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TAPS                                                         S. Gjessing
Internet-Draft                                                  M. Welzl
Intended status: Informational                        University of Oslo
Expires: December 22, 2017                                 June 20, 2017


          A Minimal Set of Transport Services for TAPS Systems
                     draft-gjessing-taps-minset-05

Abstract

   This draft recommends a minimal set of IETF Transport Services
   offered by end systems supporting TAPS, and gives guidance on
   choosing among the available mechanisms and protocols.  It is based
   on the set of transport features given in the TAPS document
   draft-ietf-taps-transports-usage-05.

Status of this Memo

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

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

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

   This Internet-Draft will expire on December 22, 2017.

Copyright Notice

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

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



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The Minimal Set of Transport Features  . . . . . . . . . . . .  5
     3.1.  Flow Creation, Connection and Termination  . . . . . . . .  5
     3.2.  Flow Group Configuration . . . . . . . . . . . . . . . . .  6
     3.3.  Flow Configuration . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Data Transfer  . . . . . . . . . . . . . . . . . . . . . .  7
       3.4.1.  The Sender . . . . . . . . . . . . . . . . . . . . . .  7
       3.4.2.  The Receiver . . . . . . . . . . . . . . . . . . . . .  8
   4.  An Abstract MinSet API . . . . . . . . . . . . . . . . . . . .  9
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Deriving the minimal set  . . . . . . . . . . . . . . 17
     A.1.  Step 1: Categorization -- The Superset of Transport
           Features . . . . . . . . . . . . . . . . . . . . . . . . . 17
       A.1.1.  CONNECTION Related Transport Features  . . . . . . . . 19
       A.1.2.  DATA Transfer Related Transport Features . . . . . . . 31
     A.2.  Step 2: Reduction -- The Reduced Set of Transport
           Features . . . . . . . . . . . . . . . . . . . . . . . . . 35
       A.2.1.  CONNECTION Related Transport Features  . . . . . . . . 36
       A.2.2.  DATA Transfer Related Transport Features . . . . . . . 37
     A.3.  Step 3: Discussion . . . . . . . . . . . . . . . . . . . . 37
       A.3.1.  Sending Messages, Receiving Bytes  . . . . . . . . . . 38
       A.3.2.  Stream Schedulers Without Streams  . . . . . . . . . . 39
       A.3.3.  Early Data Transmission  . . . . . . . . . . . . . . . 40
       A.3.4.  Sender Running Dry . . . . . . . . . . . . . . . . . . 41
       A.3.5.  Capacity Profile . . . . . . . . . . . . . . . . . . . 42
       A.3.6.  Security . . . . . . . . . . . . . . . . . . . . . . . 42
       A.3.7.  Packet Size  . . . . . . . . . . . . . . . . . . . . . 42
   Appendix B.  Revision information  . . . . . . . . . . . . . . . . 43
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43













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1.  Introduction

   The task of any system that implements TAPS is to offer transport
   services to its applications, i.e. the applications running on top of
   TAPS, without binding them to a particular transport protocol.
   Currently, the set of transport services that most applications use
   is based on TCP and UDP; this limits the ability for the network
   stack to make use of features of other 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 never utilize out-of-order message delivery:
   doing so would break a fundamental assumption of the application.

   By exposing the transport services of multiple transport protocols, a
   TAPS system can make it possible to use these services without having
   to statically bind an application 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 [TAPS2],
   which identifies 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.  The present draft is
   based on these documents and follows the same terminology (also
   listed below).

   The number of transport features of current IETF transports is large,
   and exposing all of them has a number of disadvantages: generally,
   the more functionality is exposed, the less freedom a TAPS system has
   to automate usage of the various functions of its available set of
   transport protocols.  Some functions only exist in one particular
   protocol, and if an application would use them, this would statically
   tie the application to this protocol, counteracting the purpose of a
   TAPS system.  Also, if the number of exposed features is exceedingly
   large, a TAPS system might become very hard to use for an application
   programmer.  Taking [TAPS2] as a basis, this document therefore
   develops a minimal set of transport features, removing the ones that
   could be harmful to the purpose of a TAPS system but keeping the ones
   that must be retained for applications to benefit from useful
   transport functionality.

   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 middleware if only the Berkeley Sockets
   API is extended to offer "unordered message delivery", but the
   middleware only offers an ordered bytestream.  Both the Berkeley
   Sockets API and the middleware would have to expose the "unordered
   message delivery" transport feature (alternatively, there may be



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   interesting ways for certain types of middleware to use some
   transport features without exposing them, based on knowledge about
   the applications -- but this is not the general case).  In most
   situations, in the interest of being as flexible and efficient as
   possible, the best choice will be for a middleware or 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 with a fall-back to
   TCP: i.e., a sender-side TAPS system can talk to a non-TAPS TCP
   receiver, and a receiver-side TAPS system can talk to a non-TAPS TCP
   sender.  For systems that do not have this requirement,
   [I-D.trammell-taps-post-sockets] describes a way to extend the
   functionality of the minimal set such that several of its limitations
   are removed.


2.  Terminology

   The following terms are used throughout this document, and in
   subsequent documents produced by TAPS that describe the composition
   and decomposition of transport services.

   Transport Feature:  a specific end-to-end feature that the transport
      layer provides to an application.  Examples include
      confidentiality, reliable delivery, ordered delivery, message-
      versus-stream orientation, etc.
   Transport Service:  a set of Transport Features, without an
      association to any given framing protocol, which provides a
      complete service to an application.
   Transport Protocol:  an implementation that provides one or more
      different transport services using a specific framing and header
      format on the wire.
   Transport Service Instance:  an arrangement of transport protocols
      with a selected set of features and configuration parameters that
      implements a single transport service, e.g., a protocol stack (RTP
      over UDP).
   Application:  an entity that uses the transport layer for end-to-end
      delivery data across the network (this may also be an upper layer
      protocol or tunnel encapsulation).
   Application-specific knowledge:  knowledge that only applications
      have.
   Endpoint:  an entity that communicates with one or more other
      endpoints using a transport protocol.







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   Connection:  shared state of two or more endpoints that persists
      across messages that are transmitted between these endpoints.
   Socket:  the combination of a destination IP address and a
      destination port number.


3.  The Minimal Set of Transport Features

   Based on the categorization, reduction and discussion in Appendix A,
   this section describes the minimal set of transport features that is
   offered by end systems supporting TAPS.

3.1.  Flow Creation, Connection and Termination

   A TAPS flow must be "created" before it is connected, to allow for
   initial configurations to be carried out.  All configuration
   parameters in Section 3.2 and Section 3.3 can be used initially,
   although some of them may only take effect when the flow has been
   connected.  Configuring a flow early helps a TAPS system make the
   right decisions.  In particular, the "group number" can influence the
   TAPS system to implement a TAPS flow as a stream of a multi-streaming
   protocol's existing association or not.

   A created flow can be queried for the maximum amount of data that an
   application can possibly expect to have transmitted before or during
   connection establishment.  An application can also give the flow a
   message for transmission before or during connection establishment;
   the TAPS system will try to transmit it as early as possible.  An
   application can facilitate sending the message particularly early by
   marking it as "idempotent"; in this case, the receiving application
   must be prepared to potentially receive multiple copies of the
   message.

