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In: MissingRef
TAPS                                                            M. Welzl
Internet-Draft                                               S. Gjessing
Intended status: Informational                        University of Oslo
Expires: August 10, 2018                                February 6, 2018


          A Minimal Set of Transport Services for TAPS Systems
                       draft-ietf-taps-minset-01

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-09.

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 https://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 August 10, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  The Minimal Set of Transport Features . . . . . . . . . . . .   5
     3.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION . . . . . . .   5
     3.2.  MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  DATA Transfer . . . . . . . . . . . . . . . . . . . . . .   9
       3.3.1.  Sending Data  . . . . . . . . . . . . . . . . . . . .   9
       3.3.2.  Receiving Data  . . . . . . . . . . . . . . . . . . .  10
   4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION . . . . . . .  11
     4.2.  MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . .  12
       4.2.1.  Connection groups . . . . . . . . . . . . . . . . . .  12
       4.2.2.  Individual connections  . . . . . . . . . . . . . . .  13
     4.3.  DATA Transfer . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Deriving the minimal set . . . . . . . . . . . . . .  18
     A.1.  Step 1: Categorization -- The Superset of Transport
           Features  . . . . . . . . . . . . . . . . . . . . . . . .  19
       A.1.1.  CONNECTION Related Transport Features . . . . . . . .  20
       A.1.2.  DATA Transfer Related Transport Features  . . . . . .  36
     A.2.  Step 2: Reduction -- The Reduced Set of Transport
           Features  . . . . . . . . . . . . . . . . . . . . . . . .  41
       A.2.1.  CONNECTION Related Transport Features . . . . . . . .  42
       A.2.2.  DATA Transfer Related Transport Features  . . . . . .  43
     A.3.  Step 3: Discussion  . . . . . . . . . . . . . . . . . . .  43
       A.3.1.  Sending Messages, Receiving Bytes . . . . . . . . . .  44
       A.3.2.  Stream Schedulers Without Streams . . . . . . . . . .  46
       A.3.3.  Early Data Transmission . . . . . . . . . . . . . . .  47
       A.3.4.  Sender Running Dry  . . . . . . . . . . . . . . . . .  48
       A.3.5.  Capacity Profile  . . . . . . . . . . . . . . . . . .  48
       A.3.6.  Security  . . . . . . . . . . . . . . . . . . . . . .  49
       A.3.7.  Packet Size . . . . . . . . . . . . . . . . . . . . .  49
   Appendix B.  Revision information . . . . . . . . . . . . . . . .  49
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

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.



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   Currently, the set of transport services that most applications use
   is based on TCP and UDP (and protocols running on top of them); 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] as
   well as [TAPS2UDP], which identify the specific transport features
   that are exposed to applications by the protocols TCP, MPTCP, UDP(-
   Lite) and SCTP as well as the LEDBAT congestion control mechanism.
   The present draft is based on these documents and follows the same
   terminology (also listed below).  Because the considered transport
   protocols together cover a wide range of transport features, there is
   reason to hope that the resulting set (and the reasoning that led to
   it) will also apply to many aspects of other transport protocols such
   as QUIC.

   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 ways



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   for certain types of middleware to use this transport feature without
   exposing it, 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 over TCP (or UDP, if
   certain limitations are put in place).  This means that a sender-side
   TAPS system implementing it can talk to a non-TAPS TCP (or UDP)
   receiver, and a receiver-side TAPS system implementing it can talk to
   a non-TAPS TCP (or UDP) sender.

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.
   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.

   Moreover, throughout the document, the protocol name "UDP(-Lite)" is
   used when discussing transport features that are equivalent for UDP




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   and UDP-Lite; similarly, the protocol name "TCP" refers to both TCP
   and MPTCP.

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.  This TAPS system can be
   implemented over TCP; elements of the system that may prohibit
   implementation over UDP are marked with "!UDP".  To implement a TAPS
   system that can also work over UDP, these marked transport features
   should be excluded.

