Excessive Retransmissions"

   Configure urgency: this can be done with the following parameters:

   o  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point
      Protocols: SCTP
      Functional because it allows  A number to send extra data for identify the sake type 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).

   o  Request scheduler that should be used to negotiate interleaving of user messages
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams
      operate between connections in the CONNECTION.ESTABLISHMENT
      category is automatable.
      Implementation: via a parameter group (no guarantees given).
      Schedulers are defined in LISTEN.SCTP.

   MAINTENANCE: [RFC8260].
   o  Change timeout for aborting connection (using retransmit limit or
      time value)
      Protocols: TCP, SCTP
      Functional because this is closely related  A "capacity profile" number to potentially assumed
      reliable data delivery.
      Implementation: via CHANGE_TIMEOUT.TCP or CHANGE_TIMEOUT.SCTP.
      Implementation over UDP: not identify how an application wants
      to use its available capacity.  Choices can be "lowest possible (UDP is unreliable and there
      is no
      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 timeout).

   o  Suggest timeout (e.g.  similar to table 1 in
      [I-D.ietf-tsvwg-rtcweb-qos]).
   o  A buffer limit (in bytes); when the peer
      Protocols: TCP
      Functional because this is closely related to potentially assumed
      reliable data delivery.
      Implementation: via CHANGE_TIMEOUT.TCP.
      Implementation over UDP: sender has less than the
      provided limit of bytes in the buffer, the application may be
      notified.  Notifications are not possible (UDP is unreliable guaranteed, and there it is no connection timeout).

   o  Disable Nagle algorithm
      Protocols: TCP, SCTP
      Optimizing because optional
      for a transport system to support buffer limit values greater than
      0.  Note that this decision depends on knowledge about limit and its notification should operate
      across the
      size buffers 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 whole transport system, i.e.  also any
      potential buffers that the
      Nagle algorithm). transport system itself may use on top
      of the transport's send buffer.

   Following Section 5.7, these properties can be queried:

   o  Request  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 immediate heartbeat, returning success/failure
      Protocols: SCTP
      Automatable because this informs about network-specific knowledge. error ("not available").  It
      can aid applications implementing Path MTU Discovery.
   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold)
      Protocols: TCP
      Optimizing because it  The maximum transport message size that can be sent, in bytes.
      Irrespective of fragmentation, there is an early warning to a size limit for the application,
      informing it of an impending functional event.
      Implementation: via ERROR.TCP.
      Implementation
      messages that can be handed over UDP: do nothing (there is no abortion
      threshold).

   o  Add path
      Protocols: MPTCP, SCTP
      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port to SCTP Parameters: local IP address
      Automatable or UDP(-Lite); because
      the usage service provided by a transport system is independent of multiple paths to communicate to the same end host relates
      transport protocol, it must allow an application to knowledge about query this
      value -- the network, maximum size of a message in an Application-Framed-
      Bytestream (see Section 5.1).  This may also return an error when
      data is not the
      application. delimited ("not available").
   o  Remove path
      Protocols: MPTCP, SCTP
      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port
      SCTP Parameters: local IP address
      Automatable because  The maximum transport message size that can be received from the usage
      configured interface, in bytes (or "not available").
   o  The maximum amount of multiple paths to communicate to
      the same end host relates data that can possibly be sent before or
      during connection establishment, in bytes.

   In addition to knowledge about the network, not already mentioned closing / aborting notifications
   and possible send errors, the
      application. following notifications can occur:

   o  Set primary path
      Protocols: SCTP
      Automatable because  Excessive Retransmissions: the usage configured (or a default) number of multiple paths to communicate to
      the same end host relates to knowledge about the network, not
      retransmissions has been reached, yielding this early warning
      below an abortion threshold.
   o  ICMP Arrival (parameter: ICMP message): an ICMP packet carrying
      the
      application. conveyed ICMP message has arrived.
   o  Suggest primary path to the peer
      Protocols: SCTP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates to knowledge about  ECN Arrival (parameter: ECN value): a packet carrying the network, conveyed
      ECN value has arrived.  This can be useful for applications
      implementing congestion control.
   o  Timeout (parameter: s seconds): data could not the
      application. be delivered for s
      seconds.
   o  Configure Path Switchover
      Protocols: SCTP
      Automatable because  Drain: the usage of multiple paths to communicate to send buffer has either drained below the same end host relates configured
      buffer limit or it has become completely empty.  This is a generic
      notification that tries to knowledge about enable uniform access to
      "TCP_NOTSENT_LOWAT" as well as the network, not "SENDER DRY" notification (as
      discussed in Section 5.4 -- SCTP's "SENDER DRY" is a special case
      where the
      application.

   o  Obtain status (query or notification)
      Protocols: SCTP, MPTCP
      SCTP parameters: association connection state; destination
      transport address list; destination transport address reachability
      states; current local threshold (for unsent data) is 0 and peer receiver window size; current local
      congestion window sizes; number of there is also no
      more 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 data in the
      network, not 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 application. following
   parameters, but there is no guarantee that any checksum limitations
   will indeed be enforced (the default behavior is "full coverage,
   checksum enabled"):

   o  Specify DSCP field
      Protocols: TCP, SCTP, UDP(-Lite)
      Optimizing because choosing a suitable DSCP value requires
      application-specific knowledge.
      Implementation: via SET_DSCP.TCP  A boolean to enable / SET_DSCP.SCTP disable usage of a checksum when sending
   o  The desired coverage (in bytes) of the checksum used when sending
   o  A boolean to enable / SET_DSCP.UDP(-
      Lite) disable requiring a checksum when receiving
   o  Notification  The required minimum coverage (in bytes) of ICMP error message arrival
      Protocols: TCP, UDP(-Lite)
      Optimizing because these messages can inform the checksum when
      receiving

6.3.  DATA Transfer

6.3.1.  Sending Data

   When sending a message, no guarantees are given about success or
      failure the
   preservation of functional transport features (e.g., host unreachable
      relates message boundaries to "Connect")
      Implementation: via ERROR.TCP or ERROR.UDP(-Lite).

   o  Obtain information the peer; if message boundaries
   are needed, the receiving application at the peer must know about interleaving support
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams in
   them beforehand (or the CONNECTION.ESTABLISHMENT
      category 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 automatable.
      Implementation: via STATUS.SCTP.

   o  Change authentication being handed over, the
   following 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, can be used:

   o  Reliability: this allows parameter is used to adjust key_id,
      key, convey a choice of: fully
      reliable with congestion control (not UDP), unreliable without
      congestion control, unreliable with congestion control (not UDP),
      partially reliable with congestion control (see [RFC3758] and hmac_id.  With TCP,
      [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].
   o  (not UDP) Ordered: this allows boolean parameter lets an application
      choose between ordered message delivery (true) and possibly
      unordered, potentially faster message delivery (false).
   o  Bundle: a boolean that expresses a preference for allowing to change
      bundle messages (true) or not (false).  No guarantees are given.
   o  DelAck: a boolean that, if false, lets an application request that
      the preferred
      outgoing MKT (current_key) and peer would not delay the preferred incoming MKT
      (rnext_key), respectively, acknowledgement for this message.
   o  Fragment: a segment boolean that is sent on expresses a preference for allowing to
      fragment messages (true) or not (false), at the
      connection.  Key material must be provided in IP level.  No
      guarantees are given.
   o  (not UDP) Idempotent: a way boolean that expresses whether a message
      is
      compatible with both [RFC4895] and [RFC5925].
      Implementation over UDP: not possible (UDP does idempotent (true) or not offer
      authentication).

   o  Obtain authentication information
      Protocols: SCTP
      Functional because authentication decisions (false).  Idempotent messages may have been made
      arrive multiple times at the receiver (but they will arrive at
      least once).  When data is idempotent it can be used by the peer, and this has an influence
      receiver immediately on the necessary application-
      level measures to provide a certain level of security.
      Implementation: via GET_AUTH.SCTP.
      Implementation connection establishment attempt.  Thus,
      if data is handed over TCP: With SCTP, this allows to obtain key_id
      and a chunk list.  With TCP, this allows to obtain current_key and
      rnext_key from before the transport system establishes a previously received segment.  Key material must
      be provided in
      connection with a way chosen transport protocol, stating that a
      message is compatible with both [RFC4895] and
      [RFC5925].
      Implementation over UDP: not possible (UDP does not offer
      authentication).

   o  Reset Stream
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Notification of Stream Reset
      Protocols: STCP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Reset Association
      Protocols: SCTP
      Automatable because deciding idempotent facilitates transmitting it to reset an association does not
      require application-specific knowledge.
      Implementation: via RESET_ASSOC.SCTP.

   o  Notification the peer
      application particularly early.