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

   After creation, a flow can be actively connected to the other side
   using "Connect", or passively listen for incoming connection requests
   with "Listen".  Note that "Connect" may or may not trigger a
   notification on the listening side.  It is possible that the first
   notification on the listening side is the arrival of the first data
   that the active side sends (a receiver-side TAPS system could handle



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   this by continuing a blocking "Listen" call, immediately followed by
   issuing "Receive", for example).  This also means that the active
   opening side is assumed to be the first side sending data.

   A TAPS system can actively close a connection, i.e. terminate it
   after reliably delivering all remaining data to the peer, or it can
   abort it, i.e. terminate it without delivering remaining data.
   Unless all data transfers only used unreliable frame transmission
   without congestion control, closing a connection is guaranteed to
   cause an event to notify the peer application that the connection has
   been closed.  Similarly, for anything but unreliable non-congestion-
   controlled data transfer, aborting a connection will cause an event
   to notify the peer application that the connection has been aborted.
   A timeout can be configured to abort a flow when data could not be
   delivered for too long; timeout-based abortion does not notify the
   peer application that the connection has been aborted.  Because half-
   closed connections are not supported, when a TAPS host receives a
   notification that the peer is closing or aborting the flow, the other
   side may not be able to read outstanding data.  This means that
   unacknowledged data residing in the TAPS system's send buffer may
   have to be dropped from that buffer upon arrival of a notification to
   close or abort the flow from the peer.

3.2.  Flow Group Configuration

   A flow group can be configured with a number of transport features,
   and there are some notifications to applications about a flow group.
   Here we list transport features and notifications from Appendix A.2
   that sometimes automatically apply to groups of flows (e.g., when a
   flow is mapped to a stream of a multi-streaming protocol).

   Timeout, error notifications:
   o  Change timeout for aborting connection (using retransmit limit or
      time value)
   o  Suggest timeout to the peer
   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold)
   o  Notification of ICMP error message arrival

   Others:
   o  Choose a scheduler to operate between flows of a group
   o  Obtain ECN field

   The following transport features are new or changed, based on the
   discussion in Appendix A.3:
   o  Capacity profile
      This describes how an application wants to use its available
      capacity.  Choices can be "lowest possible latency at the expense



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      of overhead" (which would disable any Nagle-like algorithm),
      "scavenger", and some more values that help determine the DSCP
      value for a flow (e.g. similar to table 1 in
      [I-D.ietf-tsvwg-rtcweb-qos]).


3.3.  Flow Configuration

   Here we list transport features and notifications from Appendix A.2
   that only apply to a single flow.

   Configure priority or weight for a scheduler

   Checksums:
   o  Disable checksum when sending
   o  Disable checksum requirement when receiving
   o  Specify checksum coverage used by the sender
   o  Specify minimum checksum coverage required by receiver

3.4.  Data Transfer

3.4.1.  The Sender

   This section discusses how to send data after flow establishment.
   Section 3.1 discusses the possiblity to hand over a message to send
   before or during establishment.

   Here we list per-frame properties that a sender can optionally
   configure if it hands over a delimited frame for sending with
   congestion control, taken from Appendix A.2:
   o  Configurable Message Reliability
   o  Choice between unordered (potentially faster) or ordered delivery
      of messages
   o  Request not to bundle messages
   o  Request not to delay the acknowledgement (SACK) of a message

   Additionally, an application can hand over delimited frames for
   unreliable transmission without congestion control (note that such
   applications should perform congestion control in accordance with
   [RFC2914]).  Then, none of the per-frame properties listed above have
   any effect, but it is possible to use the transport feature "Specify
   DF field" to allow/disallow fragmentation.

   Following Appendix A.3.7, there are three transport features (two
   old, one new) and a notification:
   o  Get max. transport frame size that may be sent without
      fragmentation from the configured interface
      This is optional for a TAPS system to offer.  It can aid



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      applications implementing Path MTU Discovery.

   o  Get max. transport frame size that may be received from the
      configured interface
      This is optional for a TAPS system to offer.

   o  Get maximum transport frame size
      Irrespective of fragmentation, there is a size limit for the
      messages that can be handed over to SCTP or UDP(-Lite); because a
      TAPS system is independent of the transport, it must allow a TAPS
      application to query this value -- the maximum size of a frame in
      an Application-Framed-Bytestream.


   There are two more sender-side notifications.  These are unreliable,
   i.e. a TAPS system cannot be assumed to implement them, but they may
   occur:
   o  Notification of send failures
      A TAPS system may inform a sender application of a failure to send
      a specific frame.  This was taken over unchanged from
      Appendix A.2.

   o  Notification of draining below a low water mark
      A TAPS system can notify a sender application when the TAPS
      system's filling level of the buffer of unsent data is below a
      configurable threshold in bytes.  Even for TAPS systems that do
      implement this notification, supporting thresholds other than 0 is
      optional.

   "Notification of draining below a low water mark" is a generic
   notification that tries to enable uniform access to
   "TCP_NOTSENT_LOWAT" as well 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) is 0 and there is also no more
   unacknowledged data in the send buffer).  Note that this threshold
   and its notification should operate across the buffers of the whole
   TAPS system, i.e. also any potential buffers that the TAPS system
   itself may use on top of the transport's send buffer.

3.4.2.  The Receiver

   A receiving application obtains an Application-Framed Bytestream.
   Similar to TCP's receiver semantics, it is just stream of bytes.  If
   frame boundaries were specified by the sender, a receiver-side TAPS
   system will still not inform the receiving application about them.
   Within the bytestream, frames themselves will always stay intact
   (partial frames are not supported - see Appendix A.3.1).  Different
   from TCP's semantics, there is no guarantee that all frames in the



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   bytestream are transmitted from the sender to the receiver, and that
   all of them are in the same sequence in which they were handed over
   by the sender.  If an application is aware of frame delimiters in the
   bytestream, and if the sender-side application has informed the TAPS
   system about these boundaries and about potentially relaxed
   requirements regarding the sequence of frames or per-frame
   reliability, frames within the receiver-side bytestream may be out-
   of-order or missing.


4.  An Abstract MinSet API

   Here we present an abstract API that a TAPS system can implement.
   This API is derived from the description in the previous section.
   The primitives of this API can be implemented in various ways.  For
   example, information that is provided to an application can either be
   offered via a primitive that is polled, or via an asynchronous
   notification.  The API offers specific primitives to configure such
   asynchronous call-backs.

   CREATE (flow-group-id)
   Returns: flow-id

   Create a flow and associate it with an existing or new flow group
   number.  The group number can influence the TAPS system to implement
   a TAPS flow as a stream of a multi-streaming protocol's existing
   association or not.


   CONFIGURE_TIMEOUT (flow-group-id [timeout] [peer_timeout]
   [retrans_notify])

   This configures timeouts for all flows in a group.  Configuration
   should generally be carried out as early as possible, ideally before
   flows are connected, to aid the TAPS system's decision taking.

