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Versions: (draft-brunstrom-taps-impl) 00 01 02 03 04

TAPS Working Group                                     A. Brunstrom, Ed.
Internet-Draft                                       Karlstad University
Intended status: Informational                             T. Pauly, Ed.
Expires: January 9, 2020                                      Apple Inc.
                                                             T. Enghardt
                                                               TU Berlin
                                                           K-J. Grinnemo
                                                     Karlstad University
                                                                T. Jones
                                                  University of Aberdeen
                                                               P. Tiesel
                                                               TU Berlin
                                                              C. Perkins
                                                   University of Glasgow
                                                                M. Welzl
                                                      University of Oslo
                                                           July 08, 2019


             Implementing Interfaces to Transport Services
                        draft-ietf-taps-impl-04

Abstract

   The Transport Services architecture [I-D.ietf-taps-arch] defines a
   system that allows applications to use transport networking protocols
   flexibly.  This document serves as a guide to implementation on how
   to build such a system.

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 January 9, 2020.






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Copyright Notice

   Copyright (c) 2019 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Implementing Basic Objects  . . . . . . . . . . . . . . . . .   3
   3.  Implementing Pre-Establishment  . . . . . . . . . . . . . . .   4
     3.1.  Configuration-time errors . . . . . . . . . . . . . . . .   5
     3.2.  Role of system policy . . . . . . . . . . . . . . . . . .   5
   4.  Implementing Connection Establishment . . . . . . . . . . . .   6
     4.1.  Candidate Gathering . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  Gathering Endpoint Candidates . . . . . . . . . . . .   7
       4.1.2.  Structuring Options as a Tree . . . . . . . . . . . .   9
       4.1.3.  Branch Types  . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Branching Order-of-Operations . . . . . . . . . . . . . .  13
     4.3.  Sorting Branches  . . . . . . . . . . . . . . . . . . . .  14
     4.4.  Candidate Racing  . . . . . . . . . . . . . . . . . . . .  15
       4.4.1.  Delayed . . . . . . . . . . . . . . . . . . . . . . .  16
       4.4.2.  Failover  . . . . . . . . . . . . . . . . . . . . . .  16
     4.5.  Completing Establishment  . . . . . . . . . . . . . . . .  17
       4.5.1.  Determining Successful Establishment  . . . . . . . .  17
     4.6.  Establishing multiplexed connections  . . . . . . . . . .  18
     4.7.  Handling racing with "unconnected" protocols  . . . . . .  19
     4.8.  Implementing listeners  . . . . . . . . . . . . . . . . .  19
       4.8.1.  Implementing listeners for Connected Protocols  . . .  20
       4.8.2.  Implementing listeners for Unconnected Protocols  . .  20
       4.8.3.  Implementing listeners for Multiplexed Protocols  . .  20
   5.  Implementing Data Transfer  . . . . . . . . . . . . . . . . .  20
     5.1.  Data transfer for streams, datagrams, and frames  . . . .  20
       5.1.1.  Sending Messages  . . . . . . . . . . . . . . . . . .  21
       5.1.2.  Receiving Messages  . . . . . . . . . . . . . . . . .  23
     5.2.  Handling of data for fast-open protocols  . . . . . . . .  23
   6.  Implementing Maintenance  . . . . . . . . . . . . . . . . . .  24
     6.1.  Managing Connections  . . . . . . . . . . . . . . . . . .  24
     6.2.  Handling Path Changes . . . . . . . . . . . . . . . . . .  26



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   7.  Implementing Termination  . . . . . . . . . . . . . . . . . .  26
   8.  Cached State  . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  Protocol state caches . . . . . . . . . . . . . . . . . .  27
     8.2.  Performance caches  . . . . . . . . . . . . . . . . . . .  28
   9.  Specific Transport Protocol Considerations  . . . . . . . . .  29
     9.1.  TCP . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.2.  UDP . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     9.3.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     9.4.  DTLS  . . . . . . . . . . . . . . . . . . . . . . . . . .  34
     9.5.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  34
     9.6.  QUIC  . . . . . . . . . . . . . . . . . . . . . . . . . .  35
     9.7.  HTTP/2 transport  . . . . . . . . . . . . . . . . . . . .  36
     9.8.  SCTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  36
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  37
     11.1.  Considerations for Candidate Gathering . . . . . . . . .  37
     11.2.  Considerations for Candidate Racing  . . . . . . . . . .  37
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     13.2.  Informative References . . . . . . . . . . . . . . . . .  39
   Appendix A.  Additional Properties  . . . . . . . . . . . . . . .  40
     A.1.  Properties Affecting Sorting of Branches  . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   The Transport Services architecture [I-D.ietf-taps-arch] defines a
   system that allows applications to use transport networking protocols
   flexibly.  The interface such a system exposes to applications is
   defined as the Transport Services API [I-D.ietf-taps-interface].
   This API is designed to be generic across multiple transport
   protocols and sets of protocols features.

   This document serves as a guide to implementation on how to build a
   system that provides a Transport Services API.  It is the job of an
   implementation of a Transport Services system to turn the requests of
   an application into decisions on how to establish connections, and
   how to transfer data over those connections once established.  The
   terminology used in this document is based on the Architecture
   [I-D.ietf-taps-arch].

2.  Implementing Basic Objects

   The basic objects that are exposed to applications for Transport
   Services are the Preconnection, the bundle of properties that
   describes the application constraints on the transport; the
   Connection, the basic object that represents a flow of data in either



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   direction between the Local and Remote Endpoints; and the Listener, a
   passive waiting object that delivers new Connections.

   Preconnection objects should be implemented as bundles of properties
   that an application can both read and write.  Once a Preconnection
   has been used to create an outbound Connection or a Listener, the
   implementation should ensure that the copy of the properties held by
   the Connection or Listener is immutable.  This may involve performing
   a deep-copy if the application is still able to modify properties on
   the original Preconnection object.

   Connection objects represent the interface between the application
   and the implementation to manage transport state, and conduct data
   transfer.  During the process of establishment (Section 4), the
   Connection will be unbound to a specific transport flow, since there
   may be multiple candidate Protocol Stacks being raced.  Once the
   Connection is established, the object should be considered mapped to
   a specific Protocol Stack.  The notion of a Connection maps to many
   different protocols, depending on the Protocol Stack.  For example,
   the Connection may ultimately represent the interface into a TCP
   connection, a TLS session over TCP, a UDP flow with fully-specified
   local and remote endpoints, a DTLS session, a SCTP stream, a QUIC
   stream, or an HTTP/2 stream.

   Listener objects are created with a Preconnection, at which point
   their configuration should be considered immutable by the
   implementation.  The process of listening is described in
   Section 4.8.

3.  Implementing Pre-Establishment

   During pre-establishment the application specifies the Endpoints to
   be used for communication as well as its preferences via Selection
   Properties and, if desired, also Connection Properties.  Generally,
   Connection Properties should be configured as early as possible, as
   they may serve as input to decisions that are made by the
   implementation (the Capacity Profile may guide usage of a protocol
   offering scavenger-type congestion control, for example).  In the
   remainder of this document, we only refer to Selection Properties
   because they are the more typical case and have to be handled by all
   implementations.

   The implementation stores these objects and properties as part of the
   Preconnection object for use during connection establishment.  For
   Selection Properties that are not provided by the application, the
   implementation must use the default values specified in the Transport
   Services API ([I-D.ietf-taps-interface]).




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3.1.  Configuration-time errors

   The transport system should have a list of supported protocols
   available, which each have transport features reflecting the
   capabilities of the protocol.  Once an application specifies its
   Transport Parameters, the transport system should match the required
   and prohibited properties against the transport features of the
   available protocols.

   In the following cases, failure should be detected during pre-
   establishment:

   o  The application requested Protocol Properties that include
      requirements or prohibitions that cannot be satisfied by any of
      the available protocols.  For example, if an application requires
      "Configure Reliability per Message", but no such protocol is
      available on the host running the transport system, e.g., because
      SCTP is not supported by the operating system, this should result
      in an error.

   o  The application requested Protocol Properties that are in conflict
      with each other, i.e., the required and prohibited properties
      cannot be satisfied by the same protocol.  For example, if an
      application prohibits "Reliable Data Transfer" but then requires
      "Configure Reliability per Message", this mismatch should result
      in an error.

   It is important to fail as early as possible in such cases in order
   to avoid allocating resources, e.g., to endpoint resolution, only to
   find out later that there is no protocol that satisfies the
   requirements.

3.2.  Role of system policy

   The properties specified during pre-establishment have a close
   connection to system policy.  The implementation is responsible for
   combining and reconciling several different sources of preferences
   when establishing Connections.  These include, but are not limited
   to:

   1.  Application preferences, i.e., preferences specified during the
       pre-establishment via Selection Properties.

   2.  Dynamic system policy, i.e., policy compiled from internally and
       externally acquired information about available network
       interfaces, supported transport protocols, and current/previous
       Connections.  Examples of ways to externally retrieve policy-
       support information are through OS-specific statistics/



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       measurement tools and tools that reside on middleboxes and
       routers.