   As in Appendix A, Appendix A.2 and [TAPS2], we categorize the minimal
   set of transport features as 1) CONNECTION related (ESTABLISHMENT,
   AVAILABILITY, MAINTENANCE, TERMINATION) and 2) DATA Transfer related
   (Sending Data, Receiving Data, Errors).  Here, the focus is on "TAPS
   Connections": connections that the TAPS system offers, as opposed to
   connections of transport protocols that the TAPS system uses.

3.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION

   A TAPS connection must first be "created" to allow for some initial
   configuration to be carried out before the TAPS system can actively
   or passively establish a transport connection.  All configuration
   parameters in Section 3.2 and can be used initially, although some of
   them may only take effect when a transport connection has been
   established.  Configuring a connection early helps a TAPS system make
   the right decisions.  In particular, grouping information can
   influence the TAPS system to implement a TAPS connection as a stream
   of a multi-streaming protocol's existing association or not.

   For ungrouped TAPS connections, early configuration is necessary
   because it allows the TAPS system to know which protocols it should
   try to use (to steer a mechanism such as "Happy Eyeballs"
   [I-D.grinnemo-taps-he]).  In particular, a TAPS system that only
   makes a one-time choice for a particular protocol must know early
   about strict requirements that must be kept, or it can end up in a
   deadlock situation (e.g., having chosen UDP and later be asked to
   support reliable transfer).  As a possibility to correctly handle
   these cases, we provide the following decision tree (this is derived
   from Appendix A.2.1 excluding authentication, as explained in
   Section 8):








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   - Will it ever be necessary to offer any of the following?
     *  Reliably transfer data
     *  Notify the peer of closing/aborting
     *  Preserve data ordering

     Yes: SCTP or TCP can be used.
     - Is any of the following useful to the application?
       * Choosing a scheduler to operate between TAPS connections
         in a group, with the possibility to configure a priority
         or weight per connection
       * Configurable message reliability
       * Unordered message delivery
       * Request not to delay the acknowledgement (SACK) of a message

       Yes: SCTP is preferred.
       No:
       - Is any of the following useful to the application?
         * Hand over a message to reliably transfer (possibly
           multiple times) before connection establishment
         * Suggest timeout to the peer
         * Notification of Excessive Retransmissions (early
           warning below abortion threshold)
         * Notification of ICMP error message arrival

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


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

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


   Note that this decision tree is not optimal for all cases.  For
   example, if an application wants to use "Specify checksum coverage
   used by the sender", which is only offered by UDP-Lite, and
   "Configure priority or weight for a scheduler", which is only offered
   by SCTP, the above decision tree will always choose UDP-Lite, making
   it impossible to use SCTP's schedulers with priorities between
   grouped TAPS connections.  The TAPS system must know which choice is
   more important for the application in order to make the best
   decision.  We caution implementers to be aware of the full set of
   trade-offs, for which we recommend consulting the list in
   Appendix A.2.1 when deciding how to initialize a TAPS connection.



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   Once a TAPS connection is created, it can be queried for the maximum
   amount of data that an application can possibly expect to have
   reliably transmitted before or during transport connection
   establishment (with zero being a possible answer).  An application
   can also give the TAPS connection a message for reliable transmission
   before or during connection establishment (!UDP); the TAPS system
   will then 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
   (because idempotent messages are reliably transferred, asking for
   idempotence is not necessary for systems that support UDP).

   After creation, a TAPS system can actively establish communication
   with a peer, or it can passively listen for incoming connection
   requests.  Note that "Establish" 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
   this by continuing to block a "Listen" call, immediately followed by
   issuing "Receive", for example; callback-based implementations could
   simply skip the equivalent of "Listen").  This also means that the
   active opening side is assumed to be the first side sending data.

   A 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 message transmission
   without congestion control (i.e., UDP-style transfer), closing a
   connection is guaranteed to cause an event to notify the peer
   application that the connection has been closed (!UDP).  Similarly,
   for anything but (UDP-style) unreliable non-congestion-controlled
   data transfer, aborting a connection will cause an event to notify
   the peer application that the connection has been aborted (!UDP).  A
   timeout can be configured to abort a TAPS connection when data could
   not be delivered for too long (!UDP); however, timeout-based abortion
   does not notify the peer application that the connection has been
   aborted.  Because half-closed connections are not supported, when a
   TAPS host receives a notification that the peer is closing or
   aborting the connection (!UDP), its peer 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 "close" or "abort" notification from the peer.