   An application can be notified of Association Reset
      Protocols: STCP
      Automatable because this notification does not relate a failure to
      application-specific knowledge.

   o  Add Streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Notification send a specific
   message.  There is no guarantee of Added Stream
      Protocols: STCP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Choose such notifications, i.e. send
   failures can also silently occur.

6.3.2.  Receiving Data

   A receiving application obtains an "Application-Framed Bytestream"
   (AFra-Bytestream); this concept is further described in Section 5.1).
   In line with TCP's receiver semantics, an AFra-Bytestream is just a scheduler to operate between streams
   stream of an association
      Protocols: SCTP
      Optimizing because bytes to the scheduling decision requires application-
      specific knowledge.  However, if receiver.  If message boundaries were
   specified by the sender, a receiver-side transport system would
   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
      this, 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 re-ordering 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.  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 Tuexen for help
   with connection 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 programme under grant agreement No. 644334
   (NEAT).

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Authentication, confidentiality protection, and integrity protection
   are identified as transport features by [RFC8095].  As currently
   deployed in the Internet, these features are generally provided by a
   protocol or wrongly configure it layer on its own, this would only affect
      the performance top 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 protocol; no guarantee is given that this current full-
   featured standards-track transport feature has any
      effect).

   o  Configure priority or weight for a scheduler
      Protocols: SCTP
      Optimizing because the priority or weight requires application-
      specific knowledge.  However, if a protocol provides all of these
   transport system would not use
      this, or wrongly configure it features on its own, own.  Therefore, these transport features
   are not considered in this would only affect document, with the performance exception of data transfers; the outcome would still be
      correct within native
   authentication capabilities of TCP and SCTP for which the "best effort" service model.
      Implementation: using CONFIGURE_STREAM_SCHEDULER.SCTP.
      Implementation over TCP: do nothing (streams are not available security
   considerations in
      TCP, but no guarantee is given that this [RFC5925] and [RFC4895] apply.  The minimum
   requirements for a secure transport feature has any
      effect).
      Implementation over UDP: do nothing (streams system are not available discussed in
      UDP, but no guarantee is given that this transport feature has any
      effect).

   o  Configure send buffer size
      Protocols: SCTP
      Automatable because this decision relates to knowledge about the
      network a
   separate document (Section 5 on Security Features and the Operating System, not the application (see also
      the discussion Transport
   Dependencies of [I-D.ietf-taps-transport-security]).

10.  References

10.1.  Normative References

   [I-D.ietf-taps-transport-security]
              Pauly, T., Perkins, C., Rose, K., and C. Wood, "A Survey
              of Transport Security Protocols", draft-ietf-taps-
              transport-security-02 (work in Appendix A.3.4).

   o  Configure receive buffer (and rwnd) size
      Protocols: SCTP
      Automatable because this decision relates to knowledge about the
      network progress), June 2018.

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and the Operating System, not the application.

   o  Configure message fragmentation
      Protocols: SCTP
      Automatable because fragmentation relates to knowledge about the
      network 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 Operating System, not the application.
      Implementation: Usage of
              Transport Features Provided by always enabling it with
      CONFIG_FRAGMENTATION.SCTP IETF Transport Protocols",
              RFC 8303, DOI 10.17487/RFC8303, February 2018,
              <https://www.rfc-editor.org/info/rfc8303>.

10.2.  Informative References

   [COBS]     Cheshire, S. and auto-setting the fragmentation size
      based M. Baker, "Consistent Overhead Byte
              Stuffing", IEEE/ACM Transactions on network or Networking Vol. 7, No.
              2, April 1999.

   [I-D.ietf-tsvwg-rtcweb-qos]
              Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
              Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
              qos-18 (work in progress), August 2016.

   [LBE-draft]
              Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
              Internet-draft draft-tsvwg-le-phb-03, February 2018.

   [POSIX]    "IEEE Standard for Information Technology--Portable
              Operating System conditions.

   o  Configure PMTUD
      Protocols: SCTP
      Automatable because Path MTU Discovery relates to knowledge about
      the network, not the application.

   o  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 Interface (POSIX(R)) Base Specifications,
              Issue 7", IEEE Std 1003.1-2017 (Revision of IEEE Std
              1003.1-2008), January 2018,
              <http://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 a sending application to request not to
      delay the SACK of a message, but this is a different transport
      feature).

   o  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 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 functionality
      is
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC7305]  Lear, E., Ed., "Report from the cookie in TCP 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; 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 this, [RFC7413] states that the server "can expire 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 cookie at any time to enhance security"
              Stream Control Transmission Protocol", RFC 8260,
              DOI 10.17487/RFC8260, November 2017,
              <https://www.rfc-editor.org/info/rfc8260>.

   [RFC8304]  Fairhurst, G. and section 4.1.2 describes an example implementation where
      updating T. Jones, "Transport Features of the key
              User Datagram Protocol (UDP) and Lightweight UDP (UDP-
              Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018,
              <https://www.rfc-editor.org/info/rfc8304>.

   [SCTP-stream-1]
              Weinrank, F. and M. Tuexen, "Transparent Flow Mapping for
              NEAT", IFIP NETWORKING Workshop 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 Future of SYN
      cookies (see Section 3.6 Internet
              Transport (FIT 2017), June 2017.

   [SCTP-stream-2]
              Welzl, M., Niederbacher, F., and S. Gjessing, "Beneficial
              Transparent Deployment of SCTP", IEEE GlobeCom 2011,
              December 2011.

   [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 [RFC4987]).
      Implementation over UDP: not possible (UDP does not offer Transport Features

   In this
      functionality).

   o  Set maximum burst
      Protocols: SCTP
      Automatable because it relates to knowledge about the network, not
      the application.

   o  Configure size where messages description, transport features are broken up for partial delivery
      Protocols: SCTP
      Functional because this is closely tied to properties of presented following the data
      that an application sends or expects
   nomenclature "CATEGORY.[SUBCATEGORY].FEATURENAME.PROTOCOL",
   equivalent 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).

   o  Disable checksum when sending
      Protocols: UDP
      Functional because application-specific knowledge "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 necessary meant to
      decide whether it can be acceptable
   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 lose data integrity.
      Implementation: via SET_CHECKSUM_ENABLED.UDP.
      Implementation implement them over TCP: do nothing (TCP does not offer to disable TCP and/or UDP is included.

   We designate some transport features as "automatable" on the checksum, but transmitting data with an intact checksum will
      not yield basis of
   a semantically wrong result). broader decision that affects multiple transport features:

   o  Disable checksum requirement when receiving
      Protocols: UDP
      Functional because application-specific knowledge is necessary  Most transport features that are related to
      decide multi-streaming were
      designated as "automatable".  This was done because the decision
      on whether it can be acceptable to lose data integrity.
      Implementation: via SET_CHECKSUM_REQUIRED.UDP.
      Implementation over TCP: do nothing (TCP does use multi-streaming or not offer to disable
      the checksum, but transmitting data with an intact checksum will does not yield a semantically wrong result).

   o  Specify checksum coverage used by the sender
      Protocols: UDP-Lite
      Functional because depend on
      application-specific knowledge knowledge.  This means that a connection that
      is necessary exhibited to
      decide for which parts an application could be implemented by using a
      single stream of the data an SCTP association instead of mapping it can be acceptable to lose
      data integrity.
      Implementation: via SET_CHECKSUM_COVERAGE.UDP-Lite.
      Implementation over TCP: do nothing (TCP does not offer to limit
      the checksum length, but transmitting 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 with an intact checksum
      will not yield (SEND.SCTP parameter "stream
      number").  Closing or aborting a semantically wrong result).
      Implementation over UDP: if checksum coverage connection could then simply free
      the stream number for future use.  This is set discussed further in
      Section 5.2.
   o  All transport features that are related to cover
      payload data, do nothing.  Else, either do nothing (transmitting
      data with an intact checksum will not yield using multiple paths or
      the choice of the network interface were designated as
      "automatable".  Choosing a semantically wrong
      result), path or an interface does not depend on
      application-specific knowledge.  For example, "Listen" could
      always listen on all available interfaces and "Connect" could use
      the transport feature "Disable checksum when
      sending".

   o  Specify minimum checksum coverage required by receiver
      Protocols: UDP-Lite
      Functional because application-specific knowledge is necessary to
      decide default interface for which parts of the data it can be acceptable to lose
      data integrity.
      Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite.
      Implementation over TCP: 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 RFC8303".  These do nothing (TCP does
   not offer to limit
      the checksum length, add any new functionality but transmitting data with an intact checksum
      will not yield just represent a semantically wrong result).
      Implementation over UDP: if checksum coverage is set simple refactoring
   step that helps to cover
      payload data, do nothing.  Else, either do nothing (transmitting
      data with an intact checksum will not yield streamline the derivation process (e.g., by
   removing a semantically wrong
      result), or use 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 RFC8303" transport feature "Disable checksum
      requirement when receiving". feature.

