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TAPS                                                         S. Gjessing
Internet-Draft                                                  M. Welzl
Intended status: Informational                        University of Oslo
Expires: September 14, 2017                               March 13, 2017


          A Minimal Set of Transport Services for TAPS Systems
                     draft-gjessing-taps-minset-04

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-03.

Status of this Memo

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

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

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

   This Internet-Draft will expire on September 14, 2017.

Copyright Notice

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

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



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Step 1: Categorization -- The Superset of Transport
       Features . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  CONNECTION Related Transport Features  . . . . . . . . . .  7
     3.2.  DATA Transfer Related Transport Features . . . . . . . . . 18
       3.2.1.  Sending Data . . . . . . . . . . . . . . . . . . . . . 18
       3.2.2.  Receiving Data . . . . . . . . . . . . . . . . . . . . 20
       3.2.3.  Errors . . . . . . . . . . . . . . . . . . . . . . . . 21
   4.  Step 2: Reduction -- The Reduced Set of Transport Features . . 22
     4.1.  CONNECTION Related Transport Features  . . . . . . . . . . 23
     4.2.  DATA Transfer Related Transport Features . . . . . . . . . 24
       4.2.1.  Sending Data . . . . . . . . . . . . . . . . . . . . . 24
       4.2.2.  Receiving Data . . . . . . . . . . . . . . . . . . . . 24
       4.2.3.  Errors . . . . . . . . . . . . . . . . . . . . . . . . 24
   5.  Step 3: Discussion . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Sending Messages, Receiving Bytes  . . . . . . . . . . . . 24
     5.2.  Stream Schedulers Without Streams  . . . . . . . . . . . . 26
     5.3.  Early Data Transmission  . . . . . . . . . . . . . . . . . 27
     5.4.  Sender Running Dry . . . . . . . . . . . . . . . . . . . . 27
     5.5.  Capacity Profile . . . . . . . . . . . . . . . . . . . . . 28
     5.6.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 28
     5.7.  Packet Size  . . . . . . . . . . . . . . . . . . . . . . . 29
   6.  Step 4: Construction -- the Minimal Set of Transport
       Features . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.1.  Flow Creation, Connection and Termination  . . . . . . . . 30
     6.2.  Flow Group Configuration . . . . . . . . . . . . . . . . . 31
     6.3.  Flow Configuration . . . . . . . . . . . . . . . . . . . . 32
     6.4.  Data Transfer  . . . . . . . . . . . . . . . . . . . . . . 32
       6.4.1.  The Sender . . . . . . . . . . . . . . . . . . . . . . 32
       6.4.2.  The Receiver . . . . . . . . . . . . . . . . . . . . . 34
   7.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 35
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     11.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Revision information  . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38









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1.  Introduction

   An application has an intended usage and demands for transport
   services, and the task of any system that implements TAPS is to offer
   these services to its applications, i.e. the applications running on
   top of TAPS, without binding them to a particular transport protocol.
   Currently, the set of transport services that most applications use
   is based on TCP and UDP; this limits the ability for the network
   stack to make use of features of other protocols.  For example, if a
   protocol supports out-of-order message delivery but applications
   always assume that the network provides an ordered bytestream, then
   the network stack can never utilize out-of-order message delivery:
   doing so would break a fundamental assumption of the application.

   By exposing the transport services of multiple transport protocols, a
   TAPS system can make it possible to use these services without having
   to statically bind an application to a specific transport protocol.
   The first step towards the design of such a system was taken by
   [RFC8095], which surveys a large number of transports, and [TAPS2],
   which identifies the specific transport features that are exposed to
   applications by the protocols TCP, MPTCP, UDP(-Lite) and SCTP as well
   as the LEDBAT congestion control mechanism.  The present draft is
   based on these documents and follows the same terminology (also
   listed below).

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

   Applications use a wide variety of APIs today.  The point of this
   document is to identify transport features that 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".  Instead, the
   middleware would have to expose the "unordered message delivery"
   transport feature to its applications (alternatively, there may be



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   interesting ways for certain types of middleware to try to use some
   of the transport features that we describe here without exposing them
   to applications, 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 all of the transport
   features that are recommended as a "minimal set" here.  As an example
   considering only TCP and UDP, a middleware or library that only
   exposes TCP's reliable bytestream cannot make use of UDP (unless it
   implements extra functionality on top of UDP) -- doing so could break
   a fundamental assumption that applications make about the data they
   send and receive.