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3.2.  MAINTENANCE

   A TAPS connection group can be configured with a number of transport
   features, and there are some notifications to applications about a
   connection group.  The following transport features and notifications
   from Appendix A.2 automatically apply to grouped TAPS connections
   (e.g., when a TAPS connection 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) (!UDP)
   o  Suggest timeout to the peer (!UDP)
   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 connections 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
      of overhead" (which would disable any Nagle-like algorithm),
      "scavenger", and values that help determine the DSCP value for a
      connection (e.g. similar to table 1 in
      [I-D.ietf-tsvwg-rtcweb-qos]).



   The following transport features and notifications from Appendix A.2
   only apply to a single TAPS connection:

   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




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

3.3.  DATA Transfer

3.3.1.  Sending Data

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

   Here we list per-message properties that a sender can optionally
   configure if it hands over a delimited message for sending with
   congestion control (!UDP), taken from Appendix A.2:

   o  Configurable Message Reliability
   o  Ordered message delivery (potentially slower than unordered)
   o  Unordered message delivery (potentially faster than ordered)
   o  Request not to bundle messages
   o  Request not to delay the acknowledgement (SACK) of a message

   Additionally, an application can hand over delimited messages for
   unreliable transmission without congestion control (note that such
   applications should perform congestion control in accordance with
   [RFC2914]).  Then, none of the per-message 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):

   o  Get max. transport message size that may be sent without
      fragmentation from the configured interface
      This is optional for a TAPS system to offer, and may return an
      error ("not available").  It can aid applications implementing
      Path MTU Discovery.


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




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      This is optional for a TAPS system to offer, and may return an
      error ("not available").


   o  Get maximum transport message 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 message
      in an Application-Framed-Bytestream (see Appendix A.3.1).  This
      may also return an error when data is not delimited ("not
      available").



   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 message.


   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.

3.3.2.  Receiving Data

   A receiving application obtains an "Application-Framed Bytestream"
   (AFra-Bytestream); this concept is further described in
   Appendix A.3.1).  In line with TCP's receiver semantics, an AFra-



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   Bytestream is just a stream of bytes to the receiver.  If message
   boundaries were specified by the sender, a receiver-side TAPS system
   implementing only the minimum set of transport services defined here
   will still not inform the receiving application about them.  Within
   the bytestream, messages themselves will always stay intact (partial
   messages are not supported).  Different from TCP's semantics, there
   is no guarantee that all messages in the 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 message 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 messages or per-message reliability, messages within the
   receiver-side bytestream may be out-of-order or missing.

4.  Summary

   Here we summarize the minimum set of transport features in a more
   compact form.

4.1.  ESTABLISHMENT, AVAILABILITY and TERMINATION

   A TAPS connection is created and associated with an existing or new
   TAPS connection group.  Grouping can influence the TAPS system to
   multiplex TAPS connections on a single transport connection or not,
   and the other parameters serve as input to the decision tree
   described in Section 3.1.  The TAPS systems gives no guarantees about
   honoring any of the requests at this stage, these parameters are just
   meant to help it choose and configure a suitable protocol.  Note that
   the parameters below affect all grouped TAPS connections.

   A TAPS connection can actively connect to a peer; this may or may not
   trigger a notification on the listening side.  If the application
   sends data (see Section 4.3) before the TAPS system establishes a
   transport connection, then such data may be transmitted early, upon
   connecting.  When a TAPS system listens for incoming connections, the
   first arriving message may already be the first block of data.

