



Internet Engineering Task Force (IETF)                 A. Brunstrom, Ed.
Request for Comments: 9623                           Karlstad University
Category: Informational                                    T. Pauly, Ed.
ISSN: 2070-1721                                               Apple Inc.
                                                             R. Enghardt
                                                                 Netflix
                                                             P.S. Tiesel
                                                                  SAP SE
                                                                M. Welzl
                                                      University of Oslo
                                                            January 2025


             Implementing Interfaces to Transport Services

Abstract

   The Transport Services System enables applications to use transport
   protocols flexibly for network communication and defines a protocol-
   independent Transport Services Application Programming Interface
   (API) that is based on an asynchronous, event-driven interaction
   pattern.  This document serves as a guide to implementing such a
   system.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9623.

Copyright Notice

   Copyright (c) 2025 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Implementing Connection Objects
   3.  Implementing Preestablishment
     3.1.  Configuration-Time Errors
     3.2.  Role of System Policy
   4.  Implementing Connection Establishment
     4.1.  Structuring Candidates as a Tree
       4.1.1.  Branch Types
       4.1.2.  Branching Order-of-Operations
       4.1.3.  Sorting Branches
     4.2.  Candidate Gathering
       4.2.1.  Gathering Endpoint Candidates
     4.3.  Candidate Racing
       4.3.1.  Simultaneous
       4.3.2.  Staggered
       4.3.3.  Failover
     4.4.  Completing Establishment
       4.4.1.  Determining Successful Establishment
     4.5.  Establishing Multiplexed Connections
     4.6.  Handling Connectionless Protocols
     4.7.  Implementing Listeners
       4.7.1.  Implementing Listeners for Connected Protocols
       4.7.2.  Implementing Listeners for Connectionless Protocols
       4.7.3.  Implementing Listeners for Multiplexed Protocols
   5.  Implementing Sending and Receiving Data
     5.1.  Sending Messages
       5.1.1.  Message Properties
       5.1.2.  Send Completion
       5.1.3.  Batching Sends
     5.2.  Receiving Messages
     5.3.  Handling of Data for Fast-Open Protocols
   6.  Implementing Message Framers
     6.1.  Defining Message Framers
     6.2.  Sender-Side Message Framing
     6.3.  Receiver-Side Message Framing
   7.  Implementing Connection Management
     7.1.  Pooled Connection
     7.2.  Handling Path Changes
   8.  Implementing Connection Termination
   9.  Cached State
     9.1.  Protocol State Caches
     9.2.  Performance Caches
   10. Specific Transport Protocol Considerations
     10.1.  TCP
     10.2.  MPTCP
     10.3.  UDP
     10.4.  UDP-Lite
     10.5.  UDP Multicast Receive
     10.6.  SCTP
   11. IANA Considerations
   12. Security Considerations
     12.1.  Considerations for Candidate Gathering
     12.2.  Considerations for Candidate Racing
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Appendix A.  API Mapping Template
   Appendix B.  Reasons for Errors
   Appendix C.  Existing Implementations
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Transport Services Architecture [RFC9621] defines a system that
   allows applications to flexibly use transport networking protocols.
   The API that such a system exposes to applications is defined as the
   Transport Services API [RFC9622].  This API is designed to be generic
   across multiple transport protocols and sets of protocol features.

   This document serves as a guide to implementing a system that
   provides a Transport Services API.  This guide offers suggestions to
   developers, but it is not prescriptive: implementations are free to
   take any desired form as long as the API specification defined in
   [RFC9622] is honored.  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 terminology defined in the Transport
   Services Architecture [RFC9621].

2.  Implementing Connection Objects

   The Connection objects that are exposed to applications for Transport
   Services are:

   *  the Preconnection, the bundle of Properties that describes the
      application constraints on, and preferences for, the transport;

   *  the Connection, the basic object that represents a flow of data as
      Messages in either 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.  A Preconnection object
   influences a Connection only at one point in time: when the
   Connection is created.  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 not necessarily be immediately bound
   to a transport protocol instance, since multiple candidate Protocol
   Stacks might be raced.

   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 cannot be mutated by
   the application making changes to the original Preconnection object.
   This may involve the implementation performing a deep-copy, copying
   the object with all the objects that it references.

   Once the Connection is established, the Transport Services
   Implementation maps actions and events to the details of the chosen
   Protocol Stack.  For example, the same Connection object may
   ultimately represent a single transport protocol instance (e.g., a
   TCP connection, a TLS session over TCP, a UDP flow with fully
   specified Local and Remote Endpoint Identifiers, a DTLS session, a
   Stream Control Transmission Protocol (SCTP) stream, a QUIC stream, or
   an HTTP/2 stream).  The Connection Properties held by a Connection or
   Listener are independent of other Connections that are not part of
   the same Connection Group.

   Connection establishment is only a local operation for connectionless
   protocols, which serves to simplify the local send/receive functions
   and to filter the traffic for the specified addresses and ports
   [RFC8085] (for example, using UDP or UDP-Lite transport without a
   connection handshake procedure).