   This document approaches the construction of a minimal set of
   transport features in the following way:
   1.  Categorization: the superset of transport features from [TAPS2]
       is presented, and transport features are categorized for later
       reduction.
   2.  Reduction: a shorter list of transport features is derived from
       the categorization in the first step.  This removes all transport
       features that do not require application-specific knowledge or
       cannot be implemented with TCP.
   3.  Discussion: the resulting list shows a number of peculiarities
       that are discussed, to provide a basis for constructing the
       minimal set.
   4.  Construction: Based on the reduced set and the discussion of the
       transport features therein, a minimal set is constructed.


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.






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   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.


3.  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

   Because QoS is out of scope of TAPS, this document assumes a "best
   effort" service model [RFC5290], [RFC7305].  Applications using a
   TAPS system can therefore not make any assumptions about e.g. the
   time it will take to send a message.  We also 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,
   unordered message delivery is a functional transport feature: it
   cannot be used without the application knowing about it because the
   application's assumption could be that messages arrive in order.



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   Failure includes any change of the application behavior that is not
   performance oriented, e.g. security.

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

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

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

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

   We designate some transport features as "automatable" on the basis of
   a broader decision that affects multiple transport features:
   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
      Section 5.2.



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   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.

3.1.  CONNECTION Related Transport Features

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

   o  Specify which IP Options must always be used
      Protocols: TCP
      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 Section 5.2.

   o  Limit the number of inbound streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

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

   o  Obtain multiple sockets
      Protocols: SCTP
      Automatable because the usage of multiple paths to communicate to



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      the same end host relates to knowledge about the network, not the
      application.

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

   o  Specify which chunk types must always be authenticated
      Protocols: SCTP
      Functional because this has a direct influence on security.
      Implementation: via a parameter in CONNECT.SCTP.
      Fall-back to TCP: TBD: this relates to the TCP Authentication
      Option in Section 7.1 of [RFC5925], which is not currently covered
      by [TAPS2].

   o  Indicate (and/or obtain upon completion) an Adaptation Layer via
      an adaptation code point
      Protocols: SCTP
      Functional because it allows to send extra data for the sake of
      identifying an adaptation layer, which by itself is application-
      specific.
      Implementation: via a parameter in CONNECT.SCTP.
      Fall-back to TCP: not possible.

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

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

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



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


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

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

   o  Listen, N specified local interfaces
      Protocols: SCTP, UDP(-Lite)
      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
      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



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      application.

   o  Specify which chunk types must always be authenticated
      Protocols: SCTP
      Functional because this has a direct influence on security.
      Implementation: via a parameter in CONNECT.SCTP.
      Fall-back to TCP: TBD: this relates to the TCP Authentication
      Option in Section 7.1 of [RFC5925], which is not currently covered
      by [TAPS2].

   o  Obtain requested number of streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Limit the number of inbound streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

   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.
      Fall-back to TCP: not possible.

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


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





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

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

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

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

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

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

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





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   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.

   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.





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   o  Change authentication parameters
      Protocols: SCTP
      Functional because this has a direct influence on security.
      Implementation: via SETAUTH.SCTP.
      Fall-back to TCP: TBD: this relates to the TCP Authentication
      Option in Section 7.1 of [RFC5925], which is not currently covered
      by [TAPS2].

   o  Obtain authentication information
      Protocols: SCTP
      Functional because authentication decisions may have been made by
      the peer, and this has an influence on the necessary application-
      level measures to provide a certain level of security.
      Implementation: via GETAUTH.SCTP.
      Fall-back to TCP: TBD: this relates to the TCP Authentication
      Option in Section 7.1 of [RFC5925], which is not currently covered
      by [TAPS2].

   o  Reset Stream
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

   o  Notification of Stream Reset
      Protocols: STCP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

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

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

   o  Add Streams
      Protocols: SCTP
      Automatable because using multi-streaming does not require
      application-specific knowledge.



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      Implementation: see Section 5.2.

   o  Notification of Added Stream
      Protocols: STCP
      Automatable because using multi-streaming does not require
      application-specific knowledge.
      Implementation: see Section 5.2.