   3.  Default implementation policy, i.e., predefined policy by OS or
       application.

   In general, any protocol or path used for a connection must conform
   to all three sources of constraints.  Any violation of any of the
   layers should cause a protocol or path to be considered ineligible
   for use.  For an example of application preferences leading to
   constraints, an application may prohibit the use of metered network
   interfaces for a given Connection to avoid user cost.  Similarly, the
   system policy at a given time may prohibit the use of such a metered
   network interface from the application's process.  Lastly, the
   implementation itself may default to disallowing certain network
   interfaces unless explicitly requested by the application and allowed
   by the system.

   It is expected that the database of system policies and the method of
   looking up these policies will vary across various platforms.  An
   implementation should attempt to look up the relevant policies for
   the system in a dynamic way to make sure it is reflecting an accurate
   version of the system policy, since the system's policy regarding the
   application's traffic may change over time due to user or
   administrative changes.

4.  Implementing Connection Establishment

   The process of establishing a network connection begins when an
   application expresses intent to communicate with a remote endpoint by
   calling Initiate.  (At this point, any constraints or requirements
   the application may have on the connection are available from pre-
   establishment.)  The process can be considered complete once there is
   at least one Protocol Stack that has completed any required setup to
   the point that it can transmit and receive the application's data.

   Connection establishment is divided into two top-level steps:
   Candidate Gathering, to identify the paths, protocols, and endpoints
   to use, and Candidate Racing, in which the necessary protocol
   handshakes are conducted so that the transport system can select
   which set to use.  This document structures candidates for racing as
   a tree.

   The most simple example of this process might involve identifying the
   single IP address to which the implementation wishes to connect,
   using the system's current default interface or path, and starting a
   TCP handshake to establish a stream to the specified IP address.
   However, each step may also vary depending on the requirements of the



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   connection: if the endpoint is defined as a hostname and port, then
   there may be multiple resolved addresses that are available; there
   may also be multiple interfaces or paths available, other than the
   default system interface; and some protocols may not need any
   transport handshake to be considered "established" (such as UDP),
   while other connections may utilize layered protocol handshakes, such
   as TLS over TCP.

   Whenever an implementation has multiple options for connection
   establishment, it can view the set of all individual connection
   establishment options as a single, aggregate connection
   establishment.  The aggregate set conceptually includes every valid
   combination of endpoints, paths, and protocols.  As an example,
   consider an implementation that initiates a TCP connection to a
   hostname + port endpoint, and has two valid interfaces available (Wi-
   Fi and LTE).  The hostname resolves to a single IPv4 address on the
   Wi-Fi network, and resolves to the same IPv4 address on the LTE
   network, as well as a single IPv6 address.  The aggregate set of
   connection establishment options can be viewed as follows:

Aggregate [Endpoint: www.example.com:80] [Interface: Any]   [Protocol: TCP]
|-> [Endpoint: 192.0.2.1:80]       [Interface: Wi-Fi] [Protocol: TCP]
|-> [Endpoint: 192.0.2.1:80]       [Interface: LTE]   [Protocol: TCP]
|-> [Endpoint: 2001:DB8::1.80]     [Interface: LTE]   [Protocol: TCP]

   Any one of these sub-entries on the aggregate connection attempt
   would satisfy the original application intent.  The concern of this
   section is the algorithm defining which of these options to try,
   when, and in what order.

4.1.  Candidate Gathering

   The step of gathering candidates involves identifying which paths,
   protocols, and endpoints may be used for a given Connection.  This
   list is determined by the requirements, prohibitions, and preferences
   of the application as specified in the Selection Properties.

4.1.1.  Gathering Endpoint Candidates

   Both Local and Remote Endpoint Candidates must be discovered during
   connection establishment.  To support ICE, or similar protocols, that
   involve out-of-band indirect signalling to exchange candidates with
   the Remote Endpoint, it's important to be able to query the set of
   candidate Local Endpoints, and give the protocol stack a set of
   candidate Remote Endpoints, before it attempts to establish
   connections.





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4.1.1.1.  Local Endpoint candidates

   The set of possible Local Endpoints is gathered.  In the simple case,
   this merely enumerates the local interfaces and protocols, allocates
   ephemeral source ports.  For example, a system that has WiFi and
   Ethernet and supports IPv4 and IPv6 might gather four candidate
   locals (IPv4 on Ethernet, IPv6 on Ethernet, IPv4 on WiFi, and IPv6 on
   WiFi) that can form the source for a transient.

   If NAT traversal is required, the process of gathering Local
   Endpoints becomes broadly equivalent to the ICE candidate gathering
   phase [RFC5245].  The endpoint determines its server reflexive Local
   Endpoints (i.e., the translated address of a local, on the other side
   of a NAT) and relayed locals (e.g., via a TURN server or other
   relay), for each interface and network protocol.  These are added to
   the set of candidate Local Endpoints for this connection.

   Gathering Local Endpoints is primarily a local operation, although it
   might involve exchanges with a STUN server to derive server reflexive
   locals, or with a TURN server or other relay to derive relayed
   locals.  It does not involve communication with the Remote Endpoint.

4.1.1.2.  Remote Endpoint Candidates

   The Remote Endpoint is typically a name that needs to be resolved
   into a set of possible addresses that can be used for communication.
   Resolving the Remote Endpoint is the process of recursively
   performing such name lookups, until fully resolved, to return the set
   of candidates for the remote of this connection.

   How this is done will depend on the type of the Remote Endpoint, and
   can also be specific to each Local Endpoint.  A common case is when
   the Remote Endpoint is a DNS name, in which case it is resolved to
   give a set of IPv4 and IPv6 addresses representing that name.  Some
   types of remote might require more complex resolution.  Resolving the
   Remote Endpoint for a peer-to-peer connection might involve
   communication with a rendezvous server, which in turn contacts the
   peer to gain consent to communicate and retrieve its set of candidate
   locals, which are returned and form the candidate remote addresses
   for contacting that peer.

   Resolving the remote is not a local operation.  It will involve a
   directory service, and can require communication with the remote to
   rendezvous and exchange peer addresses.  This can expose some or all
   of the candidate locals to the remote.






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4.1.2.  Structuring Options as a Tree

   When an implementation responsible for connection establishment needs
   to consider multiple options, it should logically structure these
   options as a hierarchical tree.  Each leaf node of the tree
   represents a single, coherent connection attempt, with an Endpoint, a
   Path, and a set of protocols that can directly negotiate and send
   data on the network.  Each node in the tree that is not a leaf
   represents a connection attempt that is either underspecified, or
   else includes multiple distinct options.  For example. when
   connecting on an IP network, a connection attempt to a hostname and
   port is underspecified, because the connection attempt requires a
   resolved IP address as its remote endpoint.  In this case, the node
   represented by the connection attempt to the hostname is a parent
   node, with child nodes for each IP address.  Similarly, an
   implementation that is allowed to connect using multiple interfaces
   will have a parent node of the tree for the decision between the
   paths, with a branch for each interface.

   The example aggregate connection attempt above can be drawn as a tree
   by grouping the addresses resolved on the same interface into
   branches:

                             ||
                +==========================+
                |  www.example.com:80/Any  |
                +==========================+
                  //                    \\
+==========================+       +==========================+
| www.example.com:80/Wi-Fi |       |  www.example.com:80/LTE  |
+==========================+       +==========================+
             ||                      //                    \\
  +====================+  +====================+  +======================+
  | 192.0.2.1:80/Wi-Fi |  |  192.0.2.1:80/LTE  |  |  2001:DB8::1.80/LTE  |
  +====================+  +====================+  +======================+

   The rest of this section will use a notation scheme to represent this
   tree.  The parent (or trunk) node of the tree will be represented by
   a single integer, such as "1".  Each child of that node will have an
   integer that identifies it, from 1 to the number of children.  That
   child node will be uniquely identified by concatenating its integer
   to it's parents identifier with a dot in between, such as "1.1" and
   "1.2".  Each node will be summarized by a tuple of three elements:
   Endpoint, Path, and Protocol.  The above example can now be written
   more succinctly as:






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   1 [www.example.com:80, Any, TCP]
     1.1 [www.example.com:80, Wi-Fi, TCP]
       1.1.1 [192.0.2.1:80, Wi-Fi, TCP]
     1.2 [www.example.com:80, LTE, TCP]
       1.2.1 [192.0.2.1:80, LTE, TCP]
       1.2.2 [2001:DB8::1.80, LTE, TCP]

   When an implementation views this aggregate set of connection
   attempts as a single connection establishment, it only will use one
   of the leaf nodes to transfer data.  Thus, when a single leaf node
   becomes ready to use, then the entire connection attempt is ready to
   use by the application.  Another way to represent this is that every
   leaf node updates the state of its parent node when it becomes ready,
   until the trunk node of the tree is ready, which then notifies the
   application that the connection as a whole is ready to use.

   A connection establishment tree may be degenerate, and only have a
   single leaf node, such as a connection attempt to an IP address over
   a single interface with a single protocol.