   Once Initiate has been called, the Selection Properties and Endpoint
   information of the created Connection are immutable (i.e., an
   application is not able to later modify the Properties of a
   Connection by manipulating the original Preconnection object).
   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 4.7.

3.  Implementing Preestablishment

   The preestablishment phase allows applications to specify Properties
   for the Connections that they are about to make or to query the API
   about potential Connections they could make.

   During preestablishment, the application specifies one or more
   Endpoints to be used for communication as well as protocol
   preferences and constraints via Selection Properties and, if desired,
   also Connection Properties.  Section 4 of [RFC9622] states that
   Connection Properties should preferably be configured during
   preestablishment because they can serve as input to decisions that
   are made by the implementation (e.g., the capacity profile can guide
   usage of a protocol offering scavenger-type congestion control).

   The implementation stores these Properties as a part of the
   Preconnection object for use during Connection establishment.  For
   Selection Properties that are not provided by the application, the
   implementation uses the default values specified in the Transport
   Services API ([RFC9622]).

3.1.  Configuration-Time Errors

   The Transport Services System should have a list of supported
   protocols available, each of which has transport features reflecting
   the capabilities of the protocol.  Once an application specifies its
   Transport Properties, the Transport Services System matches the
   required and prohibited Properties against the transport features of
   the available protocols (see Section 6.2 of [RFC9622] for the
   definition of Property Preferences).

   In the following cases, failure should be detected during
   preestablishment:

   *  A request by an application for Properties that cannot be
      satisfied by any of the available protocols.  For example, if an
      application requires perMsgReliability, but no such feature is
      available in any protocol on the host running the Transport
      Services System, this should result in an error.

   *  A request by an application for Properties that are in conflict
      with each other, such as specifying required and prohibited
      Properties that cannot be satisfied by any protocol.  For example,
      if an application prohibits reliability but then requires
      perMsgReliability, this mismatch should result in an error.

   To avoid allocating resources that are not needed, it is important
   that configuration-time errors fail as early as possible.

3.2.  Role of System Policy

   The Properties specified during preestablishment have a close
   relationship 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
       preestablishment 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/
       measurement tools and tools that reside on middleboxes and
       routers.

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

   In general, any protocol or path used for a Connection must conform
   to all three sources of constraints.  A violation that occurs at any
   of the policy layers should cause a protocol or path to be considered
   ineligible for use.  If such a violation prevents a Connection from
   being established, this should be communicated to the application,
   e.g., via the EstablishmentError event.  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.

   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 reflects 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 which point the Preconnection object contains
   all constraints or requirements the application has configured.  The
   establishment 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 (defined in Section 4.2.1 of [RFC9621]) to
      identify the paths, protocols, and endpoints to use (see
      Section 4.2) and

   *  Candidate Racing (defined in Section 4.2.2 of [RFC9621]), in which
      the necessary protocol handshakes are conducted so that the
      Transport Services System can select which set to use (see
      Section 4.3).

   Candidate Racing involves attempting multiple options for Connection
   establishment and choosing the first option to succeed as the
   Protocol Stack to use for the Connection.  These attempts are usually
   staggered, with each next option starting after a delay; however,
   they can also be performed in parallel or after failures occur.

   For ease of illustration, this document structures the candidates for
   racing as a tree (see Section 4.1).  This is not meant to restrict
   implementations from structuring racing candidates differently.

   The simplest example of this process might involve identifying the
   single IP address to which the implementation wishes to connect,
   using the system's current default path (i.e., using the default
   interface), and starting a TCP handshake to establish a stream to the
   specified IP address.  However, each step may also differ depending
   on the requirements of the connection:

   *  if the Endpoint Identifier is a hostname and port, then there may
      be multiple resolved addresses that are available;

   *  there may also be multiple paths available (in this case using an
      interface 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 Identifier and that has two valid interfaces available (Wi-
   Fi and LTE).  The hostname resolves to a single IPv4 address on the
   Wi-Fi network, to the same IPv4 address on the LTE network, and to a
   single IPv6 address.  The aggregate set of Connection establishment
   options can be viewed as follows, with the Endpoint Identifier
   abbreviated as "EId":

   Aggregate [EId: example.com:443] [Interface: Any]   [Protocol: TCP]
   |-> [EId: [3fff:23::1]:443]      [Interface: Wi-Fi] [Protocol: TCP]
   |-> [EId: 192.0.2.1:443]         [Interface: LTE]   [Protocol: TCP]
   |-> [EId: [3fff:42::1]:443]      [Interface: LTE]   [Protocol: TCP]

   Any one of these subentries 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 to try
   them, and in what order.

   During Candidate Gathering (Section 4.2), an implementation prunes
   and sorts branches according to the Selection Property Preferences
   (Section 6.2 of [RFC9622]).  First, it excludes all protocols and
   paths that match a prohibited Property or do not match all required
   Properties.  Then, it will sort branches according to preferred
   Properties, avoided Properties, and, possibly, other criteria.