   PARAMETERS:
   timeout:  a timeout value for aborting connections, in seconds
   peer_timeout:  a timeout value to be suggested to the peer (if
      possible), in seconds
   retrans_notify:  the number of retransmissions after which the
      application should be notifed of "Excessive Retransmissions"


   CONFIGURE_CHECKSUM (flow-id [send [send_length]] [receive
   [receive_length]])

   This configures the usage of checksums for a flow in a group.



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   Configuration should generally be carried out as early as possible,
   ideally before the flow is connected, to aid the TAPS system's
   decision taking. "send" parameters concern using a checksum when
   sending, "receive" parameters concern requiring a checksum when
   receiving.  There is no guarantee that any checksum limitations will
   indeed be enforced; all defaults are: "full coverage, checksum
   enabled".

   PARAMETERS:
   send:  boolean, enable / disable usage of a checksum
   send_length:  if send is true, this optional parameter can provide
      the desired coverage of the checksum in bytes
   receive:  boolean, enable / disable requiring a checksum
   receive_length:  if receive is true, this optional parameter can
      provide the required minimum coverage of the checksum in bytes


   CONFIGURE_URGENCY (flow-group-id [scheduler] [capacity_profile]
   [low_watermark])

   This carries out configuration related to the urgency of sending data
   on flows of a group.  Configuration should generally be carried out
   as early as possible, ideally before flows are connected, to aid the
   TAPS system's decision taking.

   PARAMETERS:
   scheduler:  a number to identify the type of scheduler that should be
      used to operate between flows in the group (no guarantees given).
      Future versions of this document will be self contained, but for
      now we suggest the schedulers defined in
      [I-D.ietf-tsvwg-sctp-ndata].
   capacity_profile:  a number to identify how an application wants to
      use its available capacity.  Future versions of this document will
      be self contained, but for now choices can be "lowest possible
      latency at the expense of overhead" (which would disable any
      Nagle-like algorithm), "scavenger", and some more values that help
      determine the DSCP value for a flow (e.g. similar to table 1 in
      [I-D.ietf-tsvwg-rtcweb-qos]).
   low_watermark:  a buffer limit (in bytes); when the sender has less
      then low_watermark bytes in the buffer, the application may be
      notified.  Notifications are not guaranteed, and supporting
      watermark numbers greater than 0 is not guaranteed.


   CONFIGURE_PRIORITY (flow-id priority)

   This configures a flow's priority or weight for a scheduler.
   Configuration should generally be carried out as early as possible,



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   ideally before flows are connected, to aid the TAPS system's decision
   taking.

   PARAMETERS:
   priority:  future versions of this document will be self contained,
      but for now we suggest the priority as described in
      [I-D.ietf-tsvwg-sctp-ndata].


   NOTIFICATIONS
   Returns: flow-group-id notification_type

   This is fired when an event occurs, notifying the application about
   something happening in relation to a flow group.  Notification types
   are:
   Excessive Retransmissions:  the configured (or a default) number of
      retransmissions has been reached, yielding this early warning
      below an abortion threshold
   ICMP Arrival (parameter: ICMP message):  an ICMP packet carrying the
      conveyed ICMP message has arrived.
   ECN Arrival (parameter: ECN value):  a packet carrying the conveyed
      ECN value has arrived.  This can be useful for applications
      implementing congestion control.
   Timeout (parameter: s seconds):  data could not be delivered for s
      seconds.
   Close:  the peer has closed the connection.  The peer has no more
      data to send, and will not read more data.  Data that is in
      transit or resides in the local send buffer will be discarded.
   Abort:  the peer has aborted the connection.  The peer has no more
      data to send, and will not read more data.  Data that is in
      transit or resides in the local send buffer will be discarded.
   Drain:  the send buffer has either drained below the configured low
      water mark or it has become completely empty.
   Path Change (parameter: path identifier):  the path has changed; the
      path identifier is a number that can be used to determine a
      previously used path is used again (e.g., the TAPS system has
      switched from one interface to the other and back).
   Send Failure (parameter: frame identifier):  this informs the
      application of a failure to send a specific frame.  There can be a
      send failure without this notification happening.


   QUERY_PROPERTIES (flow-group-id property_identifier)
   Returns: requested property (see below)

   This allows to query some properties of a flow group.  Return values
   per property identifier are:




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   o  The maximum frame size that may be sent without fragmentation, in
      bytes
   o  The maximum transport frame size that can be sent, in bytes
   o  The maximum transport frame size that can be received, in bytes
   o  The maximum amount of data that can possibly be sent before or
      during connection establishment, in bytes


   CONNECT (flow-id dst_addr)

   Connects a flow.  This primitive may or may not trigger a
   notification (continuing LISTEN) on the listening side.  If a send
   precedes this call, then data may be transmitted with this connect.

   PARAMETERS:
   dst_addr:  the destination transport address to connect to


   LISTEN (flow-id)

   Blocking passive connect, listening on all interfaces.  This may not
   be the direct result of the peer calling CONNECT - it may also be
   invoked upon reception of the first block of data.  In this case,
   RECEIVE_FRAME is invoked immediately after.


   SEND_FRAME (flow-id frame [reliability] [ordered] [bundle] [delack]
   [fragment] [idempotent])

   Sends an application frame.  No guarantees are given about the
   preservation of frame boundaries to the peer; if frame boundaries are
   needed, the receiving application at the peer must know about them
   beforehand.  Note that this call can already be used before a flow is
   connected.  All parameters refer to the frame that is being handed
   over.

   PARAMETERS:
   reliability:  this parameter is used to convey a choice of: fully
      reliable, unreliable without congestion control (which is
      guaranteed), unreliable, partially reliable (how to configure:
      TBD, probably using a time value).  The latter two choices are not
      guaranteed and may result in full reliability.
   ordered:  this boolean parameter lets an application choose between
      ordered message delivery (true) and possibly unordered,
      potentially faster message delivery (false).






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   bundle:  a boolean that expresses a preference for allowing to bundle
      frames (true) or not (false).  No guarantees are given.
   delack:  a boolean that, if false, lets an application request that
      the peer would not delay the acknowledgement for this frame.
   fragment:  a boolean that expresses a preference for allowing to
      fragment frames (true) or not (false), at the IP level.  No
      guarantees are given.
   idempotent:  a boolean that expresses whether a frame is idempotent
      (true) or not (false).  Idempotent frames may arrive multiple
      times at the receiver.  When data is idempotent it can be used by
      the receiver immediately on a connection establishment attempt.
      Thus, if SEND_FRAME is used before connecting, stating that a
      frame is idempotent facilitates transmitting it to the peer
      application particularly early.


   CLOSE (flow-id)

   Closes the flow after all outstanding data is reliably delivered to
   the peer (if reliable data delivery was requested).  In case reliable
   or partially reliable data delivery was requested earlier, the peer
   is notified of the CLOSE.


   ABORT (flow-id)

   Aborts the flow without delivering outstanding data to the peer.  In
   case reliable or partially reliable data delivery was requested
   earlier, the peer is notified of the ABORT.