A.1.  CONNECTION Related Transport Features

   ESTABLISHMENT:

   o  Specify DF field  Connect
      Protocols: TCP, SCTP, UDP(-Lite)
      Optimizing
      Functional because the DF field can notion of a connection is often reflected
      in applications as an expectation to be used able to carry out Path MTU
      Discovery, which can lead an application communicate after
      a "Connect" succeeded, with a communication sequence relating to choose message sizes
      this transport feature that can be transmitted more efficiently. is defined by the application
      protocol.
      Implementation: via MAINTENANCE.SET_DF.UDP(-Lite) and
      SEND_FAILURE.UDP(-Lite).
      Implementation over TCP: do nothing (with CONNECT.TCP, CONNECT.SCTP or CONNECT.UDP(-
      Lite).

   o  Specify which IP Options must always be used
      Protocols: TCP, UDP(-Lite)
      Automatable because IP Options relate to knowledge about the sending
      application is
      network, not in control of transport message sizes, making
      this functionality irrelevant). the application.

   o  Get max. transport-message size that may be sent  Request multiple streams
      Protocols: SCTP
      Automatable because using a non-
      fragmented IP packet from multi-streaming does not require
      application-specific knowledge (example implementations of using
      multi-streaming without involving the configured interface application are described in
      [SCTP-stream-1] and [SCTP-stream-2]).
      Implementation: see Section 5.2.

   o  Limit the number of inbound streams
      Protocols: UDP(-Lite)
      Optimizing SCTP
      Automatable because this can lead an application to choose message
      sizes that can be transmitted more efficiently.
      Implementation over TCP: do nothing (this information is using multi-streaming does not
      available with TCP). require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Get max. transport-message size that may be received from  Specify number of attempts and/or timeout for the
      configured interface
      Protocols: UDP(-Lite)
      Optimizing first
      establishment message
      Protocols: TCP, SCTP
      Functional because this can, is closely related to potentially assumed
      reliable data delivery for example, influence an
      application's memory management. data that is sent before or during
      connection establishment.
      Implementation: Using a parameter of CONNECT.TCP and CONNECT.SCTP.
      Implementation over TCP: do UDP: Do nothing (this information is irrelevant in case of
      UDP because there, reliable data delivery is not
      available with TCP). assumed).

   o  Specify TTL/Hop count field  Obtain multiple sockets
      Protocols: UDP(-Lite) SCTP
      Automatable because a transport system can use a large enough
      system default the usage of multiple paths to avoid communication failures.  Allowing an
      application communicate to configure it differently can produce notifications
      of ICMP error message arrivals that yield information which only
      the same end host relates to knowledge about the network, not the
      application.

   o  Obtain TTL/Hop count field  Disable MPTCP
      Protocols: UDP(-Lite) MPTCP
      Automatable because the TTL/Hop count field relates usage of multiple paths to communicate to knowledge
      about the network, not the application.

   o  Specify ECN field
      Protocols: UDP(-Lite)
      Automatable because
      the ECN field same end host relates to knowledge about the network, not the
      application.
      Implementation: via a boolean parameter in CONNECT.MPTCP.

   o  Obtain ECN field  Configure authentication
      Protocols: UDP(-Lite)
      Optimizing TCP, SCTP
      Functional because this information can has a direct influence on security.
      Implementation: via parameters in CONNECT.TCP and CONNECT.SCTP.
      With TCP, this allows to configure Master Key Tuples (MKTs) to
      authenticate complete segments (including the TCP IPv4
      pseudoheader, TCP header, and TCP data).  With SCTP, this allows
      to specify which chunk types must always be used authenticated.
      Authenticating only certain chunk types creates a reduced level of
      security that is not supported by an application TCP; to better carry out congestion control (this is relevant when
      choosing be compatible, this
      should therefore only allow to authenticate all chunk types.  Key
      material must be provided in a data transmission transport service way that does not
      already do congestion control).
      Implementation over TCP: do nothing (this information is not
      available compatible with TCP).

   o  Specify IP Options
      Protocols: UDP(-Lite)
      Automatable because IP Options relate to knowledge about the
      network, both
      [RFC4895] and [RFC5925].

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

   o  Obtain IP Options  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point
      Protocols: UDP(-Lite)
      Automatable SCTP
      Functional because IP Options relate it allows to knowledge about the
      network, not the application.

   o  Enable and configure a "Low Extra Delay Background Transfer"
      Protocols: A protocol implementing the LEDBAT congestion control
      mechanism
      Optimizing because whether this service is appropriate or not
      depends on application-specific knowledge.  However, wrongly using
      this will only affect the speed of send extra data transfers (albeit
      including other transfers that may compete with the transport
      system's transfer in for the network), so it sake of
      identifying an adaptation layer, which by itself is still correct within
      the "best effort" service model. application-
      specific.
      Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP /
      SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [LBE-draft]. a parameter in CONNECT.SCTP.
      Implementation over TCP: do nothing not possible (TCP does not support LEDBAT
      congestion control, but not implementing offer this functionality will
      not yield a semantically wrong behavior).
      functionality).
      Implementation over UDP: do nothing not possible (UDP does not offer congestion
      control).

   TERMINATION: this
      functionality).

   o  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side  Request to negotiate interleaving of user messages
      Protocols: TCP, SCTP
      Functional
      Automatable because it requires using multiple streams, but
      requesting multiple streams in the notion of a connection CONNECTION.ESTABLISHMENT
      category is often reflected automatable.
      Implementation: controlled via a parameter in applications as an expectation CONNECT.SCTP.  One
      possible implementation is to have all outstanding data
      delivered and no longer be able always try to communicate after a "Close"
      succeeded, with enable interleaving.

   o  Hand over a communication sequence relating message to reliably transfer (possibly multiple times)
      before connection establishment
      Protocols: TCP
      Functional because this
      transport feature that is defined by closely tied to properties of the data
      that an application protocol. sends or expects to receive.
      Implementation: via CLOSE.TCP and CLOSE.SCTP. a parameter in CONNECT.TCP.
      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). provide
      reliability).

   o  Abort without delivering remaining data, causing an event
      informing the application on the other side  Hand over a message to reliably transfer during connection
      establishment
      Protocols: TCP, SCTP
      Functional because this can only work if the notion of a connection message is often reflected limited in applications as an expectation to potentially not have all
      outstanding data delivered and no longer be able
      size, making it closely tied to communicate
      after an "Abort" succeeded.  On both sides properties of a connection, the data that an
      application protocol may define a communication sequence relating sends or expects to this transport feature. receive.
      Implementation: via ABORT.TCP and ABORT.SCTP. a parameter in CONNECT.SCTP.
      Implementation over UDP: TCP: not possible (UDP (TCP does not offer to cause
      an event related to aborting at allow
      identification of message boundaries because it provides a byte
      stream service)
      Implementation over UDP: not possible (UDP is unreliable).

   o  Enable UDP encapsulation with a specified remote UDP port number
      Protocols: SCTP
      Automatable because UDP encapsulation relates to knowledge about
      the peer).

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

   AVAILABILITY:

   o  Listen
      Protocols: TCP, SCTP, UDP(-Lite)
      Functional because the notion of a accepting connection requests 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 "Listen" succeeded, with a communication
      sequence relating to this transport feature.
      Implementation: via ABORT.UDP(-Lite).
      Implementation over TCP: stop using feature that is defined by the connection, wait
      application protocol.
      CHANGED FROM RFC8303.  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
      timeout.

   o  Timeout event when data could not be delivered local IP address) or LISTEN.SCTP (providing SCTP port
      number / address pairs for too long all local IP addresses).  LISTEN.UDP(-
      Lite) supports both methods.

   o  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.

   o  Listen, N specified local interfaces
      Protocols: SCTP
      Functional
      Automatable because this notifies that potentially assumed reliable
      data delivery is no longer provided.
      Implementation: via TIMEOUT.TCP decisions about local interfaces relate to
      knowledge about the network and TIMEOUT.SCTP.
      Implementation over UDP: do nothing (this event will the Operating System, not occur
      with UDP).