4.1.  Structuring Candidates as a Tree

   As noted above, the consideration of multiple candidates in a
   gathering and racing process can be conceptually structured as a
   tree; this terminological convention is used throughout this
   document.

   Each leaf node of the tree represents a single coherent connection
   attempt with an endpoint, a network 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 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 Identifier.  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 network 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:443/any path
                 +============================+
                   //                     \\
   +=========================+      +=======================+
    www.example.com:443/Wi-Fi        www.example.com:443/LTE
   +=========================+      +=======================+
               ||                      //               \\
   +======================+  +=================+  +====================+
    [3fff:23::1]:443/Wi-Fi    192.0.2.1:443/LTE    [3fff:42::1]:443/LTE
   +======================+  +=================+  +====================+

   The rest of this section will use a notation scheme to represent this
   tree.  The root node (or parent node) of the tree will be represented
   by a single integer, such as "1". ("1" is used assuming that this is
   the first connection made by the system; future connections created
   by the application would allocate numbers in an increasing manner.)
   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 its parent's identifier
   with a dot character (".") in between, such as "1.1" and "1.2".  Each
   node will be summarized by a tuple of three elements: endpoint, path
   (labeled here by interface), and protocol.  In Protocol Stacks, the
   layers are separated by a slash character ("/") and ordered with the
   protocol closest to the application first.  The above example can now
   be written more succinctly as:

   1 [www.example.com:443, any path, TCP]
     1.1 [www.example.com:443, Wi-Fi, TCP]
       1.1.1 [[2001:db8:23::1]:443, Wi-Fi, TCP]
     1.2 [www.example.com:443, LTE, TCP]
       1.2.1 [192.0.2.1:443, LTE, TCP]
       1.2.2 [[2001:db8.42::1]:443, LTE, TCP]

   When an implementation is asked to establish a single connection,
   only one of the leaf nodes in the candidate set is needed to transfer
   data.  Thus, once a single leaf node becomes ready to use, the
   Connection establishment tree is considered ready.  One way to
   implement this is by having every leaf node update the state of its
   parent node when it becomes ready until the root 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 consist of only a single node,
   such as a connection attempt to an IP address over a single interface
   with a single protocol.

   1 [[2001:db8:23::1]:443, Wi-Fi, TCP]

   A root 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:443, Wi-Fi, TCP]
     1.1 [[2001:db8:23::1]:443, Wi-Fi, TCP]

4.1.1.  Branch Types

   There are three types of branching from a parent node into one or
   more child nodes: Derived Endpoints, network paths, and protocol
   options.  Any parent node of the tree must use only one type of
   branching.

4.1.1.1.  Derived Endpoints

   If a connection originally targets a single Endpoint Identifier,
   there may be multiple endpoint candidates of different types that can
   be derived from the original.  This creates an ordered list of the
   derived endpoint candidates 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
   Identifier on an IP network, the implementation should send all
   applicable DNS queries.  Commonly, this will include both A (IPv4)
   and AAAA (IPv6) records if both address families are supported on the
   local interface.  This can also include SRV records [RFC2782], SVCB
   and HTTPS records [RFC9460], or other future record types.  The
   algorithm for ordering and racing these addresses should follow the
   recommendations in Happy Eyeballs [RFC8305].

   1 [www.example.com:443, Wi-Fi, TCP]
     1.1 [[2001:db8::1]:443, Wi-Fi, TCP]
     1.2 [192.0.2.1:443, Wi-Fi, TCP]
     1.3 [[2001:db8::2]:443, Wi-Fi, TCP]
     1.4 [[2001:db8::3]:443, Wi-Fi, TCP]

   DNS-Based Service Discovery [RFC6763] 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 [RFC6762].  These hostnames should each
   be treated as a branch that can be attempted independently from other
   hostnames.  Each of these hostnames might resolve to one or more
   addresses, which would create multiple layers of branching.

   1 [term-printer._ipp._tcp.meeting.example.com, Wi-Fi, TCP]
     1.1 [term-printer.meeting.example.com:631, Wi-Fi, TCP]
       1.1.1 [31.133.160.18:631, Wi-Fi, TCP]

   Applications can influence which derived Endpoints are allowed and
   preferred via Selection Properties set on the Preconnection.  For
   example, setting a preference for useTemporaryLocalAddress would
   prefer the use of IPv6 over IPv4, and requiring
   useTemporaryLocalAddress would eliminate IPv4 options since IPv4 does
   not support temporary addresses.

4.1.1.2.  Network Paths

   If a client has multiple network paths available to it, e.g., a
   mobile client with interfaces for both Wi-Fi and Cellular
   connectivity, it can attempt a connection over any of the paths.
   This represents a branch point in the Connection establishment.
   Similar to a derived endpoint, the paths should be ranked based on
   preference, policy, and performance.  Attempts should be started on
   one path (e.g., a specific interface) and then successively on other
   paths (or interfaces) after delays based on the expected path RTT or
   other available metrics.