   1 [192.0.2.1:80, Wi-Fi, TCP]

   A parent node may also only have one child (or leaf) node, such as a
   when a hostname resolves to only a single IP address.

   1 [www.example.com:80, Wi-Fi, TCP]
     1.1 [192.0.2.1:80, Wi-Fi, TCP]

4.1.3.  Branch Types

   There are three types of branching from a parent node into one or
   more child nodes.  Any parent node of the tree must only use one type
   of branching.

4.1.3.1.  Derived Endpoints

   If a connection originally targets a single endpoint, there may be
   multiple endpoints of different types that can be derived from the
   original.  The connection library should order the derived endpoints
   according to application preference, system policy and expected
   performance.

   DNS hostname-to-address resolution is the most common method of
   endpoint derivation.  When trying to connect to a hostname endpoint
   on a traditional IP network, the implementation should send DNS
   queries for both A (IPv4) and AAAA (IPv6) records if both are
   supported on the local link.  The algorithm for ordering and racing




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   these addresses should follow the recommendations in Happy Eyeballs
   [RFC8305].

   1 [www.example.com:80, Wi-Fi, TCP]
     1.1 [2001:DB8::1.80, Wi-Fi, TCP]
     1.2 [192.0.2.1:80, Wi-Fi, TCP]
     1.3 [2001:DB8::2.80, Wi-Fi, TCP]
     1.4 [2001:DB8::3.80, Wi-Fi, TCP]

   DNS-Based Service Discovery can also provide an endpoint derivation
   step.  When trying to connect to a named service, the client may
   discover one or more hostname and port pairs on the local network
   using multicast DNS.  These hostnames should each be treated as a
   branch which can be attempted independently from other hostnames.
   Each of these hostnames may also resolve to one or more addresses,
   thus creating multiple layers of branching.

   1 [term-printer._ipp._tcp.meeting.ietf.org, Wi-Fi, TCP]
     1.1 [term-printer.meeting.ietf.org:631, Wi-Fi, TCP]
       1.1.1 [31.133.160.18.631, Wi-Fi, TCP]

4.1.3.2.  Alternate Paths

   If a client has multiple network interfaces available to it, such as
   mobile client with both Wi-Fi and Cellular connectivity, it can
   attempt a connection over either interface.  This represents a branch
   point in the connection establishment.  Like with derived endpoints,
   the interfaces should be ranked based on preference, system policy,
   and performance.  Attempts should be started on one interface, and
   then on other interfaces successively after delays based on expected
   round-trip-time or other available metrics.

   1 [192.0.2.1:80, Any, TCP]
     1.1 [192.0.2.1:80, Wi-Fi, TCP]
     1.2 [192.0.2.1:80, LTE, TCP]

   This same approach applies to any situation in which the client is
   aware of multiple links or views of the network.  Multiple Paths,
   each with a coherent set of addresses, routes, DNS server, and more,
   may share a single interface.  A path may also represent a virtual
   interface service such as a Virtual Private Network (VPN).

   The list of available paths should be constrained by any requirements
   or prohibitions the application sets, as well as system policy.







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4.1.3.3.  Protocol Options

   Differences in possible protocol compositions and options can also
   provide a branching point in connection establishment.  This allows
   clients to be resilient to situations in which a certain protocol is
   not functioning on a server or network.

   This approach is commonly used for connections with optional proxy
   server configurations.  A single connection may be allowed to use an
   HTTP-based proxy, a SOCKS-based proxy, or connect directly.  These
   options should be ranked and attempted in succession.

   1 [www.example.com:80, Any, HTTP/TCP]
     1.1 [192.0.2.8:80, Any, HTTP/HTTP Proxy/TCP]
     1.2 [192.0.2.7:10234, Any, HTTP/SOCKS/TCP]
     1.3 [www.example.com:80, Any, HTTP/TCP]
       1.3.1 [192.0.2.1:80, Any, HTTP/TCP]

   This approach also allows a client to attempt different sets of
   application and transport protocols that may provide preferable
   characteristics when available.  For example, the protocol options
   could involve QUIC [I-D.ietf-quic-transport] over UDP on one branch,
   and HTTP/2 [RFC7540] over TLS over TCP on the other:

   1 [www.example.com:443, Any, Any HTTP]
     1.1 [www.example.com:443, Any, QUIC/UDP]
       1.1.1 [192.0.2.1:443, Any, QUIC/UDP]
     1.2 [www.example.com:443, Any, HTTP2/TLS/TCP]
       1.2.1 [192.0.2.1:443, Any, HTTP2/TLS/TCP]

   Another example is racing SCTP with TCP:

   1 [www.example.com:80, Any, Any Stream]
     1.1 [www.example.com:80, Any, SCTP]
       1.1.1 [192.0.2.1:80, Any, SCTP]
     1.2 [www.example.com:80, Any, TCP]
       1.2.1 [192.0.2.1:80, Any, TCP]

   Implementations that support racing protocols and protocol options
   should maintain a history of which protocols and protocol options
   successfully established, on a per-network basis (see Section 8.2).
   This information can influence future racing decisions to prioritize
   or prune branches.








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4.2.  Branching Order-of-Operations

   Branch types must occur in a specific order relative to one another
   to avoid creating leaf nodes with invalid or incompatible settings.
   In the example above, it would be invalid to branch for derived
   endpoints (the DNS results for www.example.com) before branching
   between interface paths, since usable DNS results on one network may
   not necessarily be the same as DNS results on another network due to
   local network entities, supported address families, or enterprise
   network configurations.  Implementations must be careful to branch in
   an order that results in usable leaf nodes whenever there are
   multiple branch types that could be used from a single node.

   The order of operations for branching, where lower numbers are acted
   upon first, should be:

   1.  Alternate Paths

   2.  Protocol Options

   3.  Derived Endpoints

   Branching between paths is the first in the list because results
   across multiple interfaces are likely not related to one another:
   endpoint resolution may return different results, especially when
   using locally resolved host and service names, and which protocols
   are supported and preferred may differ across interfaces.  Thus, if
   multiple paths are attempted, the overall connection can be seen as a
   race between the available paths or interfaces.

   Protocol options are checked next in order.  Whether or not a set of
   protocol, or protocol-specific options, can successfully connect is
   generally not dependent on which specific IP address is used.
   Furthermore, the protocol stacks being attempted may influence or
   altogether change the endpoints being used.  Adding a proxy to a
   connection's branch will change the endpoint to the proxy's IP
   address or hostname.  Choosing an alternate protocol may also modify
   the ports that should be selected.

   Branching for derived endpoints is the final step, and may have
   multiple layers of derivation or resolution, such as DNS service
   resolution and DNS hostname resolution.

   For example, if the application has indicated both a preference for
   WiFi over LTE and for a feature only available in SCTP, branches will
   be first sorted accord to path selection, with WiFi at the top.
   Then, branches with SCTP will be sorted to the top within their
   subtree according to the properties influencing protocol selection.



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   However, if the implementation has cached the information that SCTP
   is not available on the path over WiFi, there is no SCTP node in the
   WiFi subtree.  Here, the path over WiFi will be tried first, and, if
   connection establishment succeeds, TCP will be used.  So the
   Selection Property of preferring WiFi takes precedence over the
   Property that led to a preference for SCTP.

   1. [www.example.com:80, Any, Any Stream]
   1.1 [192.0.2.1:80, Wi-Fi, Any Stream]
   1.1.1 [192.0.2.1:80, Wi-Fi, TCP]
   1.2 [192.0.3.1:80, LTE, Any Stream]
   1.2.1 [192.0.3.1:80, LTE, SCTP]
   1.2.2 [192.0.3.1:80, LTE, TCP]

4.3.  Sorting Branches

   Implementations should sort the branches of the tree of connection
   options in order of their preference rank.  Leaf nodes on branches
   with higher rankings represent connection attempts that will be raced
   first.  Implementations should order the branches to reflect the
   preferences expressed by the application for its new connection,
   including Selection Properties, which are specified in
   [I-D.ietf-taps-interface].

   In addition to the properties provided by the application, an
   implementation may include additional criteria such as cached
   performance estimates, see Section 8.2, or system policy, see
   Section 3.2, in the ranking.  Two examples of how Selection and
   Connection Properties may be used to sort branches are provided
   below:

   o  "Interface Instance or Type": If the application specifies an
      interface type to be preferred or avoided, implementations should
      rank paths accordingly.  If the application specifies an interface
      type to be required or prohibited, we expect an implementation to
      not include the non-conforming paths into the three.

   o  "Capacity Profile": An implementation may use the Capacity Profile
      to prefer paths optimized for the application's expected traffic
      pattern according to cached performance estimates, see
      Section 8.2:

      *  Scavenger: Prefer paths with the highest expected available
         bandwidth, based on observed maximum throughput

      *  Low Latency/Interactive: Prefer paths with the lowest expected
         Round Trip Time




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      *  Constant-Rate Streaming: Prefer paths that can satisfy the
         requested Stream Send or Stream Receive Bitrate, based on
         observed maximum throughput

   Implementations should process properties in the following order:
   Prohibit, Require, Prefer, Avoid.  If Selection Properties contain
   any prohibited properties, the implementation should first purge
   branches containing nodes with these properties.  For required
   properties, it should only keep branches that satisfy these
   requirements.  Finally, it should order branches according to
   preferred properties, and finally use avoided properties as a
   tiebreaker.

