TAPS Working Group                                     A. Brunstrom, Ed.
Internet-Draft                                       Karlstad University
Intended status: Informational                             T. Pauly, Ed.
Expires: November 26, 2018                                    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
                                                            May 25, 2018

             Implementing Interfaces to Transport Services


   The Transport Services architecture [I-D.pauly-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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 26, 2018.

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Copyright Notice

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   document authors.  All rights reserved.

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   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 . . . . . . . . . . . . . . . .   4
     3.2.  Role of system policy . . . . . . . . . . . . . . . . . .   5
   4.  Implementing Connection Establishment . . . . . . . . . . . .   6
     4.1.  Candidate Gathering . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  Structuring Options as a Tree . . . . . . . . . . . .   7
       4.1.2.  Branch Types  . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Branching Order-of-Operations . . . . . . . . . . . . . .  11
     4.3.  Sorting Branches  . . . . . . . . . . . . . . . . . . . .  12
     4.4.  Candidate Racing  . . . . . . . . . . . . . . . . . . . .  13
       4.4.1.  Delayed Racing  . . . . . . . . . . . . . . . . . . .  14
       4.4.2.  Failover  . . . . . . . . . . . . . . . . . . . . . .  15
     4.5.  Completing Establishment  . . . . . . . . . . . . . . . .  15
       4.5.1.  Determining Successful Establishment  . . . . . . . .  16
     4.6.  Establishing multiplexed connections  . . . . . . . . . .  17
     4.7.  Handling racing with "unconnected" protocols  . . . . . .  17
     4.8.  Implementing listeners  . . . . . . . . . . . . . . . . .  18
       4.8.1.  Implementing listeners for Connected Protocols  . . .  18
       4.8.2.  Implementing listeners for Unconnected Protocols  . .  18
       4.8.3.  Implementing listeners for Multiplexed Protocols  . .  19
   5.  Implementing Data Transfer  . . . . . . . . . . . . . . . . .  19
     5.1.  Data transfer for streams, datagrams, and frames  . . . .  19
       5.1.1.  Sending Messages  . . . . . . . . . . . . . . . . . .  19
       5.1.2.  Receiving Messages  . . . . . . . . . . . . . . . . .  21
     5.2.  Handling of data for fast-open protocols  . . . . . . . .  22
   6.  Implementing Maintenance  . . . . . . . . . . . . . . . . . .  23
     6.1.  Changing Protocol Properties  . . . . . . . . . . . . . .  23
     6.2.  Handling Path Changes . . . . . . . . . . . . . . . . . .  24
   7.  Implementing Termination  . . . . . . . . . . . . . . . . . .  24

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   8.  Cached State  . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.  Protocol state caches . . . . . . . . . . . . . . . . . .  25
     8.2.  Performance caches  . . . . . . . . . . . . . . . . . . .  26
   9.  Specific Transport Protocol Considerations  . . . . . . . . .  27
     9.1.  TCP . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.2.  UDP . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.3.  SCTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.4.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.5.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.6.  QUIC  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.7.  HTTP/2 transport  . . . . . . . . . . . . . . . . . . . .  29
   10. Rendezvous and Environment Discovery  . . . . . . . . . . . .  30
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  32
     12.1.  Considerations for Candidate Gathering . . . . . . . . .  32
     12.2.  Considerations for Candidate Racing  . . . . . . . . . .  32
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  33
     14.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Appendix A.  Additional Properties  . . . . . . . . . . . . . . .  34
     A.1.  Properties Affecting Sorting of Branches  . . . . . . . .  35
     A.2.  Send Parameters . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   The Transport Services architecture [I-D.pauly-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.trammell-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

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 regarding
   Protocol and Path Selection.  The implementation stores these objects
   and properties as part of the Preconnection object for use during
   connection establishment.  For Protocol and Path Selection Properties
   that are not provided by the application, the implementation must use
   the default values specified in the Transport Services API

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.

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

   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

3.2.  Role of system policy

   The properties specified during pre-establishment has 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

   1.  Application preferences, i.e., preferences specified during the
       pre-establishment such as Local Endpoint, Remote Endpoint, Path
       Selection Properties, and Protocol 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/
       measurement tools and tools that reside on middleboxes and

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

   In general, any protocol or path used for a connection must conform
   to all three sources of constraints.  Any violation of any of the

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   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 in order to select which set to use.

   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
   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

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   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:]       [Interface: Wi-Fi] [Protocol: TCP]
|-> [Endpoint:]       [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 Path Selection Properties and
   Protocol Selection Properties.

4.1.1.  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.