   1 [192.0.2.1:443, any path, TCP]
     1.1 [192.0.2.1:443, Wi-Fi, TCP]
     1.2 [192.0.2.1:443, LTE, TCP]

   The same approach applies to any situation in which the client is
   aware of multiple links or views of the network.  A single interface
   may be shared by multiple network paths, each with a coherent set of
   addresses, routes, DNS server, and more.  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
   the application sets as well as by the System Policy.

4.1.1.3.  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 might have several
   options available: an HTTP-based proxy, a SOCKS-based proxy, or no
   proxy.  As above, these options should be ranked based on preference,
   System Policy, and performance, and should be attempted in
   succession.

   1 [www.example.com:443, any path, HTTP/TCP]
     1.1 [192.0.2.8:443, any path, HTTP/HTTP Proxy/TCP]
     1.2 [192.0.2.7:10234, any path, HTTP/SOCKS/TCP]
     1.3 [www.example.com:443, any path, HTTP/TCP]
       1.3.1 [192.0.2.1:443, any path, HTTP/TCP]

   This approach also allows a client to attempt different sets of
   application and transport protocols that, when available, could
   provide preferable features.  For example, the protocol options could
   involve QUIC [RFC9000] over UDP on one branch and HTTP/2 [RFC9113]
   over TLS over TCP on the other:

   1 [www.example.com:443, any path, HTTP]
     1.1 [www.example.com:443, any path, HTTP3/QUIC/UDP]
       1.1.1 [192.0.2.1:443, any path, HTTP3/QUIC/UDP]
     1.2 [www.example.com:443, any path, HTTP2/TLS/TCP]
       1.2.1 [192.0.2.1:443, any path, HTTP2/TLS/TCP]

   Another example is racing SCTP with TCP:

   1 [www.example.com:4740, any path, reliable-inorder-stream]
     1.1 [www.example.com:4740, any path, SCTP]
       1.1.1 [192.0.2.1:4740, any path, SCTP]
     1.2 [www.example.com:4740, any path, TCP]
       1.2.1 [192.0.2.1:4740, any path, TCP]

   Implementations that support racing protocols and protocol options
   should maintain a history of which protocols and protocol options
   were successfully established on a per-network and per-endpoint basis
   (see Section 9.2).  This information can influence future racing
   decisions to prioritize or prune branches.

4.1.2.  Branching Order-of-Operations

   Branch types ought to 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 there are situations when the
   results will be different across networks due to private names or
   different supported IP versions.  Implementations need to be careful
   to branch in a consistent order that results in usable leaf nodes
   whenever there are multiple branch types that could be used from a
   single node.

   This document recommends the following order of operations for
   branching:

   1.  Network paths

   2.  Protocol options

   3.  Derived Endpoints

   where a lower number indicates higher precedence and, therefore,
   higher placement in the tree.  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 the protocols that are supported and preferred may differ
   across interfaces.  Thus, if multiple paths are attempted, the
   overall Connection establishment process can be seen as a race
   between the available paths or interfaces.

   Protocol options are next checked in order.  Whether or not a set of
   protocols, 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 Endpoint Identifiers being used.  Adding a
   proxy to a connection's branch will change the Endpoint Identifier 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
   Wi-Fi over LTE and for a feature only available in SCTP, branches
   will first be sorted according to path selection, with Wi-Fi
   attempted as the first path.  Then, branches with SCTP will be
   attempted within their subtree according to the Properties
   influencing protocol selection.  However, if the implementation has
   current cache information that SCTP is not available on the path over
   Wi-Fi, there would be no SCTP node in the Wi-Fi subtree.  Here, the
   path over Wi-Fi will be attempted first, and, if connection
   establishment succeeds, TCP will be used.  Thus, the Selection
   Property preferring Wi-Fi takes precedence over the Property that led
   to a preference for SCTP.

   1. [www.example.com:80, any path, reliable-inorder-stream]
   1.1 [192.0.2.1:443, Wi-Fi, reliable-inorder-stream]
   1.1.1 [192.0.2.1:443, Wi-Fi, TCP]
   1.2 [192.0.3.1:443, LTE, reliable-inorder-stream]
   1.2.1 [192.0.3.1:443, LTE, SCTP]
   1.2.2 [192.0.3.1:443, LTE, TCP]

4.1.3.  Sorting Branches

   Implementations should sort the branches of the tree of connection
   options in order of their preference rank from most preferred to
   least preferred as specified by Selection Properties [RFC9622].  Leaf
   nodes on branches with higher rankings represent connection attempts
   that will be raced first.

   In addition to the Properties provided by the application, an
   implementation may include additional criteria such as cached
   performance estimates (see Section 9.2) 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:

   "Interface Instance or Type" (Property name interface):
      If the application specifies an interface type to be preferred or
      avoided, implementations should accordingly rank the paths.  If
      the application specifies an interface type to be required or
      prohibited, an implementation is expected to exclude the
      nonconforming paths.