4.4.  Candidate Racing

   The primary goal of the Candidate Racing process is to successfully
   negotiate a protocol stack to an endpoint over an interface--to
   connect a single leaf node of the tree--with as little delay and as
   few unnecessary connections attempts as possible.  Optimizing these
   two factors improves the user experience, while minimizing network
   load.

   This section covers the dynamic aspect of connection establishment.
   While the tree described above is a useful conceptual and
   architectural model, an implementation does not know what the full
   tree may become up front, nor will many of the possible branches be
   used in the common case.

   There are three different approaches to racing the attempts for
   different nodes of the connection establishment tree:

   1.  Immediate

   2.  Delayed

   3.  Failover

   Each approach is appropriate in different use-cases and branch types.
   However, to avoid consuming unnecessary network resources,
   implementations should not use immediate racing as a default
   approach.

   The timing algorithms for racing should remain independent across
   branches of the tree.  Any timers or racing logic is isolated to a
   given parent node, and is not ordered precisely with regards to other
   children of other nodes.





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4.4.1.  Delayed

   Delayed racing can be used whenever a single node of the tree has
   multiple child nodes.  Based on the order determined when building
   the tree, the first child node will be initiated immediately,
   followed by the next child node after some delay.  Once that second
   child node is initiated, the third child node (if present) will begin
   after another delay, and so on until all child nodes have been
   initiated, or one of the child nodes successfully completes its
   negotiation.

   Delayed racing attempts occur in parallel.  Implementations should
   not terminate an earlier child connection attempt upon starting a
   secondary child.

   The delay between starting child nodes should be based on the
   properties of the previously started child node.  For example, if the
   first child represents an IP address with a known route, and the
   second child represents another IP address, the delay between
   starting the first and second IP addresses can be based on the
   expected retransmission cadence for the first child's connection
   (derived from historical round-trip-time).  Alternatively, if the
   first child represents a branch on a Wi-Fi interface, and the second
   child represents a branch on an LTE interface, the delay should be
   based on the expected time in which the branch for the first
   interface would be able to establish a connection, based on link
   quality and historical round-trip-time.

   Any delay should have a defined minimum and maximum value based on
   the branch type.  Generally, branches between paths and protocols
   should have longer delays than branches between derived endpoints.
   The maximum delay should be considered with regards to how long a
   user is expected to wait for the connection to complete.

   If a child node fails to connect before the delay timer has fired for
   the next child, the next child should be started immediately.

4.4.2.  Failover

   If an implementation or application has a strong preference for one
   branch over another, the branching node may choose to wait until one
   child has failed before starting the next.  Failure of a leaf node is
   determined by its protocol negotiation failing or timing out; failure
   of a parent branching node is determined by all of its children
   failing.

   An example in which failover is recommended is a race between a
   protocol stack that uses a proxy and a protocol stack that bypasses



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   the proxy.  Failover is useful in case the proxy is down or
   misconfigured, but any more aggressive type of racing may end up
   unnecessarily avoiding a proxy that was preferred by policy.

4.5.  Completing Establishment

   The process of connection establishment completes when one leaf node
   of the tree has completed negotiation with the remote endpoint
   successfully, or else all nodes of the tree have failed to connect.
   The first leaf node to complete its connection is then used by the
   application to send and receive data.

   It is useful to process success and failure throughout the tree by
   child nodes reporting to their parent nodes (towards the trunk of the
   tree).  For example, in the following case, if 1.1.1 fails to
   connect, it reports the failure to 1.1.  Since 1.1 has no other child
   nodes, it also has failed and reports that failure to 1.  Because 1.2
   has not yet failed, 1 is not considered to have failed.  Since 1.2
   has not yet started, it is started and the process continues.
   Similarly, if 1.1.1 successfully connects, then it marks 1.1 as
   connected, which propagates to the trunk node 1.  At this point, the
   connection as a whole is considered to be successfully connected and
   ready to process application data

   1 [www.example.com:80, Any, TCP]
     1.1 [www.example.com:80, Wi-Fi, TCP]
       1.1.1 [192.0.2.1:80, Wi-Fi, TCP]
     1.2 [www.example.com:80, LTE, TCP]
   ...

   If a leaf node has successfully completed its connection, all other
   attempts should be made ineligible for use by the application for the
   original request.  New connection attempts that involve transmitting
   data on the network should not be started after another leaf node has
   completed successfully, as the connection as a whole has been
   established.  An implementation may choose to let certain handshakes
   and negotiations complete in order to gather metrics to influence
   future connections.  Similarly, an implementation may choose to hold
   onto fully established leaf nodes that were not the first to
   establish for use in future connections, but this approach is not
   recommended since those attempts were slower to connect and may
   exhibit less desirable properties.

4.5.1.  Determining Successful Establishment

   Implementations may select the criteria by which a leaf node is
   considered to be successfully connected differently on a per-protocol
   basis.  If the only protocol being used is a transport protocol with



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   a clear handshake, like TCP, then the obvious choice is to declare
   that node "connected" when the last packet of the three-way handshake
   has been received.  If the only protocol being used is an
   "unconnected" protocol, like UDP, the implementation may consider the
   node fully "connected" the moment it determines a route is present,
   before sending any packets on the network, see further Section 4.7.

   For protocol stacks with multiple handshakes, the decision becomes
   more nuanced.  If the protocol stack involves both TLS and TCP, an
   implementation could determine that a leaf node is connected after
   the TCP handshake is complete, or it can wait for the TLS handshake
   to complete as well.  The benefit of declaring completion when the
   TCP handshake finishes, and thus stopping the race for other branches
   of the tree, is that there will be less burden on the network from
   other connection attempts.  On the other hand, by waiting until the
   TLS handshake is complete, an implementation avoids the scenario in
   which a TCP handshake completes quickly, but TLS negotiation is
   either very slow or fails altogether in particular network conditions
   or to a particular endpoint.  To avoid the issue of TLS possibly
   failing, the implementation should not generate a Ready event for the
   Connection until TLS is established.

   If all of the leaf nodes fail to connect during racing, i.e. none of
   the configurations that satisfy all requirements given in the
   Transport Parameters actually work over the available paths, then the
   transport system should notify the application with an InitiateError
   event.  An InitiateError event should also be generated in case the
   transport system finds no usable candidates to race.

4.6.  Establishing multiplexed connections

   Multiplexing several Connections over a single underlying transport
   connection requires that the Connections to be multiplexed belong to
   the same Connection Group (as is indicated by the application using
   the Clone call).  When the underlying transport connection supports
   multi-streaming, the Transport System can map each Connection in the
   Connection Group to a different stream.  Thus, when the Connections
   that are offered to an application by the Transport System are
   multiplexed, the Transport System may implement the establishment of
   a new Connection by simply beginning to use a new stream of an
   already established transport connection and there is no need for a
   connection establishment procedure.  This, then, also means that
   there may not be any "establishment" message (like a TCP SYN), but
   the application can simply start sending or receiving.  Therefore,
   when the Initiate action of a Transport System is called without
   Messages being handed over, it cannot be guaranteed that the other
   endpoint will have any way to know about this, and hence a passive
   endpoint's ConnectionReceived event may not be called upon an active



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   endpoint's Inititate.  Instead, calling the ConnectionReceived event
   may be delayed until the first Message arrives.

4.7.  Handling racing with "unconnected" protocols

   While protocols that use an explicit handshake to validate a
   Connection to a peer can be used for racing multiple establishment
   attempts in parallel, "unconnected" protocols such as raw UDP do not
   offer a way to validate the presence of a peer or the usability of a
   Connection without application feedback.  An implementation should
   consider such a protocol stack to be established as soon as a local
   route to the peer endpoint is confirmed.

   However, if a peer is not reachable over the network using the
   unconnected protocol, or data cannot be exchanged for any other
   reason, the application may want to attempt using another candidate
   Protocol Stack.  The implementation should maintain the list of other
   candidate Protocol Stacks that were eligible to use.  In the case
   that the application signals that the initial Protocol Stack is
   failing for some reason and that another option should be attempted,
   the Connection can be updated to point to the next candidate Protocol
   Stack.  This can be viewed as an application-driven form of Protocol
   Stack racing.

4.8.  Implementing listeners

   When an implementation is asked to Listen, it registers with the
   system to wait for incoming traffic to the Local Endpoint.  If no
   Local Endpoint is specified, the implementation should either use an
   ephemeral port or generate an error.

   If the Selection Properties do not require a single network interface
   or path, but allow the use of multiple paths, the Listener object
   should register for incoming traffic on all of the network interfaces
   or paths that conform to the Properties.  The set of available paths
   can change over time, so the implementation should monitor network
   path changes and register and de-register the Listener across all
   usable paths.  When using multiple paths, the Listener is generally
   expected to use the same port for listening on each.