A.1.2.  DATA Transfer Related Transport Features

A.1.2.1.  Sending Data the
      application.

   o  Reliably transfer data, with congestion control  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.

   o  Specify which IP Options must always be used
      Protocols: TCP, SCTP
      Functional UDP(-Lite)
      Automatable because this is closely tied IP Options relate to properties knowledge about the
      network, not the application.

   o  Disable MPTCP
      Protocols: MPTCP
      Automatable because the usage of multiple paths to communicate to
      the data
      that an application sends or expects same end host relates to receive.
      Implementation: via SEND.TCP and SEND.SCTP.
      Implementation over UDP: knowledge about the network, not possible (UDP is unreliable). the
      application.

   o  Reliably transfer a message, with congestion control  Configure authentication
      Protocols: TCP, SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive. has a direct influence on security.
      Implementation: via SEND.SCTP. parameters in LISTEN.TCP and LISTEN.SCTP.
      Implementation over TCP: via SEND.TCP. With SEND.TCP, message
      boundaries will not be identifiable by TCP, this allows to configure Master
      Key Tuples (MKTs) to authenticate complete segments (including the receiver, because
      TCP
      provides a byte stream service.
      Implementation over UDP: not possible (UDP is unreliable).

   o  Unreliably transfer a message
      Protocols: IPv4 pseudoheader, TCP header, and TCP data).  With SCTP, UDP(-Lite)
      Optimizing because this
      allows to specify which chunk types must always be authenticated.
      Authenticating only applications know about the time
      criticality of their communication, and reliably transfering certain chunk types creates a
      message is never incorrect for the receiver reduced level of a potentially
      unreliable data transfer, it is just slower.
      ADDED.  This differs from the 2 automatable transport features
      below in
      security 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 is not supported by TCP; to be compatible, this
      should therefore only allow to authenticate all chunk types.  Key
      material must be sent reliably, provided in a way that is compatible with both
      [RFC4895] and message boundaries will [RFC5925].
      Implementation over UDP: not be
      identifiable by the receiver. possible (UDP does not offer
      authentication).

   o  Unreliably transfer a message, with congestion control  Obtain requested number of streams
      Protocols: SCTP
      Automatable because congestion control relates to knowledge about
      the network, using multi-streaming does not the application. require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Unreliably transfer a message, without congestion control  Limit the number of inbound streams
      Protocols: UDP(-Lite) SCTP
      Automatable because congestion control relates to knowledge about
      the network, using multi-streaming does not the application. require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Configurable Message Reliability  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point
      Protocols: SCTP
      Optimizing
      Functional because only applications know about the time
      criticality of their communication, and reliably transfering a
      message is never incorrect it allows to send extra data for the receiver sake of a potentially
      unreliable data transfer, it
      identifying an adaptation layer, which by itself is just slower. application-
      specific.
      Implementation: via SEND.SCTP. a parameter in LISTEN.SCTP.
      Implementation over TCP: 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. (TCP does not offer this
      functionality).
      Implementation over UDP: not possible (UDP is unreliable).

   o  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 Appendix A.3.2. does not offer this
      functionality).

   o  Choice  Request to negotiate interleaving of path (destination address) user messages
      Protocols: SCTP
      Automatable because it requires using multiple sockets, streams, but
      obtaining
      requesting multiple sockets streams in the CONNECTION.ESTABLISHMENT
      category is automatable.
      Implementation: via a parameter in LISTEN.SCTP.

   MAINTENANCE:

   o  Ordered message delivery (potentially slower than unordered)  Change timeout for aborting connection (using retransmit limit or
      time value)
      Protocols: TCP, SCTP
      Functional because this is closely tied related to properties of the potentially assumed
      reliable data
      that an application sends or expects to receive. delivery.
      Implementation: via SEND.SCTP.
      Implementation over TCP: By using SEND.TCP.  With SEND.TCP,
      messages will not be identifiable by the receiver. CHANGE_TIMEOUT.TCP or CHANGE_TIMEOUT.SCTP.
      Implementation over UDP: not possible (UDP does not offer any
      guarantees regarding ordering). is unreliable and there
      is no connection timeout).

   o  Unordered message delivery (potentially faster than ordered)  Suggest timeout to the peer
      Protocols: SCTP, UDP(-Lite) TCP
      Functional because this is closely tied related to properties of the potentially assumed
      reliable data
      that an application sends or expects to receive. delivery.
      Implementation: via SEND.SCTP. CHANGE_TIMEOUT.TCP.
      Implementation over TCP: 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 UDP: not possible to
      associate the requested delivery order to a "message" in TCP
      anyway. (UDP is unreliable and there
      is no connection timeout).

   o  Request not to bundle messages  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 SEND.SCTP. DISABLE_NAGLE.TCP and DISABLE_NAGLE.SCTP.
      Implementation over TCP: By using SEND.TCP and DISABLE_NAGLE.TCP
      to disable UDP: do nothing (UDP does not implement the
      Nagle algorithm when the request is made and enable
      it again when the request is no longer made.  Note that algorithm).

   o  Request an immediate heartbeat, returning success/failure
      Protocols: SCTP
      Automatable because this is
      not fully equivalent informs about network-specific knowledge.

   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold)
      Protocols: TCP
      Optimizing because it relates is an early warning to the time application,
      informing it of issuing the
      request rather than a specific message. an impending functional event.
      Implementation: via ERROR.TCP.
      Implementation over UDP: do nothing (UDP never bundles messages). (there is no abortion
      threshold).

   o  Specifying a "payload protocol-id" (handed over as such by the
      receiver)  Add path
      Protocols: MPTCP, SCTP
      Functional
      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port
      SCTP Parameters: local IP address
      Automatable because it allows to send extra application data with
      every message, for the sake of identification usage 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).

   o  Specifying a key id multiple paths to be used communicate to authenticate a message
      the same end host relates to knowledge about the network, not the
      application.

   o  Remove path
      Protocols: MPTCP, SCTP
      Functional
      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port
      SCTP Parameters: local IP address
      Automatable 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 usage of multiple paths to communicate to
      the same end host relates to knowledge about the time network, not the
      application.

   o  Set primary path
      Protocols: SCTP
      Automatable because the usage of issuing multiple paths to communicate to
      the request rather than a specific message.
      Implementation over UDP: not possible (UDP does same end host relates to knowledge about the network, not offer
      authentication). the
      application.

   o  Request not  Suggest primary path to delay the acknowledgement (SACK) peer
      Protocols: SCTP
      Automatable because the usage of a message multiple paths to communicate to
      the same end host relates to knowledge about the network, not the
      application.

   o  Configure Path Switchover
      Protocols: SCTP
      Optimizing
      Automatable because only an application knows for which message it
      wants the usage of multiple paths to quickly be informed communicate to
      the same end host relates to knowledge 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 network, not offer this
      functionality, but ignoring this request from the application will
      not yield a semantically wrong behavior).

A.1.2.2.  Receiving Data
      application.

   o  Receive data (with no message delimiting)  Obtain status (query or notification)
      Protocols: TCP
      Functional because a SCTP, MPTCP
      SCTP parameters: association connection state; destination
      transport system must be able to send address list; destination transport address reachability
      states; current local and
      receive data.
      Implementation: via RECEIVE.TCP.
      Implementation over UDP: do nothing (UDP only works 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 messages; 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 can be handed over, parameters relate to knowledge about the application can still ignore
      network, not the
      message boundaries). application.

   o  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)

   o  Receive a  Notification of ICMP error message arrival
      Protocols: SCTP, TCP, UDP(-Lite)
      Functional
      Optimizing because this is closely tied to properties of the data
      that an application sends these messages can inform about success or expects
      failure of functional transport features (e.g., host unreachable
      relates to receive. "Connect")
      Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite).
      Implementation over TCP: not possible (TCP does not support
      identification of message boundaries). ERROR.TCP or ERROR.UDP(-Lite).

   o  Choice of stream to receive from  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: see Appendix A.3.2. via STATUS.SCTP.

   o  Information about partial message arrival  Change authentication parameters
      Protocols: TCP, SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive. has a direct influence on security.
      Implementation: via RECEIVE.SCTP. SET_AUTH.TCP and SET_AUTH.SCTP.
      Implementation over TCP: do nothing (this information With SCTP, this allows to adjust key_id,
      key, and hmac_id.  With TCP, this allows to change the preferred
      outgoing MKT (current_key) and the preferred incoming MKT
      (rnext_key), respectively, for a segment that is not
      available sent on the
      connection.  Key material must be provided in a way that is
      compatible with TCP). both [RFC4895] and [RFC5925].
      Implementation over UDP: do nothing (this information is not
      available with UDP).