   Creation / connection / configuration parameters:

   reliability:  a boolean that should be set to true when any of the
      following will be useful to the application: reliably transfer
      data; notify the peer of closing/aborting; preserve data ordering.
   checksum_coverage:  a boolean to specify whether it will be useful to
      the application to specify checksum coverage when sending or
      receiving.
   config_msg_prio:  a boolean that should be set to true when any of
      the following per-message configuration or prioritization



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      mechanisms will be useful to the application: choosing a scheduler
      to operate between grouped connections, with the possibility to
      configure a priority or weight per connection; configurable
      message reliability; unordered message delivery; requesting not to
      delay the acknowledgement (SACK) of a message.
   earlymsg_timeout_notifications:  a boolean that should be set to true
      when any of the following will be useful to the application: hand
      over a message to reliably transfer (possibly multiple times)
      before connection establishment; suggest timeout to the peer;
      notification of excessive retransmissions (early warning below
      abortion threshold); notification of ICMP error message arrival.

   A TAPS connection can be closed after all outstanding data is
   reliably delivered to the peer (if reliable data delivery was
   requested earlier (!UDP)), in which case the peer is notified that
   the connection is closed.  Alternatively, a TAPS connection can be
   aborted without delivering outstanding data to the peer.  In case
   reliable or partially reliable data delivery was requested earlier
   (!UDP), the peer is notified that the connection is aborted.

4.2.  MAINTENANCE

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

4.2.1.  Connection groups

   The transport features below apply to all TAPS connections in the
   same group:

   (!UDP) Configure a timeout: this can be done with the following
   parameters:

   o  A timeout value for aborting connections, in seconds
   o  A timeout value to be suggested to the peer (if possible), in
      seconds
   o  The number of retransmissions after which the application should
      be notifed of "Excessive Retransmissions"

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

   o  A number to identify the type of scheduler that should be used to
      operate between connections in the group (no guarantees given).
      Schedulers are defined in [RFC8260].
   o  A "capacity profile" number to identify how an application wants
      to use its available capacity.  Choices can be "lowest possible
      latency at the expense of overhead" (which would disable any



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      Nagle-like algorithm), "scavenger", or values that help determine
      the DSCP value for a connection (e.g.  similar to table 1 in
      [I-D.ietf-tsvwg-rtcweb-qos]).
   o  A buffer limit (in bytes); when the sender has less then
      low_watermark bytes in the buffer, the application may be
      notified.  Notifications are not guaranteed, and supporting
      watermark values greater than 0 is not guaranteed.

   The following properties can be queried:

   o  The maximum message size that may be sent without fragmentation,
      in bytes (or "not available")
   o  The maximum transport message size that can be sent, in bytes (or
      "not available")
   o  The maximum transport message size that can be received, in bytes
      (or "not available")
   o  The maximum amount of data that can possibly be sent before or
      during connection establishment, in bytes (or "not available")


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

   o  Excessive Retransmissions: the configured (or a default) number of
      retransmissions has been reached, yielding this early warning
      below an abortion threshold.
   o  ICMP Arrival (parameter: ICMP message): an ICMP packet carrying
      the conveyed ICMP message has arrived.
   o  ECN Arrival (parameter: ECN value): a packet carrying the conveyed
      ECN value has arrived.  This can be useful for applications
      implementing congestion control.
   o  Timeout (parameter: s seconds): data could not be delivered for s
      seconds.
   o  Drain: the send buffer has either drained below the configured low
      water mark or it has become completely empty.

4.2.2.  Individual connections

   The transport features below apply to individual TAPS connections:

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

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




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   o  A boolean to enable / disable usage of a checksum when sending
   o  The desired coverage (in bytes) of the checksum used when sending
   o  A boolean to enable / disable requiring a checksum when receiving
   o  The required minimum coverage (in bytes) of the checksum when
      receiving