   If the Selection Properties allow multiple protocols to be used for
   listening, and the implementation supports it, the Listener object
   should register across the eligble protocols for each path.  This
   means that inbound Connections delivered by the implementation may
   have heterogeneous protocol stacks.






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4.8.1.  Implementing listeners for Connected Protocols

   Connected protocols such as TCP and TLS-over-TCP have a strong
   mapping between the Local and Remote Endpoints (five-tuple) and their
   protocol connection state.  These map well into Connection objects.
   Whenever a new inbound handshake is being started, the Listener
   should generate a new Connection object and pass it to the
   application.

4.8.2.  Implementing listeners for Unconnected Protocols

   Unconnected protocols such as UDP and UDP-lite generally do not
   provide the same mechanisms that connected protocols do to offer
   Connection objects.  Implementations should wait for incoming packets
   for unconnected protocols on a listening port and should perform
   five-tuple matching of packets to either existing Connection objects
   or the creation of new Connection objects.  On platforms with
   facilities to create a "virtual connection" for unconnected protocols
   implementations should use these mechanisms to minimise the handling
   of datagrams intended for already created Connection objects.

4.8.3.  Implementing listeners for Multiplexed Protocols

   Protocols that provide multiplexing of streams into a single five-
   tuple can listen both for entirely new connections (a new HTTP/2
   stream on a new TCP connection, for example) and for new sub-
   connections (a new HTTP/2 stream on an existing connection).  If the
   abstraction of Connection presented to the application is mapped to
   the multiplexed stream, then the Listener should deliver new
   Connection objects in the same way for either case.  The
   implementation should allow the application to introspect the
   Connection Group marked on the Connections to determine the grouping
   of the multiplexing.

5.  Implementing Data Transfer

5.1.  Data transfer for streams, datagrams, and frames

   The most basic mapping for sending a Message is an abstraction of
   datagrams, in which the transport protocol naturally deals in
   discrete packets.  Each Message here corresponds to a single
   datagram.  Generally, these will be short enough that sending and
   receiving will always use a complete Message.

   For protocols that expose byte-streams, the only delineation provided
   by the protocol is the end of the stream in a given direction.  Each
   Message in this case corresponds to the entire stream of bytes in a




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   direction.  These Messages may be quite long, in which case they can
   be sent in multiple parts.

   Protocols that provide the framing (such as length-value protocols,
   or protocols that use delimiters) provide data boundaries that may be
   longer than a traditional packet datagram.  Each Message for framing
   protocols corresponds to a single frame, which may be sent either as
   a complete Message, or in multiple parts.

5.1.1.  Sending Messages

   The effect of the application sending a Message is determined by the
   top-level protocol in the established Protocol Stack.  That is, if
   the top-level protocol provides an abstraction of framed messages
   over a connection, the receiving application will be able to obtain
   multiple Messages on that connection, even if the framing protocol is
   built on a byte-stream protocol like TCP.

5.1.1.1.  Message Properties

   o  Lifetime: this should be implemented by removing the Message from
      its queue of pending Messages after the Lifetime has expired.  A
      queue of pending Messages within the transport system
      implementation that have yet to be handed to the Protocol Stack
      can always support this property, but once a Message has been sent
      into the send buffer of a protocol, only certain protocols may
      support de-queueing a message.  For example, TCP cannot remove
      bytes from its send buffer, while in case of SCTP, such control
      over the SCTP send buffer can be exercised using the partial
      reliability extension [RFC8303].  When there is no standing queue
      of Messages within the system, and the Protocol Stack does not
      support removing a Message from its buffer, this property may be
      ignored.

   o  Priority: this represents the ability to prioritize a Message over
      other Messages.  This can be implemented by the system re-ordering
      Messages that have yet to be handed to the Protocol Stack, or by
      giving relative priority hints to protocols that support
      priorities per Message.  For example, an implementation of HTTP/2
      could choose to send Messages of different Priority on streams of
      different priority.

   o  Ordered: when this is false, it disables the requirement of in-
      order-delivery for protocols that support configurable ordering.

   o  Idempotent: when this is true, it means that the Message can be
      used by mechanisms that might transfer it multiple times - e.g.,




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      as a result of racing multiple transports or as part of TCP Fast
      Open.

   o  Final: when this is true, it means that a transport connection can
      be closed immediately after its transmission.

   o  Corruption Protection Length: when this is set to any value other
      than -1, it limits the required checksum in protocols that allow
      limiting the checksum length (e.g.  UDP-Lite).

   o  Transmission Profile: TBD - because it's not final in the API yet.
      Old text follows: when this is set to "Interactive/Low Latency",
      the Message should be sent immediately, even when this comes at
      the cost of using the network capacity less efficiently.  For
      example, small messages can sometimes be bundled to fit into a
      single data packet for the sake of reducing header overhead; such
      bundling should not be used.  For example, in case of TCP, the
      Nagle algorithm should be disabled when Interactive/Low Latency is
      selected as the capacity profile.  Scavenger/Bulk can translate
      into usage of a congestion control mechanism such as LEDBAT, and/
      or the capacity profile can lead to a choice of a DSCP value as
      described in [I-D.ietf-taps-minset]).

   o  Singular Transmission: when this is true, the application requests
      to avoid transport-layer segmentation or network-layer
      fragmentation.  Some transports implement network-layer
      fragmentation avoidance (Path MTU Discovery) without exposing this
      functionality to the application; in this case, only transport-
      layer segmentation should be avoided, by fitting the message into
      a single transport-layer segment or otherwise failing.  Otherwise,
      network-layer fragmentation should be avoided--e.g. by requesting
      the IP Don't Fragment bit to be set in case of UDP(-Lite) and IPv4
      (SET_DF in [RFC8304]).

5.1.1.2.  Send Completion

   The application should be notified whenever a Message or partial
   Message has been consumed by the Protocol Stack, or has failed to
   send.  The meaning of the Message being consumed by the stack may
   vary depending on the protocol.  For a basic datagram protocol like
   UDP, this may correspond to the time when the packet is sent into the
   interface driver.  For a protocol that buffers data in queues, like
   TCP, this may correspond to when the data has entered the send
   buffer.







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5.1.1.3.  Batching Sends

   Since sending a Message may involve a context switch between the
   application and the transport system, sending patterns that involve
   multiple small Messages can incur high overhead if each needs to be
   enqueued separately.  To avoid this, the application should have a
   way to indicate a batch of Send actions, during which time the
   implementation will hold off on processing Messages until the batch
   is complete.  This can also help context switches when enqueuing data
   in the interface driver if the operation can be batched.

5.1.2.  Receiving Messages

   Similar to sending, Receiving a Message is determined by the top-
   level protocol in the established Protocol Stack.  The main
   difference with Receiving is that the size and boundaries of the
   Message are not known beforehand.  The application can communicate in
   its Receive action the parameters for the Message, which can help the
   implementation know how much data to deliver and when.  For example,
   if the application only wants to receive a complete Message, the
   implementation should wait until an entire Message (datagram, stream,
   or frame) is read before delivering any Message content to the
   application.  This requires the implementation to understand where
   messages end, either via a supplied deframer or because the top-level
   protocol in the established Protocol Stack preserves message
   boundaries; if, on the other hand, the top-level protocol only
   supports a byte-stream and no deframers were supported, the
   application must specify the minimum number of bytes of Message
   content it wants to receive (which may be just a single byte) to
   control the flow of received data.

   If a Connection becomes finished before a requested Receive action
   can be satisfied, the implementation should deliver any partial
   Message content outstanding, or if none is available, an indication
   that there will be no more received Messages.

5.2.  Handling of data for fast-open protocols

   Several protocols allow sending higher-level protocol or application
   data within the first packet of their protocol establishment, such as
   TCP Fast Open [RFC7413] and TLS 1.3 [RFC8446].  This approach is
   referred to as sending Zero-RTT (0-RTT) data.  This is a desirable
   property, but poses challenges to an implementation that uses racing
   during connection establishment.

   If the application has 0-RTT data to send in any protocol handshakes,
   it needs to provide this data before the handshakes have begun.  When
   racing, this means that the data should be provided before the



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   process of connection establishment has begun.  If the application
   wants to send 0-RTT data, it must indicate this to the implementation
   by setting the Idempotent send parameter to true when sending the
   data.  In general, 0-RTT data may be replayed (for example, if a TCP
   SYN contains data, and the SYN is retransmitted, the data will be
   retransmitted as well), but racing means that different leaf nodes
   have the opportunity to send the same data independently.  If data is
   truly idempotent, this should be permissible.

   Once the application has provided its 0-RTT data, an implementation
   should keep a copy of this data and provide it to each new leaf node
   that is started and for which a 0-RTT protocol is being used.

   It is also possible that protocol stacks within a particular leaf
   node use 0-RTT handshakes without any idempotent application data.
   For example, TCP Fast Open could use a Client Hello from TLS as its
   0-RTT data, shortening the cumulative handshake time.