   RECEIVE_FRAME (flow-id buffer)

   This receives a block of data.  This block may or may not correspond
   to a sender-side frame, i.e. the receiving application is not
   informed about frame boundaries.  However, if the sending application
   has allowed that frames are not fully reliably transferred, or
   delivered out of order, then such re-ordering or unreliability may be
   reflected per frame in the arriving data.  Frames will always stay
   intact - i.e. if an incomplete frame is contained at the end of the
   arriving data block, this frame is guaranteed to continue in the next
   arriving data block.

   PARAMETERS:







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   buffer:  the buffer where the received data will be stored.



5.  Conclusion

   By decoupling applications from transport protocols, a TAPS system
   provides a different abstraction level than the Berkeley sockets
   interface.  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 TAPS 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 ("functional" transport features).  Other transport
   features are offered by the APIs of the protocols covered here, but
   not exposing them in a TAPS API would allow for more freedom to
   automate protocol usage in a TAPS system.

   The minimal set presented in this document is an effort to find a
   middle ground that can be recommended for TAPS systems to implement,
   on the basis of the transport features discussed in [TAPS2].  This
   middle ground eliminates a large number of transport features because
   they do not require application-specific knowledge, but rather rely
   on knowledge about the network or the Operating System.  This leaves
   us with an unanswered question about how exactly a TAPS system should
   automate using all these transport features.

   In some cases, it may be best to not entirely automate the decision
   making, but leave it up to a system-wide policy.  For example, when
   multiple paths are available, a system policy could guide the
   decision on whether to connect via a WiFi or a cellular interface.
   Such high-level guidance could also be provided by application
   developers, e.g. via a primitive that lets applications specify such
   preferences.  As long as this kind of information from applications
   is treated as advisory, it will not lead to a permanent protocol
   binding and does therefore not limit the flexibility of a TAPS
   system.  Decisions to add such primitives are therefore left open to
   TAPS system designers.


6.  Acknowledgements

   The authors would like to thank the participants of the TAPS Working
   Group and the NEAT research project for valuable input to this
   document.  We especially thank Michael Tuexen for help with TAPS flow
   connection establishment/teardown and Gorry Fairhurst for his



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   suggestions regarding fragmentation and packet sizes.  This work has
   received funding from the European Union's Horizon 2020 research and
   innovation programme under grant agreement No. 644334 (NEAT).  The
   views expressed are solely those of the author(s).


7.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.


8.  Security Considerations

   Authentication, confidentiality protection, and integrity protection
   are identified as transport features by [RFC8095].  As currently
   deployed in the Internet, these features are generally provided by a
   protocol or layer on top of the transport protocol; no current full-
   featured standards-track transport protocol provides all of these
   transport features on its own.  Therefore, these transport features
   are not considered in this document, with the exception of native
   authentication capabilities of TCP and SCTP for which the security
   considerations in [RFC5925] and [RFC4895] apply.


9.  References

9.1.  Normative References

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", RFC 8095, DOI 10.17487/
              RFC8095, March 2017,
              <http://www.rfc-editor.org/info/rfc8095>.

   [TAPS2]    Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
              Transport Features Provided by IETF Transport Protocols",
              draft-ietf-taps-transports-usage-05 (work in progress),
              May 2017.

9.2.  Informative References

   [COBS]     Cheshire, S. and M. Baker, "Consistent Overhead Byte
              Stuffing", September 1997,
              <http://stuartcheshire.org/papers/COBSforToN.pdf>.

   [I-D.ietf-tsvwg-rtcweb-qos]



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              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.

   [I-D.ietf-tsvwg-sctp-ndata]
              Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol",
              draft-ietf-tsvwg-sctp-ndata-10 (work in progress),
              April 2017.

   [I-D.trammell-taps-post-sockets]
              Trammell, B., Perkins, C., Pauly, T., and M. Kuehlewind,
              "Post Sockets, An Abstract Programming Interface for the
              Transport Layer", draft-trammell-taps-post-sockets-00
              (work in progress), March 2017.

   [LBE-draft]
              Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
              draft-tsvwg-le-phb-01 (work in progress), February 2017.

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

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

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <http://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, <http://www.rfc-editor.org/info/rfc5925>.

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

   [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              RFC 6525, DOI 10.17487/RFC6525, February 2012,



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              <http://www.rfc-editor.org/info/rfc6525>.

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

   [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: the superset of transport features from [TAPS2]
       is presented, and transport features are categorized for later
       reduction.
   2.  Reduction: 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
       cannot be implemented with TCP.
   3.  Discussion: the resulting list shows a number of peculiarities
       that are discussed, to provide a basis for constructing the
       minimal set.
   4.  Construction: Based on the reduced set and the discussion of the
       transport features therein, a minimal set is constructed.
   The first three steps as well as the underlying rationale for
   constructing the minimal set are described in this appendix.  The
   minimal set itself is described in Section 3.

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

   Following [TAPS2], 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 TAPS applications have no specific requirements that



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   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 TAPS
   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,
   unordered message delivery is a functional transport feature: it
   cannot be used without the application knowing about it because the
   application's assumption could be that messages 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 TAPS system autonomously
   decides to enable or disable them, an application will not fail, but
   a TAPS system may be able to communicate more efficiently if the
   application is in control of this optimizing transport feature.
   These transport features require application-specific knowledge
   (e.g., about delay/bandwidth requirements or the length of future
   data blocks that are to be transmitted).

   The transport features of IETF transport protocols that do not
   require application-specific knowledge and could therefore be
   transparently utilized by a TAPS system are called "Automatable".

   Finally, some transport features are aggregated and/or slightly
   changed in the TAPS API.  These transport features are marked as
   "ADDED".  The corresponding transport features are automatable, and
   they are listed immediately below the "ADDED" transport feature.

   In this description, transport services are presented following the
   nomenclature "CATEGORY.[SUBCATEGORY].SERVICENAME.PROTOCOL",
   equivalent to "pass 2" in [TAPS2].  The PROTOCOL name "UDP(-Lite)" is
   used when transport features are equivalent for UDP and UDP-Lite; the
   PROTOCOL name "TCP" refers to both TCP and MPTCP.  We also sketch how
   some of the TAPS transport services can be implemented.  For all
   transport features that are categorized as "functional" or
   "optimizing", and for which no matching TCP primitive exists in "pass
   2" of [TAPS2], a brief discussion on how to fall back to TCP is
   included.

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




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   o  Most transport features that are related to multi-streaming were
      designated as "automatable".  This was done because the decision
      on whether to use multi-streaming or not does not depend on
      application-specific knowledge.  This means that a connection that
      is exhibited to an application could be implemented by using a
      single stream of an SCTP association instead of mapping it to a
      complete SCTP association or TCP connection.  This could be
      achieved by using more than one stream when an SCTP association is
      first established (CONNECT.SCTP parameter "outbound stream
      count"), maintaining an internal stream number, and using this
      stream number when sending data (SEND.SCTP parameter "stream
      number").  Closing or aborting a connection could then simply free
      the stream number for future use.  This is discussed further in
      Appendix A.3.2.
   o  All transport features that are related to using multiple paths or
      the choice of the network interface were designated as
      "automatable".  Choosing a 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 default interface for the destination IP address.