4.3.  DATA Transfer

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

   o  (!UDP) Reliability: this parameter is used to convey a choice of:
      fully reliable, unreliable without congestion control (which is
      guaranteed), unreliable, partially reliable (see [RFC3758] and
      [RFC7496] for details on how to specify partial reliability).  The
      latter two choices are not guaranteed and may result in full
      reliability.
   o  (!UDP) Ordered: this boolean parameter lets an application choose
      between ordered message delivery (true) and possibly unordered,
      potentially faster message delivery (false).
   o  Bundle: a boolean that expresses a preference for allowing to
      bundle messages (true) or not (false).  No guarantees are given.
   o  DelAck: a boolean that, if false, lets an application request that
      the peer would not delay the acknowledgement for this message.
   o  Fragment: a boolean that expresses a preference for allowing to
      fragment messages (true) or not (false), at the IP level.  No
      guarantees are given.
   o  (!UDP) Idempotent: a boolean that expresses whether a message is
      idempotent (true) or not (false).  Idempotent messages may arrive
      multiple times at the receiver (but they will arrive at least
      once).  When data is idempotent it can be used by the receiver
      immediately on a connection establishment attempt.  Thus, if data
      is handed over before the TAPS system establishes a transport
      connection, stating that a message is idempotent facilitates
      transmitting it to the peer application particularly early.

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

   When receiving data blocks, these blocks may or may not correspond to
   a sender-side message, i.e. the receiving application is not informed
   about message boundaries (this limitation is only needed for TAPS



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   systems that are implemented to directly use TCP).  However, if the
   sending application has allowed that messages are not fully reliably
   transferred, or delivered out of order, then such re-ordering or
   unreliability may be reflected per message in the arriving data.
   Messages will always stay intact - i.e. if an incomplete message is
   contained at the end of the arriving data block, this message is
   guaranteed to continue in the next arriving data block.

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].

6.  Acknowledgements

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

7.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.








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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,
              <https://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",
              Internet-draft draft-ietf-taps-transports-usage-08, August
              2017.

   [TAPS2UDP]
              Fairhurst, G. and T. Jones, "Features of the User Datagram
              Protocol (UDP) and Lightweight UDP (UDP-Lite) Transport
              Protocols", Internet-draft draft-ietf-taps-transports-
              usage-udp-07, September 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.grinnemo-taps-he]
              Grinnemo, K., Brunstrom, A., Hurtig, P., Khademi, N., and
              Z. Bozakov, "Happy Eyeballs for Transport Selection",
              draft-grinnemo-taps-he-03 (work in progress), July 2017.

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



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   [I-D.pauly-taps-transport-security]
              Pauly, T., Rose, K., and C. Wood, "A Survey of Transport
              Security Protocols", draft-pauly-taps-transport-
              security-01 (work in progress), January 2018.

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

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

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

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

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

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

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

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

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




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

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

   [RFC8260]  Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol", RFC 8260,
              DOI 10.17487/RFC8260, November 2017,
              <https://www.rfc-editor.org/info/rfc8260>.

   [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. !!!TODO discuss UDP
   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.








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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
   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,
   ordered message delivery is a functional transport feature: it cannot
   be configured without the application knowing about it because the
   application's assumption could be that messages always arrive in
   order.  Failure includes any change of the application behavior that
   is not performance oriented, e.g. security.

   "Change DSCP" and "Disable Nagle algorithm" are examples of transport
   features that we call "Optimizing": if a 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".





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   Finally, some transport features are aggregated and/or slightly
   changed in the description below.  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].  We also sketch how some of the
   TAPS transport features can be implemented by a TAPS system.  For all
   transport features that are categorized as "functional" or
   "optimizing", and for which no matching TCP and/or UDP primitive
   exists in "pass 2" of [TAPS2], a brief discussion on how to implement
   them over TCP and/or UDP is included.

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

   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



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


   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



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      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.



   o  Configure authentication
      Protocols: TCP, SCTP
      Functional because this has a direct influence on security.
      Implementation: via parameters in CONNECT.TCP and CONNECT.SCTP.
      Implementation over 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].
      Implementation over UDP: Not possible.


   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.
      Implementation over TCP: not possible.
      Implementation over UDP: 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 CONNECT.SCTP.



   o  Hand over a message to reliably transfer (possibly multiple times)
      before connection establishment
      Protocols: TCP



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      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via a parameter in CONNECT.TCP.
      Implementation over UDP: not possible.


   o  Hand over a message to reliably transfer during connection
      establishment
      Protocols: SCTP
      Functional because this can only work if the message is limited in
      size, making it closely tied to properties of the data that an
      application sends or expects to receive.
      Implementation: via a parameter in CONNECT.SCTP.
      Implementation over UDP: not possible.