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

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

   o  Configure send buffer size
      Protocols: SCTP
      Automatable because this decision relates to knowledge about the
      network and the Operating System, not the application (see also
      the discussion in Section 5.4).

   o  Configure receive buffer (and rwnd) size
      Protocols: SCTP
      Automatable because this decision relates to knowledge about the
      network and the Operating System, not the application.

   o  Configure message fragmentation
      Protocols: SCTP
      Automatable because 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.





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   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.
      Fall-back to TCP: the closest 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; however, this is
      different from this transport feature because SCTP's cookie life
      value is set on the client side, not the server side.  The TCP
      client has no control of this value.  Thus, the recommended fall-
      back implementation is to do nothing.

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

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

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




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   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 CHECKSUM_REQUIRED.UDP.
      Fall-back to TCP: do nothing.

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

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

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

   o  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.





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

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

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

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

   o  Enable and configure a "Low Extra Delay Background Transfer"
      Protocols: A protocol implementing the LEDBAT congestion control
      mechanism
      Optimizing because whether this service is appropriate or not
      depends on application-specific knowledge.  However, wrongly using
      this will only affect the speed of data transfers (albeit
      including other transfers that may compete with the TAPS transfer
      in the network), so it is still correct within the "best effort"
      service model.
      Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP /
      SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [LBE-draft].
      Fall-back to TCP: do nothing.


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

   o  Abort without delivering remaining data, causing an event
      informing the application on the other side
      Protocols: TCP, SCTP



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      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, with a communication sequence relating
      to this transport feature that is defined by the application
      protocol.
      Implementation: via ABORT.TCP and ABORT.SCTP.

   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.


3.2.  DATA Transfer Related Transport Features

3.2.1.  Sending Data

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

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

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

   o  Unreliably transfer a message, with congestion control
      Protocols: SCTP
      Automatable because congestion control relates to knowledge about



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      the network, not the application.

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

   o  Configurable Message Reliability
      Protocols: SCTP
      Optimizing because only applications know about the time
      criticality of their communication, and reliably transfering a
      message is never incorrect for the receiver of a potentially
      unreliable data transfer, it is just slower.
      Implementation: via SEND.SCTP.
      Fall-back to TCP: By using SEND.TCP and ignoring this
      configuration: based on the assumption of the best-effort service
      model, unnecessarily delivering data does not violate application
      expectations.  Moreover, it is not possible to associate the
      requested reliability to a "message" in TCP anyway.

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

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

   o  Choice between unordered (potentially faster) or ordered delivery
      of messages
      Protocols: SCTP
      Functional because this is closely tied to properties of the data
      that an application sends or expects to receive.
      Implementation: via SEND.SCTP.
      Fall-back to TCP: By using SEND.TCP and always sending data
      ordered: based on the assumption of the best-effort service model,
      ordered delivery may just be slower and does not violate
      application expectations.  Moreover, it is not possible to
      associate the requested delivery order to a "message" in TCP
      anyway.

   o  Request not to bundle messages
      Protocols: SCTP
      Optimizing because this decision depends on knowledge about the



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      size of future data blocks and the delay between them.
      Implementation: via SEND.SCTP.
      Fall-back to TCP: By using SEND.TCP and DISABLE-NAGLE.TCP to
      disable the Nagle algorithm when the request is made and enable it
      again when the request is no longer made.

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

   o  Specifying a key id to be used to authenticate a message
      Protocols: SCTP
      Functional because this has a direct influence on security.
      Implementation: via a parameter in SEND.SCTP.
      Fall-back to TCP: TBD: this relates to the TCP Authentication
      Option in Section 7.1 of [RFC5925], which is not currently covered
      by [TAPS2].

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


3.2.2.  Receiving Data

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

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






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   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 Section 5.2.

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

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


3.2.3.  Errors

   This section describes sending failures that are associated with a
   specific call to in the "Sending Data" category (Section 3.2.1).

   o  Notification of send failures
      Protocols: SCTP, UDP(-Lite)
      Functional because this notifies that potentially assumed reliable
      data delivery is no longer provided.
      ADDED.  This differs from the 2 automatable transport features
      below in that it does not distinugish between unsent and
      unacknowledged messages.
      Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(-
      Lite).
      Fall-back to TCP: do nothing: this notification is not available
      and will therefore not occur with TCP.

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





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

   o  Notification that the stack has no more user data to send
      Protocols: SCTP
      Optimizing because reacting to this notification requires the
      application to be involved, and ensuring that the stack does not
      run dry of data (for too long) can improve performance.
      Fall-back to TCP: do nothing.  See also the discussion in
      Section 5.4.