A.1.1.  CONNECTION Related Transport Features

   ESTABLISHMENT:
   o  Connect
      Protocols: TCP, SCTP, UDP(-Lite)
      Functional because the notion of a connection is often reflected
      in applications as an expectation to be able to communicate after
      a "Connect" succeeded, with a communication sequence relating to
      this transport feature that is defined by the application
      protocol.
      Implementation: via CONNECT.TCP, CONNECT.SCTP or CONNECT.UDP(-
      Lite).

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

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

   o  Limit the number of inbound streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require



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      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Specify number of attempts and/or timeout for the first
      establishment message
      Protocols: TCP, SCTP
      Functional because this is closely related to potentially assumed
      reliable data delivery for data that is sent before or during
      connection establishment.
      Implementation: Using a parameter of CONNECT.TCP and CONNECT.SCTP.

   o  Obtain multiple sockets
      Protocols: SCTP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates to knowledge about the network, not the
      application.

   o  Disable MPTCP
      Protocols: MPTCP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates to knowledge about the network, not the
      application.
      Implementation: via a boolean parameter in CONNECT.MPTCP.
      Fall-back to TCP: Do nothing.

   o  Configure authentication
      Protocols: TCP, SCTP
      Functional because this has a direct influence on security.
      Implementation: via parameters in CONNECT.TCP and CONNECT.SCTP.
      Fall-back to TCP: 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 authenticated.
      Authenticating only certain chunk types creates a reduced level of
      security that is not supported by TCP; to be compatible, this
      should therefore only allow to authenticate all chunk types.  Key
      material must be provided in a way that is compatible with both
      [RFC4895] and [RFC5925].

   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 the sake of
      identifying an adaptation layer, which by itself is application-
      specific.
      Implementation: via a parameter in CONNECT.SCTP.
      Fall-back to TCP: not possible.




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   o  Request to negotiate interleaving of user messages
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.
      Implementation: via a parameter in CONNECT.SCTP.

   o  Hand over a message to transfer (possibly multiple times) before
      connection establishment
      Protocols: TCP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via a parameter in CONNECT.TCP.

   o  Hand over a message to transfer during connection establishment
      Protocols: SCTP
      Functional because this can only work if the message is limited in
      size, making it closely tied to properties of the data that an
      application sends or expects to receive.
      Implementation: via a parameter in CONNECT.SCTP.

   o  Enable UDP encapsulation with a specified remote UDP port number
      Protocols: SCTP
      Automatable because UDP encapsulation relates to knowledge about
      the network, not the application.


   AVAILABILITY:
   o  Listen
      Protocols: TCP, SCTP, UDP(-Lite)
      Functional because the notion of accepting connection requests is
      often reflected in applications as an expectation to be able to
      communicate after a "Listen" succeeded, with a communication
      sequence relating to this transport feature that is defined by the
      application protocol.
      ADDED.  This differs from the 3 automatable transport features
      below in that it leaves the choice of interfaces for listening
      open.
      Implementation: by listening on all interfaces via LISTEN.TCP (not
      providing a local IP address) or LISTEN.SCTP (providing SCTP port
      number / address pairs for all local IP addresses).

   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.




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

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

   o  Disable MPTCP
      Protocols: MPTCP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates to knowledge about the network, not the
      application.

   o  Configure authentication
      Protocols: TCP, SCTP
      Functional because this has a direct influence on security.
      Implementation: via parameters in LISTEN.TCP and LISTEN.SCTP.
      Fall-back to TCP: 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 authenticated.
      Authenticating only certain chunk types creates a reduced level of
      security that is not supported by TCP; to be compatible, this
      should therefore only allow to authenticate all chunk types.  Key
      material must be provided in a way that is compatible with both
      [RFC4895] and [RFC5925].

   o  Obtain requested number of streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.

   o  Limit the number of inbound streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Appendix A.3.2.



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   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 the sake of
      identifying an adaptation layer, which by itself is application-
      specific.
      Implementation: via a parameter in LISTEN.SCTP.
      Fall-back to TCP: not possible.

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


   MAINTENANCE:
   o  Change timeout for aborting connection (using retransmit limit or
      time value)
      Protocols: TCP, SCTP
      Functional because this is closely related to potentially assumed
      reliable data delivery.
      Implementation: via CHANGE-TIMEOUT.TCP or CHANGE-TIMEOUT.SCTP.

   o  Suggest timeout to the peer
      Protocols: TCP
      Functional because this is closely related to potentially assumed
      reliable data delivery.
      Implementation: via CHANGE-TIMEOUT.TCP.

   o  Disable Nagle algorithm
      Protocols: TCP, SCTP
      Optimizing because this decision depends on knowledge about the
      size of future data blocks and the delay between them.
      Implementation: via DISABLE-NAGLE.TCP and DISABLE-NAGLE.SCTP.

   o  Request an immediate heartbeat, returning success/failure
      Protocols: SCTP
      Automatable because this informs about network-specific knowledge.

   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold)
      Protocols: TCP
      Optimizing because it is an early warning to the application,
      informing it of an impending functional event.
      Implementation: via ERROR.TCP.




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

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

   o  Set primary path
      Protocols: SCTP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates to knowledge about the network, not the
      application.

   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 the network, not the
      application.

   o  Configure Path Switchover
      Protocols: SCTP
      Automatable because the usage of multiple paths to communicate to
      the same end host relates to knowledge about the network, not 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 and peer receiver window size; current local
      congestion window sizes; number of unacknowledged DATA chunks;
      number of DATA chunks pending receipt; primary path; most recent
      SRTT on primary path; RTO on primary path; SRTT and RTO on other
      destination addresses; MTU per path; interleaving supported yes/no
      MPTCP parameters: subflow-list (identified by source-IP; source-
      Port; destination-IP; destination-Port)
      Automatable because these parameters relate to knowledge about the



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      network, not the 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  Notification of ICMP error message arrival
      Protocols: TCP, UDP(-Lite)
      Optimizing because these messages can inform about success or
      failure of functional transport features (e.g., host unreachable
      relates to "Connect")
      Implementation: via ERROR.TCP or ERROR.UDP(-Lite).

   o  Obtain information about interleaving support
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.
      Implementation: via a parameter in GETINTERL.SCTP.

   o  Change authentication parameters
      Protocols: TCP, SCTP
      Functional because this has a direct influence on security.
      Implementation: via SET_AUTH.TCP and SET_AUTH.SCTP.
      Fall-back to TCP: 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 sent on the
      connection.  Key material must be provided in a way that is
      compatible with both [RFC4895] and [RFC5925].

   o  Obtain authentication information
      Protocols: SCTP
      Functional because authentication decisions may have been made by
      the peer, and this has an influence on the necessary application-
      level measures to provide a certain level of security.
      Implementation: via GETAUTH.SCTP.
      Fall-back to 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 a previously received segment.  Key material must
      be provided in a way that is compatible with both [RFC4895] and
      [RFC5925].






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   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
      Functional because it affects "Obtain a message delivery number",
      which is functional.
      Implementation: via RESETASSOC.SCTP.
      Fall-back to TCP: not possible.

   o  Notification of Association Reset
      Protocols: STCP
      Functional because it affects "Obtain a message delivery number",
      which is functional.
      Implementation: via RESETASSOC-EVENT.SCTP.
      Fall-back to TCP: not possible.