A.1.2.3.  Errors

   This section describes sending failures that are associated with a
   specific call to in the "Sending Data" category (Appendix A.1.2.1). possible (UDP does not offer
      authentication).

   o  Notification of send failures  Obtain authentication information
      Protocols: SCTP, UDP(-Lite) SCTP
      Functional because this notifies that potentially assumed reliable
      data delivery is no longer provided.
      ADDED.  This differs from the 2 automatable transport features
      below in that it does not distinugish between unsent authentication decisions may have been made by
      the peer, and
      unacknowledged messages. this has an influence on the necessary application-
      level measures to provide a certain level of security.
      Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(-
      Lite). GET_AUTH.SCTP.
      Implementation over TCP: do nothing (this notification With SCTP, this allows to obtain key_id
      and a chunk list.  With TCP, this allows to obtain current_key and
      rnext_key from a previously received segment.  Key material must
      be provided in a way that is not
      available compatible with both [RFC4895] and will therefore
      [RFC5925].
      Implementation over UDP: not occur with TCP). possible (UDP does not offer
      authentication).

   o  Notification of an unsent (part of a) message  Reset Stream
      Protocols: SCTP, UDP(-Lite) SCTP
      Automatable because the distinction between unsent and
      unacknowledged is network-specific. using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Notification of an unacknowledged (part of a) message Stream Reset
      Protocols: SCTP STCP
      Automatable because the distinction between unsent and
      unacknowledged is network-specific. using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Notification that the stack has no more user data to send  Reset Association
      Protocols: SCTP
      Optimizing
      Automatable because reacting deciding to reset an association does not
      require application-specific knowledge.
      Implementation: via RESET_ASSOC.SCTP.

   o  Notification of Association Reset
      Protocols: STCP
      Automatable because this notification requires the
      application does not relate to be involved, and ensuring that the stack
      application-specific knowledge.

   o  Add Streams
      Protocols: SCTP
      Automatable because using multi-streaming does not
      run dry require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Notification of data (for too long) can improve performance.
      Implementation over TCP: do nothing (see the discussion in
      Appendix A.3.4).
      Implementation over UDP: do nothing (this notification is not
      available and will therefore Added Stream
      Protocols: STCP
      Automatable because using multi-streaming does not occur with UDP). require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Notification to a receiver that  Choose a partial message delivery has
      been aborted scheduler to operate between streams of an association
      Protocols: SCTP
      Functional
      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 is closely tied to properties of would only affect
      the performance of data
      that an application sends or expects to receive. transfers; the outcome would still be
      correct within the "best effort" service model.
      Implementation: using SET_STREAM_SCHEDULER.SCTP.
      Implementation over TCP: do nothing (this notification is (streams are not available and will therefore not occur with TCP). in
      TCP, but no guarantee is given that this transport feature has any
      effect).
      Implementation over UDP: do nothing (this notification is (streams are not available and will therefore not occur with UDP).

A.2.  Step 2: Reduction -- 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 in
      UDP, but no guarantee is given that may not be possible this transport feature has any
      effect).

   o  Configure priority or weight 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 scheduler
      Protocols: SCTP
      Optimizing because the transport features that are
   either optimizing priority or functional.

   A weight requires application-
      specific knowledge.  However, if a transport system should be able to communicate via TCP would not use
      this, or UDP if
   alternative transport protocols 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 found not to work.  For many
   transport features, this available in
      TCP, but no guarantee is possible -- often by simply given that this transport feature has any
      effect).
      Implementation over UDP: do nothing (streams are not doing
   anything when a specific request available in
      UDP, but no guarantee is made.  For some transport
   features, however, it was identified given 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 this transport features, feature has any
      effect).

   o  Configure send buffer size
      Protocols: SCTP
      Automatable because this would eliminate the
   possibility decision relates to use TCP or UDP.  Thus, we only keep knowledge about the functional
      network and
   optimizing transport features for which an implementation over either
   TCP or UDP is possible in our reduced set.

   The "minimal set" derived the Operating System, not the application (see also
      the discussion in Section 5.4).

   o  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.

   o  Configure message fragmentation
      Protocols: SCTP
      Automatable because this document is meant relates to be
   implementable "one-sided" over TCP, and, with limitations, UDP.  In knowledge about the following list, we therefore precede a transport 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: by always enabling it with
   "T:" if an implementation over TCP is possible, "U:" if an
   implementation over UDP is possible,
      CONFIG_FRAGMENTATION.SCTP and "TU:" if an implementation
   over either TCP auto-setting the fragmentation size
      based on network or UDP is possible.

A.2.1.  CONNECTION Related Transport Features

   ESTABLISHMENT:

   o  T,U: Connect Operating System conditions.

   o  T,U: Specify number of attempts and/or timeout for  Configure PMTUD
      Protocols: SCTP
      Automatable because Path MTU Discovery relates to knowledge about
      the first
      establishment message network, not the application.

   o  T:  Configure authentication
   o  T: Hand over a message delayed SACK timer
      Protocols: SCTP
      Automatable because the receiver-side decision to reliably transfer (possibly multiple
      times) before connection establishment
   o  T: Hand over a message delay sending
      SACKs relates to reliably transfer during connection
      establishment

   AVAILABILITY:

   o  T,U: Listen
   o  T: Configure authentication

   MAINTENANCE:

   o  T: Change timeout knowledge about the network, not the application
      (it can be relevant for aborting connection (using retransmit limit
      or time value)
   o  T: Suggest timeout a sending application to request not to
      delay the peer
   o  T,U: Disable Nagle algorithm
   o  T,U: Notification of Excessive Retransmissions (early warning
      below abortion threshold)
   o  T,U: Specify DSCP field
   o  T,U: Notification SACK of ICMP error message arrival
   o  T: Change authentication parameters
   o  T: Obtain authentication information a message, but this is a different transport
      feature).

   o  T,U:  Set Cookie life value
   o  T,U: Choose a scheduler
      Protocols: SCTP
      Functional because it relates to operate between streams of an
      association
   o  T,U: Configure priority or weight for a scheduler
   o  T,U: Disable checksum when sending
   o  T,U: Disable checksum requirement when receiving
   o  T,U: Specify checksum coverage used security (possibly weakened by
      keeping a cookie very long) versus the sender
   o  T,U: Specify minimum checksum coverage required by receiver
   o  T,U: Specify DF field
   o  T,U: Get max. transport-message size that may time between connection
      establishment attempts.  Knowledge about both issues can be sent using a non-
      fragmented IP packet from
      application-specific.
      Implementation over TCP: the configured interface
   o  T,U: Get max. transport-message size closest specified TCP functionality
      is the cookie in TCP Fast Open; for this, [RFC7413] states that may be received from
      the
      configured interface
   o  T,U: Obtain ECN field
   o  T,U: Enable server "can expire the cookie at any time to enhance security"
      and configure a "Low Extra Delay Background Transfer"

   TERMINATION:

   o  T: Close after reliably delivering all remaining data, causing section 4.1.2 describes an
      event informing example implementation where
      updating the application key on the other server side
   o  T: Abort without delivering remaining data, causing an event
      informing causes the application on cookie to expire.
      Alternatively, for implementations that do not support TCP Fast
      Open, this transport feature could also affect the other side
   o  T,U: Abort without delivering remaining data, validity of SYN
      cookies (see Section 3.6 of [RFC4987]).
      Implementation over UDP: not causing an event
      informing possible (UDP does not offer this
      functionality).

   o  Set maximum burst
      Protocols: SCTP
      Automatable because it relates to knowledge about the application on network, not
      the other side application.

   o  T,U: Timeout event when data could not be delivered  Configure size where messages are broken up for too long