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



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

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

   A TAPS system should be able to fall back to TCP or UDP if
   alternative transport protocols are found not to work.  Here we only
   consider falling back to TCP.  For some transport features, it was
   identified that no fall-back to TCP is possible.  This eliminates the
   possibility to use TCP whenever an application makes use of one of
   these transport features.  Thus, we only keep the functional and
   optimizing transport features for which a fall-back to TCP is
   possible in our reduced set.  "Reset Association" and "Notification
   of Association Reset" are only functional because of their
   relationship to "Obtain a message delivery number", which is



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   functional.  Because "Obtain a message delivery number" does not have
   a fall-back to TCP, none of these three transport features are
   included in the reduced set.

4.1.  CONNECTION Related Transport Features

   ESTABLISHMENT:
   o  Connect
   o  Specify number of attempts and/or timeout for the first
      establishment message
   o  Specify which chunk types must always be authenticated
   o  Hand over a message to transfer (possibly multiple times) before
      connection establishment
   o  Hand over a message to transfer during connection establishment

   AVAILABILITY:
   o  Listen
   o  Specify which chunk types must always be authenticated

   MAINTENANCE:
   o  Change timeout for aborting connection (using retransmit limit or
      time value)
   o  Suggest timeout to the peer
   o  Disable Nagle algorithm
   o  Notification of Excessive Retransmissions (early warning below
      abortion threshold)
   o  Specify DSCP field
   o  Notification of ICMP error message arrival
   o  Change authentication parameters
   o  Obtain authentication information
   o  Choose a scheduler to operate between streams of an association
   o  Configure priority or weight for a scheduler
   o  Set Cookie life value
   o  Configure size where messages are broken up for partial delivery
   o  Disable checksum when sending
   o  Disable checksum requirement when receiving
   o  Specify checksum coverage used by the sender
   o  Specify minimum checksum coverage required by receiver
   o  Specify DF field
   o  Enable and configure a "Low Extra Delay Background Transfer"

   TERMINATION:
   o  Close after reliably delivering all remaining data, causing an
      event informing the application on the other side
   o  Abort without delivering remaining data, causing an event
      informing the application on the other side





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

4.2.  DATA Transfer Related Transport Features

4.2.1.  Sending Data

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

4.2.2.  Receiving Data

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

4.2.3.  Errors

   This section describes sending failures that are associated with a
   specific call to in the "Sending Data" category (Section 3.2.1).

   o  Notification of send failures
   o  Notification that the stack has no more user data to send
   o  Notification to a receiver that a partial message delivery has
      been aborted


5.  Step 3: Discussion

   The reduced set in the previous section exhibits a number of
   peculiarities, which we will discuss in the following.

5.1.  Sending Messages, Receiving Bytes

   There are several transport features related to sending, but only a
   single transport feature related to receiving: "Receive data (with no
   message delineation)" (and, strangely, "information about partial
   message arrival").  Notably, the transport feature "Receive a
   message" is also the only non-automatable transport feature of UDP(-
   Lite) that had to be removed because no fall-back to TCP is possible.
   It is also represents the only way that UDP(-Lite) applications can
   receive data today.




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   For the transport to operate on messages, it only needs be informed
   about them as they are handed over by a sending application; on the
   receiver side, receiving a message only differs from receiving a
   bytestream in that the application is told where messages begin and
   end in the former case but not in the latter.  The receiving
   application can still operate on these messages as long as it does
   not rely on the transport layer to inform it about message
   boundaries.

   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 possible retransmission
   delay is acceptable within the best effort service model.  Still, the
   receiving application would separate the byte stream into 100-byte
   chunks.