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

                |  www.example.com:80/Any  |
                  //                    \\
+==========================+       +==========================+
| www.example.com:80/Wi-Fi |       |  www.example.com: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:

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

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   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 [, Wi-Fi, TCP]

4.1.2.  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.  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

   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
   these addresses should follow the recommendations in Happy Eyeballs

   1 [www.example.com:80, Wi-Fi, TCP]
     1.1 [2001:DB8::1.80, Wi-Fi, TCP]
     1.2 [, 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 [, Wi-Fi, TCP]

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   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 [, Any, TCP]
     1.1 [, Wi-Fi, TCP]
     1.2 [, 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.  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 [, Any, HTTP/HTTP Proxy/TCP]
     1.2 [, Any, HTTP/SOCKS/TCP]
     1.3 [www.example.com:80, Any, HTTP/TCP]
       1.3.1 [, 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:

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   1 [www.example.com:443, Any, Any HTTP]
     1.1 [www.example.com:443, Any, QUIC/UDP]
       1.1.1 [, Any, QUIC/UDP]
     1.2 [www.example.com:443, Any, HTTP2/TLS/TCP]
       1.2.1 [, 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 [, Any, SCTP]
     1.2 [www.example.com:80, Any, TCP]
       1.2.1 [, 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.

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

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   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.

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 Protocol and Path Selection Properties, which are specified
   in [I-D.trammell-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 the Protocol and
   Path Selection Properties may be used to sort branches are provided

   o  Interface 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:

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

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

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

   [Note: See Appendix A.1 for additional examples related to Properties
   under discussion.]

   Implementations should process properties in the following order:
   Prohibit, Require, Prefer, Avoid.  If Protocol or Path 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

   For Require and Avoid, Path Selection Properties take precedence over
   Protocol Selection Properties.  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 the Path
   Selection Property, with WiFi at the top.  Then, branches with SCTP
   will be sorted to the top within their subtree according to the
   Protocol Selection Property.  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 Path Selection Property of preferring WiFi
   takes precedence over the Protocol Selection Property of preferring

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

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

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   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

   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.

4.4.1.  Delayed Racing

   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

   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

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   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

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

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

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   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
   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.

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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 Path 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 Path Selection 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 Protocol 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.

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

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.

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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
   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 delimeters) 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.

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   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

   o  Niceness: this represents the ability to de-prioritize a Message
      in favor of 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 niceness 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.,
      as a result of racing multiple transports or as part of TCP Fast

   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  Immediate Acknowledgement: this informs the implementation that
      the sender intends to execute tight control over the send buffer,
      and therefore wants to avoid delayed acknowledgements.  In case of
      SCTP, a request to immediately send acknowledgements can be
      implemented using the "sack-immediately flag" described in
      Section 4.2 of [RFC8303] for the SEND.SCTP primitive.

   o  Instantaneous Capacity Profile: 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

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      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]).

   [Note: See also Appendix A.2 for additional Send Parameters under
   discussion.]  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.  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

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   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 [I-D.ietf-tls-tls13].  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
   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

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   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.  Changing Protocol Properties

   Appendix A.1 of [I-D.ietf-taps-minset] explains, using primitives
   that are described in [RFC8303] and [RFC8304], how to implement
   changing the following protocol properties of an established
   connection with TCP and UDP.  Below, we amend this description for
   other protocols (if applicable):

   o  Relative niceness: for SCTP, this can be done using the primitive
      CONFIGURE_STREAM_SCHEDULER.SCTP described in section 4 of

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

   o  Abort timeout to suggest to the Remote Endpoint: for TCP, this can
      be done using the primitive CHANGE_TIMEOUT.TCP described in
      section 4 of [RFC8303].

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

   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

   o  Connection group transmission scheduler: for SCTP, this can be
      done using the primitive SET_STREAM_SCHEDULER.SCTP 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

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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 Path 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 Tranport
   System before calling Abort.

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   As explained in section 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

   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

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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

9.1.  TCP

   Connection lifetime for TCP translates fairly simply into the the
   abstraction presented to an application.  When the TCP three-way
   handshake is complete, its layer of the Protocol Stack can be
   considered Ready (established).  This event will cause racing of
   Protocol Stack options to complete if TCP is the top-level protocol,
   at which point the application can be notified that the Connection is
   Ready to send and receive.

   If the application sends a Close, that can translate to a graceful
   termination of the TCP connection, which is performed by sending a
   FIN to the remote endpoint.  If the application sends an Abort, then
   the TCP state can be closed abruptly, leading to a RST being sent to
   the peer.

   Without a layer of framing (a top-level protocol in the established
   Protocol Stack that preserves message boundaries, or an application-
   supplied deframer) on top of TCP, the receiver side of the transport
   system implementation can only treat the incoming stream of bytes as
   a single Message, terminated by a FIN when the Remote Endpoint closes
   the Connection.

9.2.  UDP

   UDP as a direct transport does not provide any handshake or
   connectivity state, so the notion of the transport protocol becoming
   Ready or established is degenerate.  Once the system has validated
   that there is a route on which to send and receive UDP datagrams, the
   protocol is considered Ready.  Similarly, a Close or Abort has no
   meaning to the on-the-wire protocol, but simply leads to the local
   state being torn down.