A.2.2.  DATA Transfer Related Transport Features

A.2.2.1.  Sending Data

   o  T: Reliably transfer data, with congestion control
   o  T: Reliably transfer a message, with congestion control
   o  T,U: Unreliably transfer a message
   o  T: Configurable Message Reliability
   o  T: Ordered message partial delivery (potentially slower than unordered)
   o  T,U: Unordered
      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 delivery (potentially faster than ordered)
   o  T,U: Request boundaries).
      Implementation over UDP: not to bundle messages possible (UDP does not fragment
      messages).

   o  T: Specifying a key id  Disable checksum when sending
      Protocols: UDP
      Functional because application-specific knowledge is necessary to
      decide whether it can be used acceptable to authenticate a message
   o  T,U: Request lose data integrity with
      respect to random corruption.
      Implementation: via SET_CHECKSUM_ENABLED.UDP.
      Implementation over TCP: do nothing (TCP does not offer to delay disable
      the acknowledgement (SACK) of a message

A.2.2.2.  Receiving Data

   o  T,U: Receive checksum, but transmitting data (with no message delimiting)
   o  U: Receive a message
   o  T,U: Information about partial message arrival

A.2.2.3.  Errors

   This section describes sending failures that are associated with an intact checksum will
      not yield a
   specific call to in the "Sending Data" category (Appendix A.1.2.1).

   o  T,U: Notification of send failures semantically wrong result).

   o  T,U: Notification that the stack has no more user  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 send
   o  T,U: Notification random corruption.
      Implementation: via SET_CHECKSUM_REQUIRED.UDP.
      Implementation over TCP: do nothing (TCP does not offer to a receiver that a partial message delivery
      has been aborted

A.3.  Step 3: Discussion

   The reduced set in the previous section exhibits a number of
   peculiarities, which we will discuss in disable
      the following.  This section
   focuses on TCP because, checksum, but transmitting data with the exception of one particular
   transport feature ("Receive a message" -- we an intact checksum will discuss this in
   Appendix A.3.1),
      not yield a semantically wrong result).

   o  Specify checksum coverage used by the list shows that UDP sender
      Protocols: UDP-Lite
      Functional because application-specific knowledge is strictly a subset necessary to
      decide for which parts of TCP.
   We the data it can first try be acceptable to understand how lose
      data integrity with respect to build a transport system that
   can run random corruption.
      Implementation: via SET_CHECKSUM_COVERAGE.UDP-Lite.
      Implementation over TCP, and then narrow down the result further TCP: do nothing (TCP does not offer to allow
   that limit
      the system can always run checksum length, but transmitting data with an intact checksum
      will not yield a semantically wrong result).
      Implementation over either TCP or UDP (which
   effectively means removing everything related UDP: if checksum coverage is set to reliability,
   ordering, authentication and closing/aborting cover
      payload data, do nothing.  Else, either do nothing (transmitting
      data with an intact checksum will not yield a notification to semantically wrong
      result), or use the peer).

   Note that, transport feature "Disable checksum when
      sending".

   o  Specify minimum checksum coverage required by receiver
      Protocols: UDP-Lite
      Functional because the functional transport features application-specific knowledge is necessary to
      decide for which parts of UDP are --
   with the exception of "Receive a message" -- a subset of TCP, TCP data it 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 acceptable to communicate lose
      data integrity with UDP at
   first, and falling back respect to TCP in case of a connection failure.

A.3.1.  Sending Messages, Receiving Bytes

   For implementing a transport system random corruption.
      Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite.
      Implementation over TCP, there are several
   transport features related TCP: do nothing (TCP does not offer to sending, limit
      the checksum length, but only transmitting data with an intact checksum
      will not yield a single transport
   feature related semantically wrong result).
      Implementation over UDP: if checksum coverage is set to receiving: "Receive cover
      payload data, do nothing.  Else, either do nothing (transmitting
      data (with no message
   delimiting)" (and, strangely, "information about partial message
   arrival").  Notably, the transport feature "Receive with an intact checksum will not yield a message" is
   also semantically wrong
      result), or use the only non-automatable transport feature of "Disable checksum
      requirement when receiving".

   o  Specify DF field
      Protocols: UDP(-Lite) for
   which no implementation over TCP is possible.

   To support these TCP receiver semantics, we define an "Application-
   Framed Bytestream" (AFra-Bytestream).  AFra-Bytestreams allow senders
   to operate on messages while minimizing changes to the TCP socket
   API.  In particular, nothing changes on
      Optimizing because the receiver side - data DF field can be used to carry out Path MTU
      Discovery, which can lead an application to choose message sizes
      that can be accepted transmitted more efficiently.
      Implementation: via a normal TCP socket.

   In an AFra-Bytestream, MAINTENANCE.SET_DF.UDP(-Lite) and
      SEND_FAILURE.UDP(-Lite).
      Implementation over TCP: do nothing (with TCP, the sending
      application can optionally inform
   the transport about message boundaries and required properties per
   message (configurable order and reliability, or embedding a request is not to delay the acknowledgement in control of transport message sizes, making
      this functionality irrelevant).

   o  Get max. transport-message size that may be sent using a message).  Whenever non-
      fragmented IP packet from the sending configured interface
      Protocols: UDP(-Lite)
      Optimizing because this can lead an application specifies per-message properties to choose message
      sizes that relax the notion of
   reliable in-order delivery of bytes, it must assume can be transmitted more efficiently.
      Implementation over TCP: do nothing (this information is not
      available with TCP).

   o  Get max. transport-message size that may be received from the
   receiving application
      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 1) able not
      available with TCP).

   o  Specify TTL/Hop count field
      Protocols: UDP(-Lite)
      Automatable because a transport system can use a large enough
      system default to determine message boundaries,
   provided that messages are always kept intact, and 2) able avoid communication failures.  Allowing an
      application to accept
   these relaxed per-message properties.  Any signaling configure it differently can produce notifications
      of such ICMP error message arrivals that yield information which only
      relates to knowledge about the network, not the application.

   o  Obtain TTL/Hop count field
      Protocols: UDP(-Lite)
      Automatable because the TTL/Hop count field relates to knowledge
      about the peer is up network, not the application.

   o  Specify ECN field
      Protocols: UDP(-Lite)
      Automatable because the ECN field relates to an application-layer protocol and
   considered out of scope of knowledge about the
      network, not the application.

   o  Obtain ECN field
      Protocols: UDP(-Lite)
      Optimizing because this document.

   For example, if information can be used by an application requests
      to transfer fixed-size
   messages of 100 bytes 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 partial reliability, this needs the
   receiving application TCP).

   o  Specify IP Options
      Protocols: UDP(-Lite)
      Automatable because IP Options relate to be prepared knowledge about the
      network, not the application.

   o  Obtain IP Options
      Protocols: UDP(-Lite)
      Automatable because IP Options relate to accept data in chunks of 100
   bytes.  If, then, some of these 100-byte messages are missing (e.g.,
   if SCTP with Configurable Reliability is used), this is knowledge about the expected
   application behavior.  With TCP, no messages would be missing, but
   this is also correct for
      network, not the application, application.

   o  Enable and configure a "Low Extra Delay Background Transfer"
      Protocols: A protocol implementing the possible
   retransmission delay LEDBAT congestion control
      mechanism
      Optimizing because whether this feature is acceptable within appropriate or not
      depends on application-specific knowledge.  However, wrongly using
      this will only affect the best effort service
   model (see [RFC7305], Section 3.5).  Still, speed of data transfers (albeit
      including other transfers that may compete with the receiving application
   would separate transport
      system's transfer in the network), so it is still correct within
      the byte stream into 100-byte chunks.

   Note that "best effort" service model.
      Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP /
      SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [LBE-draft].
      Implementation over TCP: do nothing (TCP does not support LEDBAT
      congestion control, but not implementing this usage of messages functionality will
      not yield a semantically wrong behavior).
      Implementation over UDP: do nothing (UDP does not require offer congestion
      control).

   TERMINATION:

   o  Close after reliably delivering all messages to be
   equal in size.  Many remaining data, causing an
      event informing the application protocols use some form on the other side
      Protocols: TCP, SCTP
      Functional because the notion of Type-
   Length-Value (TLV) encoding, e.g. by defining a header including
   length fields; another alternative connection is the use of byte stuffing
   methods such often reflected
      in applications as COBS [COBS].  If an application needs message
   numbers, e.g. expectation to restore the correct sequence of messages, these must
   also have all outstanding data
      delivered and no longer be encoded by the application itself, as the able to communicate after a "Close"
      succeeded, with a communication sequence number
   related relating to this
      transport features of SCTP are not provided feature that is defined by the "minimum
   set" (in the interest of enabling usage of TCP).