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

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

   o  Choose a scheduler to operate between streams of an association
      Protocols: SCTP
      Optimizing because the scheduling decision requires application-
      specific knowledge.  However, if a TAPS system would not use this,
      or wrongly configure it on its own, this would only affect the
      performance of data transfers; the outcome would still be correct
      within the "best effort" service model.
      Implementation: using SETSTREAMSCHEDULER.SCTP.
      Fall-back to TCP: do nothing.




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   o  Configure priority or weight for a scheduler
      Protocols: SCTP
      Optimizing because the priority or weight requires application-
      specific knowledge.  However, if a TAPS system would not use this,
      or wrongly configure it on its own, this would only affect the
      performance of data transfers; the outcome would still be correct
      within the "best effort" service model.
      Implementation: using CONFIGURESTREAMSCHEDULER.SCTP.
      Fall-back to TCP: do nothing.

   o  Configure send buffer size
      Protocols: SCTP
      Automatable because this decision relates to knowledge about the
      network and the Operating System, not the application (see also
      the discussion in Appendix A.3.4).

   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 fragmentation relates to knowledge about the
      network and the Operating System, not the application.
      Implementation: by always enabling it with
      CONFIG_FRAGMENTATION.SCTP and auto-setting the fragmentation size
      based on network or 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 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.



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

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

   o  Configure size where messages are broken up for partial delivery
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Fall-back to TCP: do nothing.  Since TCP does not deliver
      messages, partial or not, this will have no effect on TCP.

   o  Disable checksum when sending
      Protocols: UDP
      Functional because application-specific knowledge is necessary to
      decide whether it can be acceptable to lose data integrity.
      Implementation: via SET_CHECKSUM_ENABLED.UDP.
      Fall-back to TCP: do nothing.

   o  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.
      Implementation: via SET_CHECKSUM_REQUIRED.UDP.
      Fall-back to TCP: do nothing.

   o  Specify checksum coverage used by the sender
      Protocols: UDP-Lite
      Functional because application-specific knowledge is necessary to
      decide for which parts of the data it can be acceptable to lose
      data integrity.
      Implementation: via SET_CHECKSUM_COVERAGE.UDP-Lite.
      Fall-back to TCP: do nothing.

   o  Specify minimum checksum coverage required by receiver
      Protocols: UDP-Lite
      Functional because application-specific knowledge is necessary to
      decide for which parts of the data it can be acceptable to lose
      data integrity.



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      Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite.
      Fall-back to TCP: do nothing.

   o  Specify DF field
      Protocols: UDP(-Lite)
      Optimizing because the DF field can be used to carry out Path MTU
      Discovery, which can lead an application to choose message sizes
      that can be transmitted more efficiently.
      Implementation: via MAINTENANCE.SET_DF.UDP(-Lite) and
      SEND_FAILURE.UDP(-Lite).
      Fall-back to TCP: do nothing.  With TCP the sender is not in
      control of transport message sizes, making this functionality
      irrelevant.

   o  Get max. transport-message size that may be sent using a non-
      fragmented IP packet from the configured interface
      Protocols: UDP(-Lite)
      Optimizing because this can lead an application to choose message
      sizes that can be transmitted more efficiently.

   o  Get max. transport-message size that may be received from the
      configured interface
      Protocols: UDP(-Lite)
      Optimizing because this can, for example, influence an
      application's memory management.

   o  Specify TTL/Hop count field
      Protocols: UDP(-Lite)
      Automatable because a TAPS system can use a large enough system
      default to avoid communication failures.  Allowing an application
      to configure it differently can produce notifications of ICMP
      error message arrivals that yield information 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 network, not the application.

   o  Specify ECN field
      Protocols: UDP(-Lite)
      Automatable because the ECN field relates to knowledge about the
      network, not the application.

   o  Obtain ECN field
      Protocols: UDP(-Lite)
      Optimizing because this information can be used by an application
      to better carry out congestion control (this is relevant when



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      choosing a data transmission transport service that does not
      already do congestion control).

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

   o  Obtain IP Options
      Protocols: UDP(-Lite)
      Automatable because IP Options relate 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 data transfers (albeit
      including other transfers that may compete with the TAPS transfer
      in the network), so it is still correct within the "best effort"
      service model.
      Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP /
      SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [LBE-draft].
      Fall-back to TCP: do nothing.


   TERMINATION:
   o  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side
      Protocols: TCP, SCTP
      Functional because the notion of a connection is often reflected
      in applications as an expectation to have all outstanding data
      delivered and no longer be able to communicate after a "Close"
      succeeded, with a communication sequence relating to this
      transport feature that is defined by the application protocol.
      Implementation: via CLOSE.TCP and CLOSE.SCTP.

   o  Abort without delivering remaining data, causing an event
      informing the application on the other side
      Protocols: TCP, SCTP
      Functional because the notion of a connection is often reflected
      in applications as an expectation to potentially not have all
      outstanding data delivered and no longer be able to communicate
      after an "Abort" succeeded.  On both sides of a connection, an
      application protocol may define a communication sequence relating
      to this transport feature.
      Implementation: via ABORT.TCP and ABORT.SCTP.



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   o  Abort without delivering remaining data, not causing an event
      informing the application on the other side
      Protocols: UDP(-Lite)
      Functional because the notion of a connection is often reflected
      in applications as an expectation to potentially not have all
      outstanding data delivered and no longer be able to communicate
      after an "Abort" succeeded.  On both sides of a connection, an
      application protocol may define a communication sequence relating
      to this transport feature.
      Implementation: via ABORT.UDP(-Lite).
      Fall-back to TCP: stop using the connection, wait for a timeout.

   o  Timeout event when data could not be delivered for too long
      Protocols: TCP, SCTP
      Functional because this notifies that potentially assumed reliable
      data delivery is no longer provided.
      Implementation: via TIMEOUT.TCP and TIMEOUT.SCTP.


A.1.2.  DATA Transfer Related Transport Features

A.1.2.1.  Sending Data

   o  Reliably transfer data, with congestion control
      Protocols: TCP, SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via SEND.TCP and SEND.SCTP.

   o  Reliably transfer a message, with congestion control
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via SEND.SCTP and SEND.TCP.  With SEND.TCP,
      messages will not be identifiable by the receiver.  Inform the
      application of the result.

   o  Unreliably transfer a message
      Protocols: SCTP, UDP(-Lite)
      Optimizing because only applications know about the time
      criticality of their communication, and reliably transfering a
      message is never incorrect for the receiver of a potentially
      unreliable data transfer, it is just slower.
      ADDED.  This differs from the 2 automatable transport features
      below in that it leaves the choice of congestion control open.
      Implementation: via SEND.SCTP or SEND.UDP or SEND.TCP.  With
      SEND.TCP, messages will not be identifiable by the receiver.
      Inform the application of the result.