   When sending and receiving messages over UDP, each Message should
   correspond to a single UDP datagram.  The Message can contain

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   metadata about the packet, such as the ECN bits applied to the

9.3.  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-
   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

9.4.  TLS

   The mapping of a TLS stream abstraction into the application is
   equivalent to the contract provided by TCP (see Section 9.1).  The
   Ready state should be determined by the completion of the TLS
   handshake, which involves potentially several more round trips beyond
   the TCP handshake.  The application should not be notified that the
   Connection is Ready until TLS is established.

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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.

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 (possibly after some
   timeout), or after an individual stream Connection sends an Abort.

   Messages over a direct QUIC stream should be represented similarly to
   the TCP stream (one Message per direction, see Section 9.1), unless a
   framing mapping is used on top of QUIC.

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).

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10.  Rendezvous and Environment Discovery

   The connection establishment process outlined in Section 4 is
   appropriate for client-server connections, but needs to be expanded
   in peer-to-peer Rendezvous scenarios, as follows:

   o  Gathering 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 locals is primarily an endpoint 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.

   o  Gathering 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.

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      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.

   o  Establishing Connections

      The set of candidate Local Endpoints and the set of candidate
      Remote Endpoints are paired, to derive a priority ordered set of
      Candidate Paths that can potentially be used to establish a

      Then, communication is attempted over each candidate path, in
      priority order.  If there are multiple candidates with the same
      priority, then connection establishment proceeds simultaneously
      and uses the transient that wins the race to be established.
      Otherwise, connection establishment is sequential, paced at a rate
      that should not congest the network.  Depending on the chosen
      transport, this phase might involve racing TCP connections to a
      server over IPv4 and IPv6 [RFC8305], or it could involve a STUN
      exchange to establish peer-to-peer UDP connectivity [RFC5245], or
      some other means.

   o  Confirming and Maintaining Connections

      Once connectivity has been established, unused resources can be
      released and the chosen path can be confirmed.  This is primarily
      required when establishing peer-to-peer connectivity, where
      connections supporting relayed locals that were not required can
      be closed, and where an associated signalling operation might be
      needed to inform middleboxes and proxies of the chosen path.
      Keep-alive messages may also be sent, as appropriate, to ensure
      NAT and firewall state is maintained, so the Connection remains

   To support ICE, or similar protocols, that involve an out-of-band
   indirect signalling exchange 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.

   (TO-DO: It is expected that a single abstract algorithm can be
   identified that supports both the peer-to-peer and client-server
   connection racing, allowing this text to be merged with Section 4)

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11.  IANA Considerations

   RFC-EDITOR: Please remove this section before publication.

   This document has no actions for IANA.

12.  Security Considerations

12.1.  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.

12.2.  Considerations for Candidate Racing

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

   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.

13.  Acknowledgements

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

   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.

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   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.

14.  References

14.1.  Normative References

              Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for TAPS Systems", draft-ietf-taps-minset-03
              (work in progress), March 2018.

              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
              Perkins, C., Tiesel, P., and C. Wood, "An Architecture for
              Transport Services", draft-pauly-taps-arch-00 (work in
              progress), February 2018.

              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-trammell-taps-interface-00 (work in
              progress), March 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,

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [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,

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   [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,

   [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,

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

14.2.  Informative References

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-12 (work
              in progress), May 2018.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

              "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,

   [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

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   to whether they should be added to a future revision of the base

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  Size to be Sent or Received: An implementation may use the Size to
      be Sent or Received in combination with cached performance
      estimates, see Section 8.2, e.g. the observed Round Trip Time and
      the observed maximum throughput, to compute an estimate of the
      completion time of a transfer over different available paths.  It
      may then prefer the path with the shorter expected completion
      time.  This property may be used instead of the Capacity profile,
      as the application does not always know whether its transfer will
      be latency-bound or bandwidth-bound, and thus may not be able to
      specify a Capacity Profile.  However, the application may know the
      Size to be Sent or Received from metadata, e.g., in adaptive HTTP
      streaming such as MPEG-DASH, or in operating system upgrades.  A
      related paper is currently under submission.

   o  Send / Receive Bitrate: If the application indicates an 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

   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.

A.2.  Send Parameters

   In addition to the Send Parameters listed in Section, the
   following Send Parameters are under discussion:

   o  Send Bitrate: If an application indicates a certain bitrate it
      wants to send on the connection, the implementation may limit the
      bitrate of the outgoing communication to that rate, for example by
      setting an upper bound for the TCP congestion window of a
      connection calculated from the Send Bitrate and the Round Trip

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      Time.  This helps to avoid bursty traffic patterns on video
      streaming servers, see [Trickle].

Authors' Addresses

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

   Email: anna.brunstrom@kau.se

   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

   Email: theresa@inet.tu-berlin.de

   Karl-Johan Grinnemo
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad

   Email: karl-johan.grinnemo@kau.se

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

   Email: tom@erg.abdn.ac.uk

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   Philipp S. Tiesel
   TU Berlin
   Marchstrasse 23
   10587 Berlin

   Email: philipp@inet.tu-berlin.de

   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

   Email: michawe@ifi.uio.no

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