   "Capacity Profile" (Property name connCapacityProfile):
      An implementation can use the capacity profile to prefer paths
      that match an application's expected traffic profile.  This match
      will use cached performance estimates; see Section 9.2.  Some
      examples of path preferences based on capacity profiles include:

      Low Latency/Interactive:  Prefer paths with the lowest expected
         Round-Trip Time (RTT), based on observed RTT estimates;

      Low Latency/Non-Interactive:  Prefer paths with a low expected
         Round-Trip Time (RTT) and possible delay variation;

      Constant-Rate Streaming:  Prefer paths that are expected to
         satisfy the requested stream send or receive bitrate based on
         the observed maximum throughput;

      Capacity-Seeking:  Prefer adapting to paths to determine the
         highest available capacity based on the observed maximum
         throughput.

   As another example, branch sorting can also be influenced by bounds
   on the send or receive rate (Selection Properties minSendRate /
   minRecvRate / maxSendRate / maxRecvRate): 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 9.2).  The
   application may know the send or receive bitrate from metadata in
   adaptive HTTP streaming, such as MPEG-DASH.

   Implementations process the Properties (Section 6.2 of [RFC9622]) 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 the branches according
   to the preferred Properties and use any avoided Properties as a
   tiebreaker.  When ordering branches, an implementation can give more
   weight to Properties that the application has explicitly set rather
   than to the Properties that are set by default.

   The available protocols and paths on a specific system and in a
   specific context can change; therefore, the result of sorting and the
   outcome of racing may vary, even when using the same Selection and
   Connection Properties.  However, an implementation ought to provide a
   consistent outcome to applications, e.g., by preferring protocols and
   paths that are already used by existing Connections that specified
   similar Properties.

4.2.  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, preferences,
   and avoidances of the application as specified in the Selection
   Properties.

4.2.1.  Gathering Endpoint Candidates

   Both Local and Remote Endpoint Candidates must be discovered during
   Connection establishment.  To support Interactive Connectivity
   Establishment (ICE) [RFC8445], or similar protocols that involve out-
   of-band indirect signaling to exchange candidates with the Remote
   Endpoint, it is important to query the set of candidate Local
   Endpoints and provide the Protocol Stack with a set of candidate
   Remote Endpoints before the Local Endpoint attempts to establish
   connections.

4.2.1.1.  Local Endpoint Candidates

   The set of possible Local Endpoints is gathered.  In a simple case,
   this merely enumerates the local interfaces and protocols and
   allocates ephemeral source ports.  For example, a system that has Wi-
   Fi and Ethernet and supports IPv4 and IPv6 might gather four
   candidate Local Endpoints (IPv4 on Ethernet, IPv6 on Ethernet, IPv4
   on Wi-Fi, and IPv6 on Wi-Fi) 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 (see Section 5.1.1 of [RFC8445]).  The endpoint determines its
   server-reflexive Local Endpoints (i.e., the translated address of a
   Local Endpoint, on the other side of a NAT, e.g., via a STUN server
   [RFC8489]) and relayed Local Endpoints (e.g., via a TURN server
   [RFC8656] or other relay) for each interface and network protocol.
   These are added to the set of candidate Local Endpoint Identifiers
   for this connection.

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

4.2.1.2.  Remote Endpoint Candidates

   The Remote Endpoint Identifier 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 Endpoint of this
   Connection.

   How this resolution 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 Identifier 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 Endpoint Identifiers
   might require more complex resolution.  Resolving the Remote Endpoint
   for a peer-to-peer connection might involve communication with a
   rendezvous server.  The server, in turn, contacts the peer to gain
   consent to communicate and retrieve its set of candidate Local
   Endpoints.  These Endpoints are returned and form the candidate
   remote addresses for contacting that peer.

   Resolving the Remote Endpoint is not a local operation.  It will
   involve a directory service and can require communication between the
   Remote Endpoint and a rendezvous server as well as the exchange of
   peer addresses.  This can expose some or all of the candidate Local
   Endpoints to the Remote Endpoint.

4.3.  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 connection attempts as possible.  Optimizing these
   two factors improves the user experience, while minimizing network
   load.

   This section covers the dynamic aspect of Connection establishment.
   The tree described above is a useful conceptual and architectural
   model.  However, an implementation is unable to know all of the nodes
   that will be used until steps like name resolution have occurred;
   many of the possible branches ultimately might not be attempted.

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

   1.  Simultaneous

   2.  Staggered

   3.  Failover

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

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

4.3.1.  Simultaneous

   Simultaneous racing is when multiple alternate branches are started
   without waiting for any one branch to make progress before starting
   the next alternative.  This means the attempts are effectively
   simultaneous.  Simultaneous racing should be avoided by
   implementations since it consumes extra network resources and
   establishes state that might not be used.

4.3.2.  Staggered

   Staggered 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.

   Staggered racing attempts can proceed in parallel.  Implementations
   should not terminate an earlier child connection attempt upon
   starting a secondary child.

   If a child node fails to establish connectivity (as in Section 4.4.1)
   before the delay time has expired for the next child, the next child
   should be started immediately.