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   o  Unreliably transfer a message, with congestion control
      Protocols: SCTP
      Automatable because congestion control relates to knowledge about
      the network, not the application.

   o  Unreliably transfer a message, without congestion control
      Protocols: UDP(-Lite)
      Automatable because congestion control relates to knowledge about
      the network, not the application.

   o  Configurable Message Reliability
      Protocols: SCTP
      Optimizing because only applications know about the time
      criticality of their communication, and reliably transfering a
      message is never incorrect for the receiver of a potentially
      unreliable data transfer, it is just slower.
      Implementation: via SEND.SCTP.
      Fall-back to 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.

   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.

   o  Choice of path (destination address)
      Protocols: SCTP
      Automatable because it requires using multiple sockets, but
      obtaining multiple sockets in the CONNECTION.ESTABLISHMENT
      category is automatable.

   o  Choice between unordered (potentially faster) or ordered delivery
      of messages
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via SEND.SCTP.
      Fall-back to 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 not possible to
      associate the requested delivery order to a "message" in TCP
      anyway.




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   o  Request not to bundle messages
      Protocols: SCTP
      Optimizing because this decision depends on knowledge about the
      size of future data blocks and the delay between them.
      Implementation: via SEND.SCTP.
      Fall-back to TCP: By using SEND.TCP and DISABLE-NAGLE.TCP to
      disable the Nagle algorithm when the request is made and enable it
      again when the request is no longer made.  Note that this is not
      fully equivalent because it relates to the time of issuing the
      request rather than a specific message.

   o  Specifying a "payload protocol-id" (handed over as such by the
      receiver)
      Protocols: SCTP
      Functional because it allows to send extra application data with
      every message, for the sake of identification of data, which by
      itself is application-specific.
      Implementation: SEND.SCTP.
      Fall-back to TCP: not possible.

   o  Specifying a key id to be used to authenticate a message
      Protocols: SCTP
      Functional because this has a direct influence on security.
      Implementation: via a parameter in SEND.SCTP.
      Fall-back to TCP: This could be emulated by using SET_AUTH.TCP
      before and after the message is sent.  Note that this is not fully
      equivalent because it relates to the time of issuing the request
      rather than a specific message.

   o  Request not to delay the acknowledgement (SACK) of a message
      Protocols: SCTP
      Optimizing because only an application knows for which message it
      wants to quickly be informed about success / failure of its
      delivery.
      Fall-back to TCP: do nothing.


A.1.2.2.  Receiving Data

   o  Receive data (with no message delineation)
      Protocols: TCP
      Functional because a TAPS system must be able to send and receive
      data.
      Implementation: via RECEIVE.TCP

   o  Receive a message
      Protocols: SCTP, UDP(-Lite)
      Functional because this is closely tied to properties of the data



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      that an application sends or expects to receive.
      Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite).
      Fall-back to TCP: not possible.

   o  Choice of stream to receive from
      Protocols: SCTP
      Automatable because it requires using multiple streams, but
      requesting multiple streams in the CONNECTION.ESTABLISHMENT
      category is automatable.
      Implementation: see Appendix A.3.2.

   o  Information about partial message arrival
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via RECEIVE.SCTP.
      Fall-back to TCP: do nothing: this information is not available
      with TCP.

   o  Obtain a message delivery number
      Protocols: SCTP
      Functional because this number can let applications detect and, if
      desired, correct reordering.  Whether messages are in the correct
      order or not is closely tied to properties of the data that an
      application sends or expects to receive.
      Implementation: via RECEIVE.SCTP.
      Fall-back to TCP: not possible.


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).

   o  Notification of send failures
      Protocols: SCTP, UDP(-Lite)
      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 and
      unacknowledged messages.
      Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(-
      Lite).
      Fall-back to TCP: do nothing: this notification is not available
      and will therefore not occur with TCP.






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   o  Notification of an unsent (part of a) message
      Protocols: SCTP, UDP(-Lite)
      Automatable because the distinction between unsent and
      unacknowledged is network-specific.

   o  Notification of an unacknowledged (part of a) message
      Protocols: SCTP
      Automatable because the distinction between unsent and
      unacknowledged is network-specific.

   o  Notification that the stack has no more user data to send
      Protocols: SCTP
      Optimizing because reacting to this notification requires the
      application to be involved, and ensuring that the stack does not
      run dry of data (for too long) can improve performance.
      Fall-back to TCP: do nothing.  See also the discussion in
      Appendix A.3.4.

   o  Notification to a receiver that a partial message delivery has
      been aborted
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Fall-back to TCP: do nothing.  This notification is not available
      and will therefore not occur with TCP.


A.2.  Step 2: Reduction -- The Reduced Set of Transport Features

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

   A TAPS system should be able to fall back to TCP or UDP if
   alternative transport protocols are found not to work.  Here we only
   consider falling back to TCP.  For some transport features, it was
   identified that no fall-back to TCP is possible.  This eliminates the
   possibility to use TCP whenever an application makes use of one of
   these transport features.  Thus, we only keep the functional and



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   optimizing transport features for which a fall-back to TCP is
   possible in our reduced set.  "Reset Association" and "Notification
   of Association Reset" are only functional because of their
   relationship to "Obtain a message delivery number", which is
   functional.  Because "Obtain a message delivery number" does not have
   a fall-back to TCP, none of these three transport features are
   included in the reduced set.

A.2.1.  CONNECTION Related Transport Features

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

   AVAILABILITY:
   o  Listen
   o  Configure authentication

   MAINTENANCE:
   o  Change timeout for aborting connection (using retransmit limit or
      time value)
   o  Suggest timeout to the peer
   o  Disable Nagle algorithm
   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold)
   o  Specify DSCP field
   o  Notification of ICMP error message arrival
   o  Change authentication parameters
   o  Obtain authentication information
   o  Set Cookie life value
   o  Choose a scheduler to operate between streams of an association
   o  Configure priority or weight for a scheduler
   o  Configure size where messages are broken up for partial delivery
   o  Disable checksum when sending
   o  Disable checksum requirement when receiving
   o  Specify checksum coverage used by the sender
   o  Specify minimum checksum coverage required by receiver
   o  Specify DF field
   o  Get max. transport-message size that may be sent using a non-
      fragmented IP packet from the configured interface
   o  Get max. transport-message size that may be received from the
      configured interface




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   o  Obtain ECN field
   o  Enable and configure a "Low Extra Delay Background Transfer"

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

A.2.2.  DATA Transfer Related Transport Features

A.2.2.1.  Sending Data

   o  Reliably transfer data, with congestion control
   o  Reliably transfer a message, with congestion control
   o  Unreliably transfer a message
   o  Configurable Message Reliability
   o  Choice between unordered (potentially faster) or ordered delivery
      of messages
   o  Request not to bundle messages
   o  Specifying a key id to be used to authenticate a message
   o  Request not to delay the acknowledgement (SACK) of a message

A.2.2.2.  Receiving Data

   o  Receive data (with no message delineation)
   o  Information about partial message arrival

A.2.2.3.  Errors

   This section describes sending failures that are associated with a
   specific call to in the "Sending Data" category (Appendix A.1.2.1).

   o  Notification of send failures
   o  Notification that the stack has no more user data to send
   o  Notification 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 the following.