A.3.2.  Stream Schedulers Without Streams

   We have already stated that multi-streaming application protocol.
      Implementation: via CLOSE.TCP and CLOSE.SCTP.
      Implementation over UDP: not possible (UDP is unreliable and hence
      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 know when all remaining data is delivered; it does also
      not offer to cause an event related to
   knowledge about closing at the network and peer).

   o  Abort without delivering remaining data, causing an event
      informing the particular transport protocol in
   use, not application on the application.  However, other side
      Protocols: TCP, SCTP
      Functional because the transport features "Choose notion of a
   scheduler connection is often reflected
      in applications as an expectation to operate between streams 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 association" and
   "Configure priority or weight for
      application protocol may define a scheduler" operate on streams.
   Here, streams identify communication channels between which a
   scheduler operates, sequence relating
      to this transport feature.
      Implementation: via ABORT.TCP and they can be assigned a priority.  Moreover, ABORT.SCTP.

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

   o  Abort without delivering remaining data, not causing an event
      informing the MAINTENANCE category all operate application on
   assocations in case of SCTP, i.e. they apply to all streams in that
   assocation.

   With only these semantics necessary to represent, the interface to a
   transport system becomes easier if we assume that connections may be other side
      Protocols: UDP(-Lite)
      Functional because the notion of a transport protocol's connection or association, but could also be a
   stream of is often reflected
      in applications as an existing SCTP association, for example.  We only need expectation to
   allow for a way 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 possible grouping of connections.  Then,
   all MAINTENANCE transport features can be said communication sequence relating
      to operate on
   connection groups, not connections, and a scheduler operates on this transport feature.
      Implementation: via ABORT.UDP(-Lite).
      Implementation over TCP: stop using the
   connections within connection, wait for a group.

   To
      timeout.

   o  Timeout event when data could not be compatible with multiple transport protocols 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 uniformly
   allow access 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

   o  Reliably transfer data, with congestion control
      Protocols: TCP, SCTP
      Functional because this is closely tied to both transport connections and streams properties of a multi-
   streaming protocol, the semantics of opening data
      that an application sends or expects to receive.
      Implementation: via SEND.TCP and closing need SEND.SCTP.
      Implementation over UDP: not possible (UDP is unreliable).

   o  Reliably transfer a message, with congestion control
      Protocols: SCTP
      Functional because this is closely tied to be
   the most restrictive subset of all properties of the underlying options.  For
   example, TCP's support of half-closed connections can 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 seen as a
   feature on top of identifiable by the more restrictive "ABORT"; this feature cannot
   be supported receiver, because TCP
      provides a byte stream service.
      Implementation over UDP: not possible (UDP is unreliable).

   o  Unreliably transfer a message
      Protocols: SCTP, UDP(-Lite)
      Optimizing because not all protocols used by only applications know about the time
      criticality of their communication, and reliably transfering a transport system
   (including streams
      message is never incorrect for the receiver of an association) support half-closed
   connections.

A.3.3.  Early Data Transmission

   There are two a potentially
      unreliable data transfer, it is just slower.
      CHANGED FROM RFC8303.  This differs from the 2 automatable
      transport features related to transferring a message
   early: "Hand below in that it leaves the choice of
      congestion control open.
      Implementation: via SEND.SCTP or SEND.UDP(-Lite).
      Implementation over a TCP: use SEND.TCP.  With SEND.TCP, messages
      will be sent reliably, and message to reliably boundaries will not be
      identifiable by the receiver.

   o  Unreliably transfer (possibly multiple
   times) before connection establishment", which relates to TCP Fast
   Open [RFC7413], and "Hand over a message message, with congestion control
      Protocols: SCTP
      Automatable because congestion control relates to reliably knowledge about
      the network, not the application.

   o  Unreliably transfer during
   connection establishment", which a message, without congestion control
      Protocols: UDP(-Lite)
      Automatable because congestion control relates to SCTP's ability to
   transfer data together with knowledge about
      the COOKIE-Echo chunk.  Also without TCP
   Fast Open, TCP can transfer data during network, not the handshake, together with application.

   o  Configurable Message Reliability
      Protocols: SCTP
      Optimizing because only applications know about the SYN packet -- however, time
      criticality of their communication, and reliably transfering a
      message is never incorrect for the receiver of this a potentially
      unreliable data may not hand transfer, it is just slower.
      Implementation: via SEND.SCTP.
      Implementation over to TCP: By using SEND.TCP and ignoring this
      configuration: based on the application until assumption of the handshake has completed.  Also,
   different from TCP Fast Open, this best-effort service
      model, unnecessarily delivering data does not violate application
      expectations.  Moreover, it is not delimited as possible to associate the
      requested reliability to a message
   by "message" in TCP (thus, anyway.
      Implementation over UDP: not visible as a ``message'').  This functionality possible (UDP is
   commonly available in TCP and supported unreliable).

   o  Choice of stream
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams in several implementations,
   even though the TCP specification does not explain how to provide CONNECTION.ESTABLISHMENT
      category is automatable.  Implementation: see Section 5.2.

   o  Choice of path (destination address)
      Protocols: SCTP
      Automatable because it
   to applications.

   A transport system could differentiate between requires using multiple sockets, but
      obtaining multiple sockets in the cases CONNECTION.ESTABLISHMENT
      category is automatable.

   o  Ordered message delivery (potentially slower than unordered)
      Protocols: SCTP
      Functional because this is closely tied to properties of
   transmitting data "before" (possibly multiple times) or "during" the
   handshake.  Alternatively, it could also assume that data
      that are
   handed an application sends or expects to receive.
      Implementation: via SEND.SCTP.
      Implementation over early TCP: By using SEND.TCP.  With SEND.TCP,
      messages will not be transmitted as early as possible, and
   "before" identifiable by the handshake would only be used for messages that are
   explicitly marked as "idempotent" (i.e., it would be acceptable receiver.
      Implementation over UDP: not possible (UDP does not offer any
      guarantees regarding ordering).

   o  Unordered message delivery (potentially faster than ordered)
      Protocols: SCTP, UDP(-Lite)
      Functional because this is closely tied to
   transfer them multiple times).

   The amount properties of the data
      that can successfully be transmitted before an application sends or
   during the handshake depends expects to receive.
      Implementation: via SEND.SCTP.
      Implementation over TCP: By using SEND.TCP and always sending data
      ordered: based on various factors: the transport
   protocol, the use assumption of header options, the choice of IPv4 and IPv6 best-effort service model,
      ordered delivery may just be slower and does not violate
      application expectations.  Moreover, it is not possible to
      associate the Path MTU.  A transport system should therefore allow requested delivery order to a sending
   application "message" in TCP
      anyway.

   o  Request not to query bundle messages
      Protocols: SCTP
      Optimizing because this decision depends on knowledge about the maximum amount
      size of future data blocks and the delay between them.
      Implementation: via SEND.SCTP.
      Implementation over TCP: By using SEND.TCP and DISABLE_NAGLE.TCP
      to disable the Nagle algorithm when the request is made and enable
      it can possibly
   transmit before (or, if exposed, during) connection establishment.

A.3.4.  Sender Running Dry

   The transport feature "Notification that again when the stack has request is no more user
   data to send" longer made.  Note that this is
      not fully equivalent because it relates to SCTP's "SENDER DRY" notification.  Such
   notifications can, in principle, be used the time of issuing the
      request rather than a specific message.
      Implementation over UDP: do nothing (UDP never bundles messages).

   o  Specifying a "payload protocol-id" (handed over as such by the
      receiver)
      Protocols: SCTP
      Functional because it allows to avoid having an
   unnecessarily large send buffer, yet ensure that the transport sender
   always has 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 when it has an opportunity in TCP).
      Implementation over UDP: not possible (this functionality is not
      available in UDP).

   o  Specifying a key id to transmit it.
   This has been found 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 very beneficial for some applications
   [WWDC2015].  However, "SENDER DRY" truly means that the entire send
   buffer (including both unsent emulated by using
      SET_AUTH.TCP before and unacknowledged data) has emptied --
   i.e., when it notifies after the sender, it message 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 sent.  Note that TCP can keep in the
   socket buffer; this allows
      is not fully equivalent because it relates to specify at which buffer filling level the socket becomes writable, time of issuing
      the request rather than waiting for the buffer a specific message.
      Implementation over UDP: not possible (UDP does not offer
      authentication).

   o  Request not to
   run empty. delay the acknowledgement (SACK) of a message
      Protocols: SCTP allows
      Optimizing because only an application knows for which message it
      wants to configure the sender-side buffer too: the automatable
   Transport Feature "Configure send buffer size" provides quickly be informed about success / failure of its
      delivery.
      Implementation over TCP: do nothing (TCP does not offer this
      functionality, but only for ignoring this request from the complete buffer, which includes both
   unsent and unacknowledged data.  SCTP application will
      not yield a semantically wrong behavior).