   0-RTT handshakes often rely on previous state, such as TCP Fast Open
   cookies, previously established TLS tickets, or out-of-band
   distributed pre-shared keys (PSKs).  Implementations should be aware
   of security concerns around using these tokens across multiple
   addresses or paths when racing.  In the case of TLS, any given ticket
   or PSK should only be used on one leaf node.  If implementations have
   multiple tickets available from a previous connection, each leaf node
   attempt must use a different ticket.  In effect, each leaf node will
   send the same early application data, yet encoded (encrypted)
   differently on the wire.

6.  Implementing Maintenance

   Maintenance encompasses changes that the application can request to a
   Connection, or that a Connection can react to based on system and
   network changes.

6.1.  Managing Connections

   Appendix A.1 of [I-D.ietf-taps-minset] explains, using primitives
   from [RFC8303] and [RFC8304], how to implement changing some of the
   following protocol properties of an established connection with TCP
   and UDP.  Below, we amend this description for other protocols (if
   applicable) and extend it with Connection Properties that are not
   contained in [I-D.ietf-taps-minset].

   o  Notification of excessive retransmissions: TODO






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   o  Retransmission threshold before excessive retransmission
      notification: TODO; for TCP, this can be done using ERROR.TCP
      described in section 4 of [RFC8303].

   o  Notification of ICMP soft error message arrival: TODO

   o  Required minimum coverage of the checksum for receiving: for UDP-
      Lite, this can be done using the primitive
      SET_MIN_CHECKSUM_COVERAGE.UDP-Lite described in section 4 of
      [RFC8303].

   o  Priority (Connection): TODO; for SCTP, this can be done using the
      primitive CONFIGURE_STREAM_SCHEDULER.SCTP described in section 4
      of [RFC8303].

   o  Timeout for aborting Connection: for SCTP, this can be done using
      the primitive CHANGE_TIMEOUT.SCTP described in section 4 of
      [RFC8303].

   o  Connection group transmission scheduler: for SCTP, this can be
      done using the primitive SET_STREAM_SCHEDULER.SCTP described in
      section 4 of [RFC8303].

   o  Maximum message size concurrent with Connection establishment:
      TODO

   o  Maximum Message size before fragmentation or segmentation: TODO

   o  Maximum Message size on send: TODO

   o  Maximum Message size on receive: TODO

   o  Capacity Profile: TODO

   o  Bounds on Send or Receive Rate: TODO

   o  TCP-specific Property: User Timeout: for TCP, this can be
      configured using the primitive CHANGE_TIMEOUT.TCP described in
      section 4 of [RFC8303].

   It may happen that the application attempts to set a Protocol
   Property which does not apply to the actually chosen protocol.  In
   this case, the implementation should fail gracefully, i.e., it may
   give a warning to the application, but it should not terminate the
   Connection.






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6.2.  Handling Path Changes

   When a path change occurs, the Transport Services implementation is
   responsible for notifying Protocol Instances in the Protocol Stack.
   If the Protocol Stack includes a transport protocol that supports
   multipath connectivity, an update to the available paths should
   inform the Protocol Instance of the new set of paths that are
   permissible based on the Selection Properties passed by the
   application.  A multipath protocol can establish new subflows over
   new paths, and should tear down subflows over paths that are no
   longer available.  If the Protocol Stack includes a transport
   protocol that does not support multipath, but support migrating
   between paths, the update to available paths can be used as the
   trigger to migrating the connection.  For protocols that do not
   support multipath or migration, the Protocol Instances may be
   informed of the path change, but should not be forcibly disconnected
   if the previously used path becomes unavailable.  An exception to
   this case is if the System Policy changes to prohibit traffic from
   the Connection based on its properties, in which case the Protocol
   Stack should be disconnected.

7.  Implementing Termination

   With TCP, when an application closes a connection, this means that it
   has no more data to send (but expects all data that has been handed
   over to be reliably delivered).  However, with TCP only, "close" does
   not mean that the application will stop receiving data.  This is
   related to TCP's ability to support half-closed connections.

   SCTP is an example of a protocol that does not support such half-
   closed connections.  Hence, with SCTP, the meaning of "close" is
   stricter: an application has no more data to send (but expects all
   data that has been handed over to be reliably delivered), and will
   also not receive any more data.

   Implementing a protocol independent transport system means that the
   exposed semantics must be the strictest subset of the semantics of
   all supported protocols.  Hence, as is common with all reliable
   transport protocols, after a Close action, the application can expect
   to have its reliability requirements honored regarding the data it
   has given to the Transport System, but it cannot expect to be able to
   read any more data after calling Close.

   Abort differs from Close only in that no guarantees are given
   regarding data that the application has handed over to the Transport
   System before calling Abort.





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   As explained in Section 4.6, when a new stream is multiplexed on an
   already existing connection of a Transport Protocol Instance, there
   is no need for a connection establishment procedure.  Because the
   Connections that are offered by the Transport System can be
   implemented as streams that are multiplexed on a transport protocol's
   connection, it can therefore not be guaranteed that one Endpoint's
   Initiate action provokes a ConnectionReceived event at its peer.

   For Close (provoking a Finished event) and Abort (provoking a
   ConnectionError event), the same logic applies: while it is desirable
   to be informed when a peer closes or aborts a Connection, whether
   this is possible depends on the underlying protocol, and no
   guarantees can be given.  With SCTP, the transport system can use the
   stream reset procedure to cause a Finish event upon a Close action
   from the peer [NEAT-flow-mapping].

8.  Cached State

   Beyond a single Connection's lifetime, it is useful for an
   implementation to keep state and history.  This cached state can help
   improve future Connection establishment due to re-using results and
   credentials, and favoring paths and protocols that performed well in
   the past.

   Cached state may be associated with different Endpoints for the same
   Connection, depending on the protocol generating the cached content.
   For example, session tickets for TLS are associated with specific
   endpoints, and thus should be cached based on a Connection's hostname
   Endpoint (if applicable).  On the other hand, performance
   characteristics of a path are more likely tied to the IP address and
   subnet being used.

8.1.  Protocol state caches

   Some protocols will have long-term state to be cached in association
   with Endpoints.  This state often has some time after which it is
   expired, so the implementation should allow each protocol to specify
   an expiration for cached content.

   Examples of cached protocol state include:

   o  The DNS protocol can cache resolution answers (A and AAAA queries,
      for example), associated with a Time To Live (TTL) to be used for
      future hostname resolutions without requiring asking the DNS
      resolver again.

   o  TLS caches session state and tickets based on a hostname, which
      can be used for resuming sessions with a server.



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   o  TCP can cache cookies for use in TCP Fast Open.

   Cached protocol state is primarily used during Connection
   establishment for a single Protocol Stack, but may be used to
   influence an implementation's preference between several candidate
   Protocol Stacks.  For example, if two IP address Endpoints are
   otherwise equally preferred, an implementation may choose to attempt
   a connection to an address for which it has a TCP Fast Open cookie.

   Applications must have a way to flush protocol cache state if
   desired.  This may be necessary, for example, if application-layer
   identifiers rotate and clients wish to avoid linkability via
   trackable TLS tickets or TFO cookies.

8.2.  Performance caches

   In addition to protocol state, Protocol Instances should provide data
   into a performance-oriented cache to help guide future protocol and
   path selection.  Some performance information can be gathered
   generically across several protocols to allow predictive comparisons
   between protocols on given paths:

   o  Observed Round Trip Time

   o  Connection Establishment latency

   o  Connection Establishment success rate

   These items can be cached on a per-address and per-subnet
   granularity, and averaged between different values.  The information
   should be cached on a per-network basis, since it is expected that
   different network attachments will have different performance
   characteristics.  Besides Protocol Instances, other system entities
   may also provide data into performance-oriented caches.  This could
   for instance be signal strength information reported by radio modems
   like Wi-Fi and mobile broadband or information about the battery-
   level of the device.  Furthermore, the system may cache the observed
   maximum throughput on a path as an estimate of the available
   bandwidth.

   An implementation should use this information, when possible, to
   determine preference between candidate paths, endpoints, and protocol
   options.  Eligible options that historically had significantly better
   performance than others should be selected first when gathering
   candidates (see Section 4.1) to ensure better performance for the
   application.





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   The reasonable lifetime for cached performance values will vary
   depending on the nature of the value.  Certain information, like the
   connection establishment success rate to a Remote Endpoint using a
   given protocol stack, can be stored for a long period of time (hours
   or longer), since it is expected that the capabilities of the Remote
   Endpoint are not changing very quickly.  On the other hand, Round
   Trip Time observed by TCP over a particular network path may vary
   over a relatively short time interval.  For such values, the
   implementation should remove them from the cache more quickly, or
   treat older values with less confidence/weight.

9.  Specific Transport Protocol Considerations

   Each protocol that can run as part of a Transport Services
   implementation defines both its API mapping as well as implementation
   details.

   API mappings for a protocol apply most to Connections in which the
   given protocol is the "top" of the Protocol Stack.  For example, the
   mapping of the "Send" function for TCP applies to Connections in
   which the application directly sends over TCP.  If HTTP/2 is used on
   top of TCP, the HTTP/2 mappings take precendence.