   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).  LISTEN.UDP(-
      Lite) supports both methods.


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




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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.
      Implementation over 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].
      Implementation over UDP: not possible.


   o  Obtain requested number of streams



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      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.


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



   o  Suggest timeout to the peer



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      Protocols: TCP
      Functional because this is closely related to potentially assumed
      reliable data delivery.
      Implementation: via CHANGE-TIMEOUT.TCP.
      Implementation over UDP: not possible (UDP is unreliable and there
      is no connection timeout).



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



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



   o  Add path
      Protocols: MPTCP, SCTP
      MPTCP Parameters: source-IP; source-Port; destination-IP;
      destination-Port
      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



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      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
      network, not the application.




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   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.
      Implementation over 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].
      Implementation over UDP: not possible.



   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.




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      Implementation over TCP: With SCTP, this allows to obtain key_id
      and a chunk list.  With TCP, this allows to obtain current_key and
      rnext_key from a previously received segment.  Key material must
      be provided in a way that is compatible with both [RFC4895] and
      [RFC5925].
      Implementation over UDP: not possible.



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


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


   o  Reset Association
      Protocols: SCTP
      Automatable because deciding to reset an association does not
      require application-specific knowledge.
      Implementation: via RESETASSOC.SCTP.



   o  Notification of Association Reset
      Protocols: STCP
      Automatable because this notification does not relate to
      application-specific knowledge.



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


   o  Notification of Added Stream
      Protocols: STCP




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      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.
      Implementation over TCP: do nothing.
      Implementation over UDP: do nothing.



   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.
      Implementation over TCP: do nothing.
      Implementation over UDP: 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



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      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.
      Implementation over TCP: the closest specified TCP functionality
      is the cookie in TCP Fast Open; for this, [RFC7413] states that
      the server "can expire the cookie at any time to enhance security"
      and section 4.1.2 describes an example implementation where
      updating the key on the server side causes the cookie to expire.
      Alternatively, for implementations that do not support TCP Fast
      Open, this transport feature could also affect the validity of SYN
      cookies (see Section 3.6 of [RFC4987]).
      Implementation over UDP: do nothing.



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




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   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.
      Implementation over TCP: not possible.
      Implementation over UDP: not possible.



   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.
      Implementation over 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.
      Implementation over 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.
      Implementation over 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.
      Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite.
      Implementation over 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.



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      Implementation: via MAINTENANCE.SET_DF.UDP(-Lite) and
      SEND_FAILURE.UDP(-Lite).
      Implementation over 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.
      Implementation over TCP: do nothing: this information is not
      available with TCP.



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



   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.



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   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
      choosing a data transmission transport service that does not
      already do congestion control).
      Implementation over TCP: do nothing: this information is not
      available with TCP.



   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].
      Implementation over TCP: do nothing.
      Implementation over UDP: do nothing.




   TERMINATION:

   o  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side
      Protocols: TCP, SCTP




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      Functional because the notion of a connection is often reflected
      in applications as an expectation to have all outstanding data
      delivered and no longer be able to communicate after a "Close"
      succeeded, with a communication sequence relating to this
      transport feature that is defined by the application protocol.
      Implementation: via CLOSE.TCP and CLOSE.SCTP.
      Implementation over UDP: not possible.



   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.
      Implementation over UDP: not possible.



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





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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.
      Implementation over UDP: not possible.



   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.
      Implementation over TCP: via SEND.TCP.  With SEND.TCP, messages
      will not be identifiable by the receiver.
      Implementation over UDP: not possible.



   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(-Lite).
      Implementation over TCP: use SEND.TCP.  With SEND.TCP, messages
      will be sent reliably, and they will not be identifiable by the
      receiver.



   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)



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



   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  Ordered message delivery (potentially slower than unordered)
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via SEND.SCTP.
      Implementation over TCP: By using SEND.TCP.  With SEND.TCP,
      messages will not be identifiable by the receiver.
      Implementation over UDP: not possible.



   o  Unordered message delivery (potentially faster than ordered)



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      Protocols: SCTP, UDP(-Lite)
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via SEND.SCTP.
      Implementation over TCP: 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.