   Staggered racing between IP addresses for a generic Connection should
   follow the Happy Eyeballs algorithm described in [RFC8305].  Guidance
   for racing when performing ICE can be found in [RFC8421].

   Generally, the delay before starting a given child node ought to be
   based on the length of time the previously started child node is
   expected to take before it succeeds or makes progress in connection
   establishment.  Algorithms like Happy Eyeballs choose a delay based
   on how long the transport connection handshake is expected to take.
   When performing staggered races in multiple branch types (such as
   racing between network interfaces and then racing between IP
   addresses), a longer delay may be chosen for some branch types.  For
   example, when racing between network interfaces, the delay should
   also take into account the amount of time it takes to prepare the
   network interface (such as radio association) and name resolution
   over that interface in addition to the delay that would be added for
   a single transport connection handshake.

   Since the staggered delay can be chosen based on dynamic information,
   such as predicted RTT, implementations should define upper and lower
   bounds for delay times.  These bounds are implementation specific and
   may differ based on which branch type is being used.

4.3.3.  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
   preferred Protocol Stack that uses a proxy and an alternate Protocol
   Stack that bypasses the proxy.  Failover is useful if 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.

4.4.  Completing Establishment

   The process of Connection establishment completes when one leaf node
   of the tree has successfully completed negotiation with the Remote
   Endpoint or when 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.  This is signaled to the
   application using the Ready event in the API (Section 7.1 of
   [RFC9622]).

   Successes and failures of a given attempt should be reported up to
   parent nodes (toward the root 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
   root 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:443, Any, TCP]
     1.1 [www.example.com:443, Wi-Fi, TCP]
       1.1.1 [192.0.2.1:443, Wi-Fi, TCP]
     1.2 [www.example.com:443, 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 ought not to be started after another leaf node
   has already successfully completed because the Connection as a whole
   has now been established.  An implementation could choose to let
   certain handshakes and negotiations complete to gather metrics that
   influence future connections.  Keeping additional connections is
   generally not recommended because those attempts were slower to
   connect and may exhibit less desirable properties.

4.4.1.  Determining Successful Establishment

   On a per-protocol basis, implementations may select different
   criteria by which a leaf node is considered to be successfully
   connected.  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 three-way handshake completes.
   If the only protocol being used is a connectionless 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 in Section 4.6.

   Depending on the protocols involved, there is no guarantee that the
   Remote Endpoint will be notified when the Initiate action is called
   without any Messages being sent at the same time.  Therefore, a
   passive Endpoint's application may not receive a ConnectionReceived
   event until it receives the first Message on the new Connection.

   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 reduced burden on the network and Remote Endpoints
   from further connection attempts that are likely to be abandoned.  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 the TLS handshake
   is complete.

   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 Properties actually work over the available paths, then the
   Transport Services System should report an EstablishmentError to the
   application.  An EstablishmentError event should also be generated if
   the Transport Services System finds no usable candidates to race.

4.5.  Establishing Multiplexed Connections

   Multiplexing several Connections over a single underlying transport
   connection requires that the multiplexed Connections belong to the
   same Connection Group (as is indicated by the application using the
   Clone action).  When the underlying transport connection supports
   multistreaming, the Transport Services System can map each Connection
   in the Connection Group to a different stream of this connection.

   For such streams, there is often no explicit connection establishment
   procedure for the new stream prior to sending data on it (e.g., with
   SCTP).  In this case, the same considerations apply to determining
   stream establishment as apply to establishing a UDP connection, as
   discussed in Section 4.4.1.  This means that there might not be any
   "establishment" message (like a TCP SYN).

4.6.  Handling Connectionless 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, connectionless 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 the
   Transport Services System has selected a path on which to send data.

   However, this can cause a problem if a specific peer is not reachable
   over the network using the connectionless protocol or data cannot be
   exchanged with the peer for any other reason.  To handle the lack of
   an explicit handshake in the underlying protocol, an application can
   use a Message Framer (Section 6) on top of a connectionless protocol
   to only mark a specific connection attempt as ready when some data
   has been received or after some application-level handshake has been
   performed by the Message Framer.

4.7.  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 Identifier is specified, the implementation should use
   an ephemeral port.

   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 change the registration of the Listener across all
   usable paths as appropriate.  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 support receiving inbound connections for each eligible
   protocol on each eligible path.

4.7.1.  Implementing Listeners for Connected Protocols

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

4.7.2.  Implementing Listeners for Connectionless Protocols

   Connectionless 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 connectionless protocols on a listening port and should perform
   four-tuple matching of packets to existing Connection objects if
   possible.  If a matching Connection object does not exist, an
   incoming packet from a connectionless protocol should cause a new
   Connection object to be created.

4.7.3.  Implementing Listeners for Multiplexed Protocols

   Protocols that provide multiplexing of streams can listen for
   entirely new connections as well as for new subconnections (streams
   of an already-existing connection).  A new stream arrival on an
   existing connection is presented to the application as a new
   Connection.  This new Connection is grouped with all other
   Connections that are multiplexed via the same protocol.