      Implementation over UDP: do nothing (UDP does not allow to control these
   two sizes separately.  It therefore makes sense for offer this
      functionality, but ignoring this request from the application will
      not yield a semantically wrong behavior).

A.2.2.  Receiving Data

   o  Receive data (with no message delimiting)
      Protocols: TCP
      Functional because a transport system must be able to allow for uniform access to "TCP_NOTSENT_LOWAT" as well as
   the "SENDER DRY" notification.

A.3.5.  Capacity Profile

   The transport features:

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

   all relate to a QoS-like
      receive data.
      Implementation: via RECEIVE.TCP.
      Implementation over UDP: do nothing (UDP only works on messages;
      these can be handed over, the application need such as "low latency" or
   "scavenger".  In can still ignore the interest of flexibility of a transport system,
   they could therefore be offered in a uniform, more abstract way,
   where
      message boundaries).

   o  Receive a transport system could e.g. decide by itself how message
      Protocols: SCTP, UDP(-Lite)
      Functional because this is closely tied to use
   combinations properties of LEDBAT-like congestion control and certain DSCP
   values, and the data
      that an application would only specify a general "capacity
   profile" (a description of how it wants sends or expects to use the available
   capacity).  A need for "lowest receive.
      Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite).
      Implementation over TCP: not possible latency at the expense (TCP does not support
      identification of
   overhead" could then translate into automatically disabling the Nagle
   algorithm.

   In some cases, the Nagle algorithm is best controlled directly by message boundaries).

   o  Choice of stream to receive from
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams in the
   application CONNECTION.ESTABLISHMENT
      category is automatable.
      Implementation: see Section 5.2.

   o  Information about partial message arrival
      Protocols: SCTP
      Functional because it this is not only related to a general profile but
   also closely tied to knowledge about the size properties of future messages.  For fine-grain
   control over Nagle-like functionality, the "Request not data
      that an application sends or expects to bundle
   messages" receive.
      Implementation: via RECEIVE.SCTP.
      Implementation over TCP: do nothing (this information is available.

A.3.6.  Security

   Both TCP and SCTP offer authentication.  TCP authenticates complete
   segments.  SCTP allows 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 configure which in the "Sending Data" category (Appendix A.2.1).

   o  Notification of SCTP's chunk types must
   always be authenticated -- if send failures
      Protocols: SCTP, UDP(-Lite)
      Functional because this notifies that potentially assumed reliable
      data delivery is exposed as such, it creates an
   undesirable dependency on no longer provided.
      CHANGED FROM RFC8303.  This differs from the 2 automatable
      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) features below in that it does not distinugish between
      unsent and payload.

   Security unacknowledged messages.
      Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(-
      Lite).
      Implementation over TCP: do nothing (this notification is discussed in a separate document
   [I-D.ietf-taps-transport-security].  The minimal set presented in the
   present document excludes all security related transport features:
   "Configure authentication", "Change authentication parameters",
   "Obtain authentication information" not
      available and will therefore not occur with TCP).

   o  Notification of an unsent (part of a) message
      Protocols: SCTP, UDP(-Lite)
      Automatable because the distinction between unsent and "Set Cookie life value"
   as well as "Specifying a key id to be used
      unacknowledged does not relate to authenticate a
   message".

A.3.7.  Packet Size

   UDP(-Lite) has a transport feature called "Specify DF field".  This
   yields application-specific knowledge.

   o  Notification of an error message in case unacknowledged (part of sending a a) message
      Protocols: SCTP
      Automatable because the distinction between unsent and
      unacknowledged does not relate to application-specific knowledge.

   o  Notification that exceeds the
   Path MTU, which is necessary for a UDP-based stack has no more user data to send
      Protocols: SCTP
      Optimizing because reacting to this notification requires the
      application to be able
   to implement Path MTU Discovery (a function involved, and ensuring that UDP-based
   applications must the stack does not
      run dry of data (for too long) can improve performance.
      Implementation over TCP: do by themselves).  The "Get max. transport-message
   size 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).

   o  Notification to a receiver that may be sent using a non-fragmented IP packet from partial message delivery has
      been aborted
      Protocols: SCTP
      Functional because this is closely tied to properties of the
   configured interface" transport feature yields data
      that an upper limit for the
   Path MTU (minus headers) application sends or expects to receive.
      Implementation over TCP: do nothing (this notification is not
      available and can will therefore help to implement Path MTU
   Discovery more efficiently. not occur with TCP).
      Implementation over UDP: do nothing (this notification is not
      available and will therefore not occur with UDP).

Appendix B.  Revision information

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

   -02: implementation suggestions added, discussion section added,
   terminology extended, DELETED category removed, various other fixes;
   list of Transport Features adjusted to -01 version of [RFC8303]
   except that MPTCP is not included.

   -03: updated to be consistent with -02 version of [RFC8303].

   -04: updated to be consistent with -03 version of [RFC8303].
   Reorganized document, rewrote intro and conclusion, and made a first
   stab at creating a real "minimal set".

   -05: updated to be consistent with -05 version of [RFC8303] (minor
   changes).  Fixed a mistake regarding Cookie Life value.  Exclusion of
   security related transport features (to be covered in a separate
   document).  Reorganized the document (now begins with the minset,
   derivation is in the appendix).  First stab at an abstract API for
   the minset.

   draft-ietf-taps-minset-00: updated to be consistent with -08 version
   of [RFC8303] ("obtain message delivery number" was removed, as this
   has also been removed in [RFC8303] because it was a mistake in
   RFC4960.  This led to the removal of two more transport features that
   were only designated as functional because they affected "obtain
   message delivery number").  Fall-back to UDP incorporated (this was
   requested at IETF-99); this also affected the transport feature
   "Choice between unordered (potentially faster) or ordered delivery of
   messages" because this is a boolean which is always true for one
   fall-back protocol, and always false for the other one.  This was
   therefore now divided into two features, one for ordered, one for
   unordered delivery.  The word "reliably" was added to the transport
   features "Hand over a message to reliably transfer (possibly multiple
   times) before connection establishment" and "Hand over a message to
   reliably transfer during connection establishment" to make it clearer
   why this is not supported by UDP.  Clarified that the "minset
   abstract interface" is not proposing a specific API for all TAPS
   systems to implement, but it is just a way to describe the minimum
   set.  Author order changed.

   WG -01: "fall-back to" (TCP or UDP) replaced (mostly with
   "implementation over").  References to post-sockets removed (these
   were statments that assumed that post-sockets requires two-sided
   implementation).  Replaced "flow" with "TAPS Connection" and "frame"
   with "message" to avoid introducing new terminology.  Made sections 3
   and 4 in line with the categorization that is already used in the
   appendix and [RFC8303], and changed style of section 4 to be even
   shorter and less interface-like.  Updated reference draft-ietf-tsvwg-
   sctp-ndata to RFC8260.

   WG -02: rephrased "the TAPS system" and "TAPS connection" etc. to
   more generally talk about transport after the intro (mostly replacing
   "TAPS system" with "transport system" and "TAPS connection" with
   "connection".  Merged sections 3 and 4 to form a new section 3.

   WG -03: updated sentence referencing
   [I-D.ietf-taps-transport-security] to say that "the minimum security
   requirements for a taps system are discussed in a separate security
   document", wrote "example" in the paragraph introducing the decision
   tree.  Removed reference draft-grinnemo-taps-he-03 and the sentence
   that referred to it.

   WG -04: addressed comments from Theresa Enghardt and Tommy Pauly.  As
   part of that, removed "TAPS" as a term everywhere (abstract, intro,
   ..).

   WG -05: addressed comments from Spencer Dawkins.

   WG -06: Fixed nits.

   WG -07: Addressed Genart comments from Robert Sparks.

   WG -08: Addressed one more Genart comment from Robert Sparks.

   WG -09: Addressed comments from Mirja Kuehlewind, Alvaro Retana, Ben
   Campbell, Benjamin Kaduk and Eric Rescorla.

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

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

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

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