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A.3.1.  Sending Messages, Receiving Bytes

   There are several transport features related to sending, but only a
   single transport feature related to receiving: "Receive data (with no
   message delineation)" (and, strangely, "information about partial
   message arrival").  Notably, the transport feature "Receive a
   message" is also the only non-automatable transport feature of UDP(-
   Lite) that had to be removed because no fall-back to 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 the receiver side - data can
   be accepted via a normal TCP socket.

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

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

   Note that this usage of messages does not require all messages to be
   equal in size.  Many application protocols use some form of Type-
   Length-Value (TLV) encoding, e.g. by defining a header including
   length fields; another alternative is the use of byte stuffing
   methods such as COBS [COBS].  If an application needs message
   numbers, e.g. to restore the correct sequence of messages, these must
   also be encoded by the application itself, as the sequence number
   related transport features of SCTP are no longer provided (in the
   interest of enabling a fall-back to TCP).




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   !!!NOTE: IMPLEMENTATION DETAILS BELOW WILL BE MOVED TO A SEPARATE
   DRAFT IN A FUTURE VERSION.!!!

   For the implementation of a TAPS system, this has the following
   consequences:
   o  Because the receiver-side transport leaves it up to the
      application to delineate messages, messages must always remain
      intact as they are handed over by the transport receiver.  Data
      can be handed over at any time as they arrive, but the byte stream
      must never "skip ahead" to the beginning of the next message.
   o  With SCTP, a "partial flag" informs a receiving application that a
      message is incomplete.  Then, the next receive calls will only
      deliver remaining parts of the same message (i.e., no messages or
      partial messages will arrive on other streams until the message is
      complete) (see Section 8.1.20 in [RFC6458]).  This can facilitate
      the implementation of the receiver buffer in the receiving
      application, but then such an application does not support message
      interleaving (which is required by stream schedulers).  However,
      receiving a byte stream from multiple SCTP streams requires a per-
      stream receiver buffer anyway, so this potential benefit is lost
      and the "partial flag" (the transport feature "Information about
      partial message arrival") becomes unnecessary for a TAPS system.
      With it, the transport features "Configure size where messages are
      broken up for partial delivery" and "Notification to a receiver
      that a partial message delivery has been aborted" become
      unnecessary too.
   o  From the above, a TAPS system should always support message
      interleaving because it enables the use of stream schedulers and
      comes at no additional implementation cost on the receiver side.
      Stream schedulers operate on the sender side.  Hence, because a
      TAPS sender-side application may talk to an SCTP receiver that
      does not support interleaving, it cannot assume that stream
      schedulers will always work as expected.

A.3.2.  Stream Schedulers Without Streams

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



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   assocation.

   With only these semantics necessary to represent, the interface to a
   TAPS system becomes easier if we rename connections into "TAPS flows"
   (the TAPS equivalent of a connection which may be a transport
   connection or association, but could also become a stream of an
   existing SCTP association, for example) and allow assigning a "Group
   Number" to a TAPS flow.  Then, all MAINTENANCE transport features can
   be said to operate on flow groups, not connections, and a scheduler
   also operates on the flows within a group.

   !!!NOTE: IMPLEMENTATION DETAILS BELOW WILL BE MOVED TO A SEPARATE
   DRAFT IN A FUTURE VERSION.!!!

   For the implementation of a TAPS system, this has the following
   consequences:
   o  Streams may be identified in different ways across different
      protocols.  The only multi-streaming protocol considered in this
      document, SCTP, uses a stream id.  The transport association below
      still uses a Transport Address (which includes one port number)
      for each communicating endpoint.  To implement a TAPS system
      without exposed streams, an application must be given an
      identifier for each TAPS flow (akin to a socket), and depending on
      whether streams are used or not, there will be a 1:1 mapping
      between this identifier and local ports or not.
   o  In SCTP, a fixed number of streams exists from the beginning of an
      association; streams are not "established", there is no handshake
      or any other form of signaling to create them: they can just be
      used.  They are also not "gracefully shut down" -- at best, an
      "SSN Reset Request Parameter" in a "RE-CONFIG" chunk [RFC6525] can
      be used to inform the peer that of a "Stream Reset", as a rough
      equivalent of an "Abort".  This has an impact on the semantics
      connection establishment and teardown (see Section 3.1).
   o  To support stream schedulers, a receiver-side TAPS system should
      always support message interleaving because it comes at no
      additional implementation cost (because of the receiver-side
      stream reception discussed in Appendix A.3.1).  Note, however,
      that Stream schedulers operate on the sender side.  Hence, because
      a TAPS sender-side application may talk to a native TCP-based
      receiver-side application, it cannot assume that stream schedulers
      will always work as expected.

A.3.3.  Early Data Transmission

   There are two transport features related to transferring a message
   early: "Hand over a message to transfer (possibly multiple times)
   before connection establishment", which relates to TCP Fast Open
   [RFC7413], and "Hand over a message to transfer during connection



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   establishment", which relates to SCTP's ability to transfer data
   together with the COOKIE-Echo chunk.  Also without TCP Fast Open, TCP
   can transfer data during the handshake, together with the SYN packet
   -- however, the receiver of this data may not hand it over to the
   application until the handshake has completed.  This functionality is
   commonly available in TCP and supported in several implementations,
   even though the TCP specification does not explain how to provide it
   to applications.

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

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

A.3.4.  Sender Running Dry

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

   SCTP allows to configure the sender-side buffer too: the automatable
   Transport Feature "Configure send buffer size" provides this
   functionality, but only for the complete buffer, which includes both
   unsent and unacknowledged data.  SCTP does not allow to control these
   two sizes separately.  A TAPS system should allow for uniform access



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

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

A.3.6.  Security

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

   Security will be discussed in a separate TAPS document (to be
   referenced here when it appears).  The minimal set presented in the
   present document therefore excludes all security related transport
   features: "Configure authentication", "Change authentication
   parameters", "Obtain authentication information" and and "Set Cookie
   life value" as well as "Specifying a key id to be used to
   authenticate a message".

A.3.7.  Packet Size

   UDP(-Lite) has a transport feature called "Specify DF field".  This
   yields an error message in case of sending a message that exceeds the
   Path MTU, which is necessary for a UDP-based application to be able
   to implement Path MTU Discovery (a function that UDP-based



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   applications must do by themselves).  The "Get max. transport-message
   size that may be sent using a non-fragmented IP packet from the
   configured interface" transport feature yields an upper limit for the
   Path MTU (minus headers) and can therefore help to implement Path MTU
   Discovery more efficiently.

   This also relates to the fact that the choice of path is automatable:
   if a TAPS system can switch a path at any time, unknown to an
   application, yet the application intends to do Path MTU Discovery,
   this could yield a very inefficient behavior.  Thus, a TAPS system
   should probably avoid automatically switching paths, and inform the
   application about any unavoidable path changes, when applications
   request to disallow fragmentation with the "Specify DF field"
   feature.


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 [TAPS2] except
   that MPTCP is not included.

   -03: updated to be consistent with -02 version of [TAPS2].

   -04: updated to be consistent with -03 version of [TAPS2].
   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 [TAPS2] (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.














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Authors' Addresses

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

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


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

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































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