5.  Implementing Sending and Receiving Data

   The most basic mapping for sending a Message is an abstraction of
   datagrams, in which the transport protocol naturally deals in
   discrete packets (such as UDP).  Each Message here corresponds to a
   single datagram.

   For protocols that expose byte-streams (such as TCP), 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 framing (such as length-value protocols, or
   protocols that use delimiters like HTTP/1.1) may support Message
   sizes that do not fit within a single datagram.  Each Message for
   framing protocols corresponds to a single frame, which may be sent
   either as a complete Message in the underlying protocol or in
   multiple parts.

   Messages themselves generally consist of bytes passed in the
   messageData parameter intended to be processed at an application
   layer.  However, Message objects presented through the API can carry
   associated Message Properties passed through the messageContext
   parameter.  When these are Protocol-specific Properties, they can
   include metadata that exists separately from a byte encoding.  For
   example, these Properties can include name-value pairs of
   information, like HTTP header fields.  In such cases, Messages might
   be "empty" insofar as they contain zero bytes in the messageData
   parameter, but they can still include data in the messageContext that
   is interpreted by the Protocol Stack.

5.1.  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.  Message Properties

   The API allows various Properties to be associated with each Message,
   which should be implemented as discussed below.

   msgLifetime:  This should be implemented by removing the Message from
      the queue of pending Messages after the Lifetime has expired.  A
      queue of pending Messages within the Transport Services
      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 removing it from their send buffer.  For example, a
      Transport Services Implementation cannot remove bytes from a TCP
      send buffer, while it can remove data from an SCTP send buffer
      using the partial reliability extension [RFC8303].  When there is
      no standing queue of Messages within the system, and the Protocol
      Stack does not support the removal of a Message from the stack's
      send buffer, this Property may be ignored.

   msgPriority:  This represents the ability to prioritize a Message
      over other Messages.  This can be implemented by the Transport
      Services System by reordering 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.

   msgOrdered:  When this is false, it disables the requirement of in-
      order delivery for protocols that support configurable ordering.
      When the Protocol Stack does not support configurable ordering,
      this Property may be ignored.

   safelyReplayable:  When this is true, it means that the Message can
      be used by a transport mechanism that might deliver it multiple
      times -- e.g., as a result of racing multiple transports or as
      part of TCP Fast Open (TFO).  Also, protocols that do not protect
      against duplicated Messages, such as UDP (when used directly,
      without a protocol layered atop), can only be used with Messages
      that are safely replayable.  When a Transport Services System is
      permitted to replay Messages, replay protection could be provided
      by the application.

   final:  When this is true, it means that the sender will not send any
      further Messages.  The Connection need not be closed (if the
      Protocol Stack supports half-closed operations, like TCP).  Any
      Messages sent after a Message marked Final will result in a
      SendError.

   msgChecksumLen:  When this is set to any value other than Full
      Coverage, it sets the minimum protection in protocols that allow
      limiting the checksum length (e.g., UDP-Lite).  If the Protocol
      Stack does not support checksum length limitation, this Property
      may be ignored.

   msgReliable:  When true, this Property specifies that the Message
      must be reliably transmitted.  When false, and if unreliable
      transmission is supported by the underlying protocol, then the
      Message should be unreliably transmitted.  If the underlying
      protocol does not support unreliable transmission, the Message
      should be reliably transmitted.

   msgCapacityProfile:  When true, this expresses a wish to override the
      Generic Connection Property connCapacityProfile for this Message.
      Depending on the value, this can, for example, be implemented by
      changing the Differentiated Services Code Point (DSCP) value of
      the associated packet (note that the guidelines in Section 6 of
      [RFC7657] apply; for example, the DSCP value should not be changed
      for different packets within a reliable transport protocol session
      or DCCP connection).

   noFragmentation:  Setting this avoids network-layer fragmentation.
      Messages exceeding the transport's current estimate of its maximum
      packet size (the singularTransmissionMsgMaxLen Connection
      Property) can result in transport segmentation when permitted or
      generate an error.  When used with transports running over IPv4,
      the Don't Fragment (DF) bit should be set to avoid on-path IP
      fragmentation [RFC8304].

   noSegmentation:  When set, this Property limits the Message size to
      the transport's current estimate of its maximum packet size (the
      singularTransmissionMsgMaxLen Connection Property).  Messages
      larger than this size generate an error.  Setting this avoids
      transport-layer segmentation and network-layer fragmentation.
      When used with transports running over IPv4, the DF bit should be
      set to avoid on-path IP fragmentation ([RFC8304]).

5.1.2.  Send Completion

   The application should be notified (using a Sent, Expired, or
   SendError event) whenever a Message or partial Message has been
   consumed by the Protocol Stack or has failed to send.  The time at
   which a Message is considered to have been consumed by the Protocol
   Stack may vary depending on the protocol.  For example, 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.  The time at which a Message failed
   to send is when the Transport Services Implementation (including the
   Protocol Stack) has experienced a failure related to sending; this
   can depend on protocol-specific timeouts.