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

   For the implementation of a TAPS system, this has the following
   consequences:
   o  Because the receiver-side transport leaves it up to the
      application to delineate messages, messages must always remain
      intact as they are handed over by the transport receiver.  Data
      can be handed over at any time as they arrive, but the byte stream
      must never "skip ahead" to the beginning of the next message.
   o  With SCTP, a "partial flag" informs a receiving application that a
      message is incomplete.  Then, the next receive calls will only
      deliver remaining parts of the same message (i.e., no messages or
      partial messages will arrive on other streams until the message is
      complete) (see Section 8.1.20 in [RFC6458]).  This can facilitate
      the implementation of the receiver buffer in the receiving
      application, but then such an application does not support message
      interleaving (which is required by stream schedulers).  However,
      receiving a byte stream from multiple SCTP streams requires a per-
      stream receiver buffer anyway, so this potential benefit is lost
      and the "partial flag" (the transport feature "Information about



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      partial message arrival") becomes unnecessary for a TAPS system.
      With it, the transport features "Configure size where messages are
      broken up for partial delivery" and "Notification to a receiver
      that a partial message delivery has been aborted" become
      unnecessary too.
   o  From the above, a TAPS system should always support message
      interleaving because it enables the use of stream schedulers and
      comes at no additional implementation cost on the receiver side.
      Stream schedulers operate on the sender side.  Hence, because a
      TAPS sender-side application may talk to an SCTP receiver that
      does not support interleaving, it cannot assume that stream
      schedulers will always work as expected.

5.2.  Stream Schedulers Without Streams

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

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

   For the implementation of a TAPS system, this has the following
   consequences:
   o  Streams may be identified in different ways across different
      protocols.  The only multi-streaming protocol considered in this
      document, SCTP, uses a stream id.  The transport association below
      still uses a Transport Address (which includes one port number)
      for each communicating endpoint.  To implement a TAPS system
      without exposed streams, an application must be given an
      identifier for each TAPS flow (akin to a socket), and depending on
      whether streams are used or not, there will be a 1:1 mapping



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      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 6.1).
   o  To support stream schedulers, a receiver-side TAPS system should
      always support message interleaving because it comes at no
      additional implementation cost (because of the receiver-side
      stream reception discussed in Section 5.1).  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.

5.3.  Early Data Transmission

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

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

5.4.  Sender Running Dry

   The transport feature "Notification that the stack has no more user
   data to send" relates to SCTP's "SENDER DRY" notification.  Such
   notifications can, in principle, be used to avoid having an
   unnecessarily large send buffer, yet ensure that the transport sender
   always has data available when it has an opportunity to transmit it.
   This has been found to be very beneficial for some applications



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   [WWDC2015].  However, "SENDER DRY" truly means that the buffer has
   emptied -- i.e., when it notifies the sender, it is already too late,
   the transport protocol already missed an opportunity to send data.
   Some modern TCP implementations now include the unspecified
   "TCP_NOTSENT_LOWAT" socket option proposed in [WWDC2015], which
   limits the amount of unsent data that TCP can keep in the socket
   buffer; this allows to specify at which buffer filling level the
   socket becomes writable, rather than waiting for the buffer to run
   empty.

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

5.5.  Capacity Profile

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

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

5.6.  Security

   Both TCP and SCTP offer authentication.  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.  Generally, to an application it is relevant
   whether the transport protocol authenticates its own control data,
   the user data, or both, and a TAPS system should therefore allow to



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   configure and query these three cases.

   TBD -- more to come in the next version.  This relates to the TCP
   Authentication Option in Section 7.1 of [RFC5925], which is not
   currently covered.

   Set Cookie life value -- TBD in the next version: SCTP is client-
   side, TCP is server-side.

5.7.  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).  This is the only transport
   feature related to packet sizes.  UDP applications typically make use
   of IP-layer functionality to obtain the size of the link MTU; it
   would therefore seem that offering such functionality to TAPS
   applications could be useful, albeit in a transport protocol
   independent way.

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


6.  Step 4: Construction -- the Minimal Set of Transport Features

   Based on the categorization, reduction and discussion in the previous
   sections, this section presents the minimal set of transport features
   that is offered by end systems supporting TAPS.  They are described
   in an abstract fashion, i.e. they can be implemented in various
   different ways.  For example, information that is provided to an
   application can either be offered via a primitive that is polled, or
   via an asynchronous notification.

   Future versions of this document will probably describe the transport
   features in this section in greater detail; for now, we only specify
   how they differ from the transport features they are based upon.  We
   carry out an additional simplification: CONNECTION.ESTABLISHMENT
   "Specify number of attempts and/or timeout for the first
   establishment message" and CONNECTION.MAINTENANCE "Change timeout for



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   aborting connection (using retransmit limit or time value)" are
   essentially the same, just applied upon connection establishment or
   during the lifetime of a connection.  The same is the case for
   CONNECTION.ESTABLISHMENT "Specify which chunk types must always be
   authenticated" and CONNECTION.MAINTENANCE "Change authentication
   parameters".  We therefore state that connections (called TAPS flows)
   must be instantiated before connecting them, and allow configurations
   to be carried out before connecting (in cases where this is not
   allowed by the transport protocol, a TAPS system will have to
   internall delay this configuration until the flow has been
   connected).