   Each protocol has a notion of Connectedness.  Possible values for
   Connectedness are:

   o  Unconnected.  Unconnected protocols do not establish explicit
      state between endpoints, and do not perform a handshake during
      Connection establishment.

   o  Connected.  Connected protocols establish state between endpoints,
      and perform a handshake during Connection establishment.  The
      handshake may be 0-RTT to send data or resume a session, but
      bidirectional traffic is required to confirm connectedness.

   o  Multiplexing Connected.  Multiplexing Connected protocols share
      properties with Connected protocols, but also explictly support
      opening multiple application-level flows.  This means that they
      can support cloning new Connection objects without a new explicit
      handshake.

   Protocols also define a notion of Data Unit.  Possible values for
   Data Unit are:

   o  Byte-stream.  Byte-stream protocols do not define any Message
      boundaries of their own apart from the end of a stream in each
      direction.




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   o  Datagram.  Datagram protocols define Message boundaries at the
      same level of transmission, such that only complete (not partial)
      Messages are supported.

   o  Message.  Message protocols support Message boundaries that can be
      sent and received either as complete or partial Messages.  Maximum
      Message lengths can be defined, and Messages can be partially
      reliable.

9.1.  TCP

   Connectedness: Connected

   Data Unit: Byte-stream

   API mappings for TCP are as follows:

   Connection Object:  TCP connections between two hosts map directly to
      Connection objects.

   Initiate:  Calling "Initiate" on a TCP Connection causes it to
      reserve a local port, and send a SYN to the Remote Endpoint.

   InitiateWithSend:  Early idempotent data is sent on a TCP Connection
      in the SYN, as TCP Fast Open data.

   Ready:  A TCP Connection is ready once the three-way handshake is
      complete.

   InitiateError:  TCP can throw various errors during connection setup.
      Specifically, it is important to handle a RST being sent by the
      peer during the handshake.

   ConnectionError:  Once established, TCP throws errors whenever the
      connection is disconnected, such as due to receive a RST from the
      peer; or hitting a TCP retransmission timeout.

   Listen:  Calling "Listen" for TCP binds a local port and prepares it
      to receive inbound SYN packets from peers.

   ConnectionReceived:  TCP Listeners will deliver new connections once
      they have replied to an inbound SYN with a SYN-ACK.

   Clone:  Calling "Clone" on a TCP Connection creates a new Connection
      with equivalent parameters.  The two Connections are otherwise
      independent.





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   Send:  TCP does not on its own preserve Message boundaries.  Calling
      "Send" on a TCP connection lays out the bytes on the TCP send
      stream without any other delineation.  Any Message marked as Final
      will cause TCP to send a FIN once the Message has been completely
      written.

   Receive:  TCP delivers a stream of bytes without any Message
      delineation.  All data delivered in the "Received" or
      "ReceivedPartial" event will be part of a single stream-wide
      Message that is marked Final (unless a MessageFramer is used).
      EndOfMessage will be delivered when the TCP Connection has
      received a FIN from the peer.

   Close:  Calling "Close" on a TCP Connection indicates that the
      Connection should be gracefully closed by sending a FIN to the
      peer and waiting for a FIN-ACK before delivering the "Closed"
      event.

   Abort:  Calling "Abort" on a TCP Connection indicates that the
      Connection should be immediately closed by sending a RST to the
      peer.

9.2.  UDP

   Connectedness: Unconnected

   Data Unit: Datagram

   API mappings for UDP are as follows:

   Connection Object:  UDP connections represent a pair of specific IP
      addresses and ports on two hosts.

   Initiate:  Calling "Initiate" on a UDP Connection causes it to
      reserve a local port, but does not generate any traffic.

   InitiateWithSend:  Early data on a UDP Connection does not have any
      special meaning.  The data is sent whenever the Connection is
      Ready.

   Ready:  A UDP Connection is ready once the system has reserved a
      local port and has a path to send to the Remote Endpoint.

   InitiateError:  UDP Connections can only generate errors on
      initiation due to port conflicts on the local system.

   ConnectionError:  Once in use, UDP throws errors upon receiving ICMP
      notifications indicating failures in the network.



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   Listen:  Calling "Listen" for UDP binds a local port and prepares it
      to receive inbound UDP datagrams from peers.

   ConnectionReceived:  UDP Listeners will deliver new connections once
      they have received traffic from a new Remote Endpoint.

   Clone:  Calling "Clone" on a UDP Connection creates a new Connection
      with equivalent parameters.  The two Connections are otherwise
      independent.

   Send:  Calling "Send" on a UDP connection sends the data as the
      payload of a complete UDP datagram.  Marking Messages as Final
      does not change anything in the datagram's contents.

   Receive:  UDP only delivers complete Messages to "Received", each of
      which represents a single datagram received in a UDP packet.

   Close:  Calling "Close" on a UDP Connection releases the local port
      reservation.

   Abort:  Calling "Abort" on a UDP Connection is identical to calling
      "Close".

9.3.  TLS

   The mapping of a TLS stream abstraction into the application is
   equivalent to the contract provided by TCP (see Section 9.1), and
   builds upon many of the actions of TCP connections.

   Connectedness: Connected

   Data Unit: Byte-stream

   Connection Object:  Connection objects represent a single TLS
      connection running over a TCP connection between two hosts.

   Initiate:  Calling "Initiate" on a TLS Connection causes it to first
      initiate a TCP connection.  Once the TCP protocol is Ready, the
      TLS handshake will be performed as a client (starting by sending a
      "client_hello", and so on).

   InitiateWithSend:  Early idempotent data is supported by TLS 1.3, and
      sends encrypted application data in the first TLS message when
      performing session resumption.  For older versions of TLS, or if a
      session is not being resumed, the initial data will be delayed
      until the TLS handshake is complete.  TCP Fast Option can also be
      enabled automatically.




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   Ready:  A TLS Connection is ready once the underlying TCP connection
      is Ready, and TLS handshake is also complete and keys have been
      established to encrypt application data.

   InitiateError:  In addition to TCP initiation errors, TLS can
      generate errors during its handshake.  Examples of error include a
      failure of the peer to successfully authenticate, the peer
      rejecting the local authentication, or a failure to match versions
      or algorithms.

   ConnectionError:  TLS connections will generate TCP errors, or errors
      due to failures to rekey or decrypt received messages.

   Listen:  Calling "Listen" for TLS listens on TCP, and sets up
      received connections to perform server-side TLS handshakes.

   ConnectionReceived:  TLS Listeners will deliver new connections once
      they have successfully completed both TCP and TLS handshakes.

   Clone:  As with TCP, calling "Clone" on a TLS Connection creates a
      new Connection with equivalent parameters.  The two Connections
      are otherwise independent.

   Send:  Like TCP, TLS does not preserve message boundaries.  Although
      application data is framed natively in TLS, there is not a general
      guarantee that these TLS messages represent semantically
      meaningful application stream boundaries.  Rather, sending data on
      a TLS Connection only guarantees that the application data will be
      transmitted in an encrypted form.  Marking Messages as Final
      causes a "close_notify" to be generated once the data has been
      written.

   Receive:  Like TCP, TLS delivers a stream of bytes without any
      Message delineation.  The data is decrypted prior to being
      delivered to the application.  If a "close_notify" is received,
      the stream-wide Message will be delivered with EndOfMessage set.

   Close:  Calling "Close" on a TLS Connection indicates that the
      Connection should be gracefully closed by sending a "close_notify"
      to the peer and waiting for a corresponding "close_notify" before
      delivering the "Closed" event.

   Abort:  Calling "Abort" on a TCP Connection indicates that the
      Connection should be immediately closed by sending a
      "close_notify", optionally preceded by "user_canceled", to the
      peer.  Implementations do not need to wait to receive
      "close_notify" before delivering the "Closed" event.




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9.4.  DTLS

   DTLS follows the same behavior as TLS (Section 9.3), with the notable
   exception of not inheriting behavior directly from TCP.  Differences
   from TLS are detailed below, and all cases not explicitly mentioned
   should be considered the same as TLS.

   Connectedness: Connected

   Data Unit: Datagram

   Connection Object:  Connection objects represent a single DTLS
      connection running over a set of UDP ports between two hosts.

   Initiate:  Calling "Initiate" on a DTLS Connection causes it reserve
      a UDP local port, and begin sending handshake messages to the peer
      over UDP.  These messages are reliable, and will be automatically
      retransmitted.

   Ready:  A DTLS Connection is ready once the TLS handshake is complete
      and keys have been established to encrypt application data.

   Send:  Sending over DTLS does preserve message boundaries in the same
      way that UDP datagrams do.  Marking a Message as Final does send a
      "close_notify" like TLS.

   Receive:  Receiving over DTLS delivers one decrypted Message for each
      received DTLS datagram.  If a "close_notify" is received, a
      Message will be delivered that is marked as Final.

9.5.  HTTP

   HTTP requests and responses map naturally into Messages, since they
   are delineated chunks of data with metadata that can be sent over a
   transport.  To that end, HTTP can be seen as the most prevalent
   framing protocol that runs on top of streams like TCP, TLS, etc.