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



   o  Specifying a "payload protocol-id" (handed over as such by the
      receiver)
      Protocols: SCTP
      Functional because it allows to send extra application data with
      every message, for the sake of identification of data, which by
      itself is application-specific.
      Implementation: SEND.SCTP.
      Implementation over TCP: not possible.
      Implementation over UDP: 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.
      Implementation over TCP: This could be emulated by using
      SET_AUTH.TCP before and after the message is sent.  Note that this
      is not fully equivalent because it relates to the time of issuing
      the request rather than a specific message.
      Implementation over UDP: not possible.



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   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.
      Implementation over TCP: do nothing.
      Implementation over UDP: do nothing.




A.1.2.2.  Receiving Data

   o  Receive data (with no message delimiting)
      Protocols: TCP
      Functional because a TAPS system must be able to send and receive
      data.
      Implementation: via RECEIVE.TCP.
      Implementation over UDP: do nothing (hand over a message, let the
      application ignore message boundaries).



   o  Receive a message
      Protocols: SCTP, UDP(-Lite)
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite).
      Implementation over TCP: not possible.



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




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




A.1.2.3.  Errors

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

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



   o  Notification of an unsent (part of a) message
      Protocols: SCTP, UDP(-Lite)
      Automatable because the distinction between unsent and
      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.
      Implementation over TCP: do nothing.  See also the discussion in
      Appendix A.3.4.
      Implementation over UDP: do nothing.  This notification is not
      available and will therefore not occur with UDP.



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   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.
      Implementation over TCP: do nothing.  This notification is not
      available and will therefore not occur with TCP.
      Implementation over UDP: do nothing.  This notification is not
      available and will therefore not occur with UDP.




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

   In the following list, we precede a transport feature with "T:" if an
   implementation over TCP is possible, "U:" if an implementation over
   UDP is possible, and "TU:" if an implementation over either TCP or
   UDP is possible.







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A.2.1.  CONNECTION Related Transport Features

   ESTABLISHMENT:

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

   AVAILABILITY:

   o  T,U: Listen
   o  T: Configure authentication

   MAINTENANCE:

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

   TERMINATION:




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   o  T: Close after reliably delivering all remaining data, causing an
      event informing the application on the other side
   o  T: Abort without delivering remaining data, causing an event
      informing the application on the other side
   o  T,U: Abort without delivering remaining data, not causing an event
      informing the application on the other side
   o  T,U: 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  T: Reliably transfer data, with congestion control
   o  T: Reliably transfer a message, with congestion control
   o  T,U: Unreliably transfer a message
   o  T: Configurable Message Reliability
   o  T: Ordered message delivery (potentially slower than unordered)
   o  T,U: Unordered message delivery (potentially faster than ordered)
   o  T,U: Request not to bundle messages
   o  T: Specifying a key id to be used to authenticate a message
   o  T,U: Request not to delay the acknowledgement (SACK) of a message

A.2.2.2.  Receiving Data

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

A.2.2.3.  Errors

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

   o  T,U: Notification of send failures
   o  T,U: Notification that the stack has no more user data to send
   o  T,U: 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.  This section
   focuses on TCP because, with the exception of one particular
   transport feature ("Receive a message" -- we will discuss this in
   Appendix A.3.1), the list shows that UDP is strictly a subset of TCP.
   We can first try to understand how to build a TAPS system that can
   run over TCP, and then narrow down the result further to allow that
   the system can always run over either TCP or UDP (which effectively



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   means removing everything related to reliability, ordering,
   authentication and closing/aborting with a notification to the peer).