6.1.  Flow Creation, Connection and Termination

   A TAPS flow must be "created" before it is connected, to allow for
   initial configurations to be carried out.  All configuration
   parameters in Section 6.2 and Section 6.3 can be used initially,
   although some of them may only take effect when the flow has been
   connected.  Configuring a flow early helps a TAPS system make the
   right decisions.  In particular, the "group number" can influence the
   the TAPS system to implement a TAPS flow as a stream of a multi-
   streaming protocol's existing association or not.

   A created flow can be queried for the maximum amount of data that an
   application can possibly expect to have transmitted before or during
   connection establishment.  An application can also give the flow a
   message for transmission before or during connection establishment,
   and specify which case is preferred (before / during).  In case of
   transmission before establishment, the receiving application must be
   prepared to potentially receive multiple copies of the message.

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

   After creation, a flow can be actively connected to the other side
   using "Connect", or passively listen for incoming connection requests
   with "Listen".  Note that "Connect" may or may not trigger a
   notification on the listening side.  It is possible that the first
   notification on the listening side is the arrival of the first data
   that the active side sends (a receiver-side TAPS system could handle
   this by continuing a blocking "Listen" call, immediately followed by
   issuing "Receive", for example).  This also means that the active



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   opening side is assumed to be the first side sending data.

   A flow can be actively closed, i.e. terminated after reliably
   delivering all remaining data, or aborted, i.e. terminated without
   delivering remaining data.  A timeout can be configured to abort a
   flow when data could not be delivered for too long.  Because half-
   closed connections are not supported, when a TAPS host receives a
   notification that the peer is closing or aborting the flow, the other
   side may not be able to read outstanding data.  This means that
   unacknowledged data residing in the TAPS system's send buffer may
   have to be dropped from that buffer upon arrival of a notification to
   close or abort the flow from the peer.  In case of SCTP streams,
   "Stream Reset" (a "SSN Reset Request Parameter" in a "RE-CONFIG"
   chunk [RFC6525]) can be used to notify a peer of an intention to
   close a flow.

6.2.  Flow Group Configuration

   A flow group can be configured with a number of transport features,
   and there are some notifications to applications about a flow group.
   Here we list transport features and notifications that are taken from
   Section 4 unchanged, with the exception that some of them can also be
   applied initially (before a flow is connected).

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

   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

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

   The following transport features are new or changed, based on the
   discussion in Section 5:
   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", "scavenger", and some more values that help
      determine the DSCP value for a flow (e.g. similar to table 1 in



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      [I-D.ietf-tsvwg-rtcweb-qos]). (details TBD)

   o  Authentication
      TBD in the next version: Different from SCTP's original transport
      features, this will only allow to configure authenticating the
      whole transport, all control information, or user data (not to
      distinguish between various SCTP chunks, to avoid this protocol
      dependency).  It will also have to be made in line with TCP
      Authentication [RFC5925].  For SCTP, this functionality will be
      based on the transport features "Change authentication
      parameters", "Obtain authentication information" and the initially
      available "Specify which chunk types must always be
      authenticated".  Note that SCTP also allows per-message
      configuration via "Specifying a key id to be used to authenticate
      a message", which may affect Section 6.4.

   o  Set Cookie life value
      TBD in the next version (not yet sure how to handle the client vs.
      server semantics of SCTP and TCP, respectively)


6.3.  Flow Configuration

   A flow can be assigned a priority or weight for a scheduler.

6.4.  Data Transfer

6.4.1.  The Sender

   This section discusses how to send data after flow establishment.
   Section 6.1 discusses the possiblity to hand over a message to send
   before or during establishment.