   In order to use a transport Connection that provides HTTP Message
   support, the establishment and closing of the connection can be
   treated as it would without the framing protocol.  Sending and
   receiving of Messages, however, changes to treat each Message as a
   well-delineated HTTP request or response, with the content of the
   Message representing the body, and the Headers being provided in
   Message metadata.

   Connectedness: Multiplexing Connected

   Data Unit: Message



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   Connection Object:  Connection objects represent a flow of HTTP
      messages between a client and a server, which may be an HTTP/1.1
      connection over TCP, or a single stream in an HTTP/2 connection.

   Initiate:  Calling "Initiate" on an HTTP connection intiates a TCP or
      TLS connection as a client.

   Clone:  Calling "Clone" on an HTTP Connection opens a new stream on
      an existing HTTP/2 connection when possible.  If the underlying
      version does not support multiplexed streams, calling "Clone"
      simply creates a new parallel connection.

   Send:  When an application sends an HTTP Message, it is expected to
      provide HTTP header values as a MessageContext in a canonical
      form, along with any associated HTTP message body as the Message
      data.  The HTTP header values are encoded in the specific version
      format upon sending.

   Receive:  HTTP Connections deliver Messages in which HTTP header
      values attached to MessageContexts, and HTTP bodies in Message
      data.

   Close:  Calling "Close" on an HTTP Connection will only close the
      underlying TLS or TCP connection if the HTTP version does not
      support multiplexing.  For HTTP/2, for example, closing the
      connection only closes a specific stream.

9.6.  QUIC

   QUIC provides a multi-streaming interface to an encrypted transport.
   Each stream can be viewed as equivalent to a TLS stream over TCP, so
   a natural mapping is to present each QUIC stream as an individual
   Connection.  The protocol for the stream will be considered Ready
   whenever the underlying QUIC connection is established to the point
   that this stream's data can be sent.  For streams after the first
   stream, this will likely be an immediate operation.

   Closing a single QUIC stream, presented to the application as a
   Connection, does not imply closing the underlying QUIC connection
   itself.  Rather, the implementation may choose to close the QUIC
   connection once all streams have been closed (often after some
   timeout), or after an individual stream Connection sends an Abort.

   Connectedness: Multiplexing Connected

   Data Unit: Stream





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   Connection Object:  Connection objects represent a single QUIC stream
      on a QUIC connection.

9.7.  HTTP/2 transport

   Similar to QUIC (Section 9.6), HTTP/2 provides a multi-streaming
   interface.  This will generally use HTTP as the unit of Messages over
   the streams, in which each stream can be represented as a transport
   Connection.  The lifetime of streams and the HTTP/2 connection should
   be managed as described for QUIC.

   It is possible to treat each HTTP/2 stream as a raw byte-stream
   instead of a carrier for HTTP messages, in which case the Messages
   over the streams can be represented similarly to the TCP stream (one
   Message per direction, see Section 9.1).

   Connectedness: Multiplexing Connected

   Data Unit: Stream

   Connection Object:  Connection objects represent a single HTTP/2
      stream on a HTTP/2 connection.

9.8.  SCTP

   To support sender-side stream schedulers (which are implemented on
   the sender side), a receiver-side Transport System should always
   support message interleaving [RFC8260].

   SCTP messages can be very large.  To allow the reception of large
   messages in pieces, a "partial flag" can be used to inform a (native
   SCTP) receiving application that a message is incomplete.  After
   receiving the "partial flag", this application would know that 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]).  The "partial flag" can therefore facilitate the
   implementation of the receiver buffer in the receiving application,
   at the cost of limiting multiplexing and temporarily creating head-
   of-line blocking delay at the receiver.

   When a Transport System transfers a Message, it seems natural to map
   the Message object to SCTP messages in order to support properties
   such as "Ordered" or "Lifetime" (which maps onto partially reliable
   delivery with a SCTP_PR_SCTP_TTL policy [RFC6458]).  However, since
   multiplexing of Connections onto SCTP streams may happen, and would
   be hidden from the application, the Transport System requires a per-




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   stream receiver buffer anyway, so this potential benefit is lost and
   the "partial flag" becomes unnecessary for the system.

   The problem of long messages either requiring large receiver-side
   buffers or getting in the way of multiplexing is addressed by message
   interleaving [RFC8260], which is yet another reason why a receivers-
   side transport system supporting SCTP should implement this
   mechanism.

10.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no actions for IANA.

11.  Security Considerations

11.1.  Considerations for Candidate Gathering

   Implementations should avoid downgrade attacks that allow network
   interference to cause the implementation to select less secure, or
   entirely insecure, combinations of paths and protocols.

11.2.  Considerations for Candidate Racing

   See Section 5.2 for security considerations around racing with 0-RTT
   data.

   An attacker that knows a particular device is racing several options
   during connection establishment may be able to block packets for the
   first connection attempt, thus inducing the device to fall back to a
   secondary attempt.  This is a problem if the secondary attempts have
   worse security properties that enable further attacks.
   Implementations should ensure that all options have equivalent
   security properties to avoid incentivizing attacks.

   Since results from the network can determine how a connection attempt
   tree is built, such as when DNS returns a list of resolved endpoints,
   it is possible for the network to cause an implementation to consume
   significant on-device resources.  Implementations should limit the
   maximum amount of state allowed for any given node, including the
   number of child nodes, especially when the state is based on results
   from the network.








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12.  Acknowledgements

   This work has received funding from the European Union's Horizon 2020
   research and innovation programme under grant agreement No. 644334
   (NEAT).

   This work has been supported by Leibniz Prize project funds of DFG -
   German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
   FE 570/4-1).

   This work has been supported by the UK Engineering and Physical
   Sciences Research Council under grant EP/R04144X/1.

   Thanks to Stuart Cheshire, Josh Graessley, David Schinazi, and Eric
   Kinnear for their implementation and design efforts, including Happy
   Eyeballs, that heavily influenced this work.

13.  References

13.1.  Normative References

   [I-D.ietf-taps-arch]
              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
              Perkins, C., Tiesel, P., and C. Wood, "An Architecture for
              Transport Services", draft-ietf-taps-arch-03 (work in
              progress), March 2019.

   [I-D.ietf-taps-interface]
              Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
              Kuehlewind, M., Perkins, C., Tiesel, P., and C. Wood, "An
              Abstract Application Layer Interface to Transport
              Services", draft-ietf-taps-interface-03 (work in
              progress), March 2019.

   [I-D.ietf-taps-minset]
              Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for End Systems", draft-ietf-taps-minset-11 (work
              in progress), September 2018.

   [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>.

   [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>.



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   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [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>.

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

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

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

13.2.  Informative References

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), April 2019.

   [NEAT-flow-mapping]
              "Transparent Flow Mapping for NEAT (in Workshop on Future
              of Internet Transport (FIT 2017))", n.d..

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <https://www.rfc-editor.org/info/rfc5245>.





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   [Trickle]  "Trickle - Rate Limiting YouTube Video Streaming (ATC
              2012)", n.d..

Appendix A.  Additional Properties

   This appendix discusses implementation considerations for additional
   parameters and properties that could be used to enhance transport
   protocol and/or path selection, or the transmission of messages given
   a Protocol Stack that implements them.  These are not part of the
   interface, and may be removed from the final document, but are
   presented here to support discussion within the TAPS working group as
   to whether they should be added to a future revision of the base
   specification.

A.1.  Properties Affecting Sorting of Branches

   In addition to the Protocol and Path Selection Properties discussed
   in Section 4.3, the following properties under discussion can
   influence branch sorting:

   o  Bounds on Send or Receive Rate: If the application indicates a
      bound on the expected Send or Receive bitrate, an implementation
      may prefer a path that can likely provide the desired bandwidth,
      based on cached maximum throughput, see Section 8.2.  The
      application may know the Send or Receive Bitrate from metadata in
      adaptive HTTP streaming, such as MPEG-DASH.

   o  Cost Preferences: If the application indicates a preference to
      avoid expensive paths, and some paths are associated with a
      monetary cost, an implementation should decrease the ranking of
      such paths.  If the application indicates that it prohibits using
      expensive paths, paths that are associated with a cost should be
      purged from the decision tree.

Authors' Addresses

   Anna Brunstrom (editor)
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad
   Sweden

   Email: anna.brunstrom@kau.se








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   Tommy Pauly (editor)
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: tpauly@apple.com


   Theresa Enghardt
   TU Berlin
   Marchstrasse 23
   10587 Berlin
   Germany

   Email: theresa@inet.tu-berlin.de


   Karl-Johan Grinnemo
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad
   Sweden

   Email: karl-johan.grinnemo@kau.se


   Tom Jones
   University of Aberdeen
   Fraser Noble Building
   Aberdeen, AB24 3UE
   UK

   Email: tom@erg.abdn.ac.uk


   Philipp S. Tiesel
   TU Berlin
   Einsteinufer 25
   10587 Berlin
   Germany

   Email: philipp@tiesel.net








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   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow G12 8QQ
   United Kingdom

   Email: csp@csperkins.org


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

   Email: michawe@ifi.uio.no



































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