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

A.3.1.  Sending Messages, Receiving Bytes

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

   To support these TCP receiver semantics, we define an "Application-
   Framed 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 message boundaries and required properties per
   message (configurable order and reliability, or embedding a request
   not to delay the acknowledgement of a message).  Whenever the sending
   application specifies per-message properties that relax the notion of
   reliable in-order delivery of bytes, it must assume that the
   receiving application is 1) able to determine message boundaries,
   provided that messages are always kept intact, and 2) able to accept
   these relaxed per-message properties.  Any signaling of such
   information to the peer is up to an application-layer protocol and
   considered out of scope of this document.

   For example, if an application requests to transfer fixed-size
   messages of 100 bytes with partial reliability, this needs the
   receiving application to be prepared to accept data in chunks of 100
   bytes.  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




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   model [RFC7305].  Still, the receiving application would separate the
   byte stream into 100-byte chunks.

   Note that this usage of messages does not require all messages to be
   equal in size.  Many application protocols use some form of Type-
   Length-Value (TLV) encoding, e.g. by defining a header including
   length fields; another alternative is the use of byte stuffing
   methods such as 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 not provided by the "minimum
   set" (in the interest of enabling usage of TCP).

   !!!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 delimit 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 feature "Notification to a receiver that a
      partial message delivery has been aborted" becomes 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.





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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 of an SCTP association versus using two
   separate SCTP associations or TCP connections are related to
   knowledge about the network and the particular transport protocol in
   use, not the application.  However, the transport features "Choose a
   scheduler to operate between streams of an association" and
   "Configure priority or weight for a scheduler" operate on streams.
   Here, streams identify communication channels between which a
   scheduler operates, and they can be assigned a priority.  Moreover,
   the transport features in the MAINTENANCE category all operate on
   assocations in case of SCTP, i.e. they apply to all streams in that
   assocation.

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



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   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.

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

A.3.3.  Early Data Transmission

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

   A 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 messages that are explicitly marked
   as "idempotent" (i.e., it would be acceptable to transfer them
   multiple times).

   The amount of data that can successfully be transmitted before or
   during the handshake depends on various factors: the transport
   protocol, the use of header options, the choice of IPv4 and IPv6 and
   the Path MTU.  A TAPS system should therefore allow a sending



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   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 that was 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.  It therefore makes sense for a TAPS system to
   allow for uniform access to "TCP_NOTSENT_LOWAT" as well as the
   "SENDER DRY" notification.

A.3.5.  Capacity Profile

   The transport features:

   o  Disable Nagle algorithm
   o  Enable 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.




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   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 is discussed in a separate TAPS document
   [I-D.pauly-taps-transport-security].  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
   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.

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].





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   -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.

   draft-ietf-taps-minset-00: updated to be consistent with -08 version
   of [TAPS2] ("obtain message delivery number" was removed, as this has
   also been removed in [TAPS2] because it was a mistake in RFC4960.
   This led to the removal of two more transport features that were only
   designated as functional because they affected "obtain message
   delivery number").  Fall-back to UDP incorporated (this was requested
   at IETF-99); this also affected the transport feature "Choice between
   unordered (potentially faster) or ordered delivery of messages"
   because this is a boolean which is always true for one fall-back
   protocol, and always false for the other one.  This was therefore now
   divided into two features, one for ordered, one for unordered
   delivery.  The word "reliably" was added to the transport features
   "Hand over a message to reliably transfer (possibly multiple times)
   before connection establishment" and "Hand over a message to reliably
   transfer during connection establishment" to make it clearer why this
   is not supported by UDP.  Clarified that the "minset abstract
   interface" is not proposing a specific API for all TAPS systems to
   implement, but it is just a way to describe the minimum set.  Author
   order changed.

   draft-ietf-taps-minset-01: "fall-back to" (TCP or UDP) replaced
   (mostly with "implementation over").  References to post-sockets
   removed (these were statments that assumed that post-sockets requires
   two-sided implementation).  Replaced "flow" with "TAPS Connection"
   and "frame" with "message" to avoid introducing new terminology.
   Made sections 3 and 4 in line with the categorization that is already
   used in the appendix and [TAPS2], and changed style of section 4 to
   be even shorter and less interface-like.  Updated reference draft-
   ietf-tsvwg-sctp-ndata to RFC8260.

Authors' Addresses









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

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


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

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

































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