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

   Here we list per-frame properties that a sender can optionally



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   configure if it hands over a delimited frame for sending with
   congestion control, taken from Section 4:
   o  Configurable Message Reliability
   o  Choice between unordered (potentially faster) or ordered delivery
      of messages
   o  Request not to bundle messages
   o  Request not to delay the acknowledgement (SACK) of a message

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

   AUTHOR'S NOTE: do folks agree with this design?  It ties
   fragmentation to UDP only, because we called SCTP's "Configure
   message fragmentation" transport feature "automatable".  It is indeed
   questionable whether applications need control over fragmentation
   when they work with SCTP -- doing so creates a complication for app
   writers that may not be necessary, especially when messages can be
   interleaved.

   Following Section 5.7, there are two new transport features and a
   notification:
   o  Query maximum unfragmented frame size
      This is optional for a TAPS system to offer, and if it is offered,
      it informs the sender about the maximum expected size of a data
      frame that it can send without fragmentation.  This can aid
      applications implementing Path MTU Discovery.

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

   o  Notify the application of a path change
      If an application has disallowed fragmentation via the "Specify DF
      field" transport feature, this notification may optionally tell it
      that a path has changed (with a means to identify the path, so
      that the application can e.g. tell two flipping paths apart from
      completely diverse path changes).  This informs the application
      that it may have to repeat Path MTU Discovery, and it can have
      relevance for application-level congestion control.  For MPTCP and
      SCTP, a TAPS system can implement this functionality using the
      "Obtain status (query or notification)" transport feature.



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   There are two more sender-side notifications.  These are unreliable,
   i.e. a TAPS system cannot be assumed to implement them, but they may
   occur:
   o  Notification of send failures
      A TAPS system may inform a sender application of a failure to send
      a specific frame.  This was taken over unchanged from Section 4.

   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 Section 5.4 -- SCTP's "SENDER DRY" is a special case
   where the threshold is 0).  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.

6.4.2.  The Receiver

   A receiving application obtains an Application-Framed Bytestream.
   Similar to TCP's receiver semantics, it is just stream of bytes.  If
   frame boundaries were specified by the sender, a TAPS system will
   still not inform the receiving application about them, but frames
   themselves will always stay intact (partial frames are not supported
   - see Section 5.1).  Different from TCP's semantics, there is no
   guarantee that all bytes in the bytestream are received, and that all
   of them are in the same sequence in which they were handed over by
   the sender.  If an application is aware of frame delimiters in the
   bytestream, and if the sender-side application has informed the TAPS
   system about these boundaries and about potentially relaxed
   requirements regarding the sequence of frames or per-frame
   reliability, frames within the receiver-side bytestream may be out-
   of-order or missing.


7.  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



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   programmer.  This is the design trade-off that a TAPS system
   developer is facing, and this document provides guidance on the
   design of this abstraction level.  Some transport features are
   currently rarely offered by APIs, yet they must be offered or they
   can never be used ("functional" transport features).  Other transport
   features are offered by the APIs of the protocols covered here, but
   not exposing them in a TAPS API would allow for more freedom to
   automate protocol usage in a TAPS system.

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

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


8.  Acknowledgements

   The authors would like to thank the participants of the TAPS Working
   Group and the NEAT research project for valuable input to this
   document.  We especially thank Michael Tuexen for help with TAPS flow
   connection establishment/teardown and Gorry Fairhurst for his
   suggestions regarding fragmentation and packet sizes.  This work has
   received funding from the European Union's Horizon 2020 research and
   innovation programme under grant agreement No. 644334 (NEAT).  The
   views expressed are solely those of the author(s).


9.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   This memo includes no request to IANA.



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10.  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.


11.  References

11.1.  Normative References

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

   [TAPS2]    Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
              Transport Features Provided by IETF Transport Protocols",
              draft-ietf-taps-transports-usage-03 (work in progress),
              March 2017.

11.2.  Informative References

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

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

   [LBE-draft]
              Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
              draft-tsvwg-le-phb-00 (work in progress), October 2016.

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




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

   [RFC5290]  Floyd, S. and M. Allman, "Comments on the Usefulness of
              Simple Best-Effort Traffic", RFC 5290, DOI 10.17487/
              RFC5290, July 2008,
              <http://www.rfc-editor.org/info/rfc5290>.

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

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

   [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              RFC 6525, DOI 10.17487/RFC6525, February 2012,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7305>.

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

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


Appendix A.  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



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   that MPTCP is not included.

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

   -04: updated to be consistent with -03 version of [TAPS2].
   Reorganized document, rewrote intro and conclusion, and made a first
   stab at creating a real "minimal set".


Authors' Addresses

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

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


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

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






















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