5.1.3.  Batching Sends

   Sending multiple Messages can incur high overhead if each needs to be
   enqueued separately (e.g., each Message might involve a context
   switch between the application and the Transport Services System).
   To avoid this, the application can indicate a batch of Send actions
   through the API.  When this is used, the implementation can defer the
   processing of Messages until the batch is complete.

5.2.  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
   the parameters for the Message in its Receive action, which can help
   the Transport Services 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 Message
   Framer or because the top-level protocol in the established Protocol
   Stack preserves Message boundaries.  The application can also control
   the flow of received data by specifying the minimum and maximum
   number of bytes of Message content it wants to receive at one time.

   If a Connection finishes before a requested Receive action can be
   satisfied, the Transport Services System should deliver any
   outstanding partial Message content; if none is available, the system
   should indicate that there will be no additional received Messages.

5.3.  Handling of Data for Fast-Open Protocols

   Several protocols allow sending higher-level protocol or application
   data during their protocol establishment, such as TFO [RFC7413] and
   TLS 1.3 [RFC8446].  This approach is referred to as sending Zero-RTT
   (0-RTT) data.  This is a desirable feature, but it poses challenges
   to an implementation that uses racing during Connection
   establishment.

   The application can express its preference for sending Messages as
   0-RTT data by using the zeroRttMsg Selection Property on the
   Preconnection.  Then, the application can provide the Message to send
   as 0-RTT data via the InitiateWithSend action.  In order to be sent
   as 0-RTT data, the Message needs to be marked with the
   safelyReplayable Property.  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 may be
   considered a new connection instead of a retransmission).  When
   racing connections, different leaf nodes have the opportunity to send
   the same data independently.  If data is truly safely replayable,
   this is permissible.

   Once the application has provided its 0-RTT data, a Transport
   Services Implementation should keep a copy of this data and provide
   it to each new leaf node that is started and for which a protocol
   instance supporting 0-RTT is being used.  Note that the amount of
   data that can actually be sent as 0-RTT data varies by protocol, so
   any given Protocol Stack might only consume part of the saved data
   prior to becoming established.  The implementation needs to keep
   track of how much data a particular Protocol Stack has consumed and
   ensure that any pending 0-RTT-eligible data from the application is
   handled before subsequent Messages.

   It is also possible for Protocol Stacks within a particular leaf node
   to use a 0-RTT handshake in a lower-level protocol without any safely
   replayable application data if a higher-level protocol in the stack
   has idempotent handshake data to send.  For example, TFO could use a
   Client Hello from TLS as its 0-RTT data without any data being
   provided by the application.

   0-RTT handshakes often rely on previous state, such as TFO 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, since servers will likely reject duplicate
   tickets in order to prevent replays (see Section 8.1 of [RFC8446]).
   If implementations have multiple tickets available from a previous
   connection, each leaf node attempt can use a different ticket.  In
   effect, each leaf node will send the same early application data, but
   the data will be encoded (encrypted) differently on the wire.

6.  Implementing Message Framers

   Message Framers are functions that define simple transformations
   between application Message data and raw transport protocol data.
   Generally, a Message Framer implements a simple application protocol
   that can be provided either by the Transport Services implementation
   or by the application.  It is optional for Transport Services
   Implementations to provide Message Framers: the API specification
   [RFC9622] does not prescribe any particular Message Framers to be
   implemented.  A Framer can encapsulate or encode outbound Messages,
   decapsulate or decode inbound data into Messages, and implement parts
   of protocols that do not directly map to application Messages (such
   as protocol handshakes or preludes before Message exchange).

   While many protocols can be represented as Message Framers, for the
   purposes of the Transport Services API, these are ways for
   applications or application frameworks to define their own Message
   parsing to be included within a Connection's Protocol Stack.  As an
   example, TLS is a protocol that is by default built into the
   Transport Services API, even though it could also serve the purpose
   of framing data over TCP.

   Most Message Framers fall into one of two categories:

   *  Header-prefixed record formats, such as a basic Type-Length-Value
      (TLV) structure

   *  Delimiter-separated formats, such as HTTP/1.1

   Common Message Framers can be provided by a Transport Services
   Implementation, but an implementation ought to allow custom Message
   Framers to be defined by the application or some other piece of
   software.  This section describes one possible API for defining
   Message Framers as an example.

6.1.  Defining Message Framers

   A Message Framer is primarily defined by the code that handles events
   for a Framer implementation, specifically how it handles inbound and
   outbound data parsing.  The function that implements custom framing
   logic will be referred to as the "Framer implementation", which may
   be provided by a Transport Services Implementation or the application
   itself.  The Message Framer holds a reference to the object or
   function within the main Connection implementation that delivers
   events to the custom Framer implementation whenever data is ready to
   be parsed or framed.

   The API examples in this section use the notation conventions for the
   Transport Services API defined in Section 1.1 of [RFC9622].

   The Transport Services Implementation needs to ensure that all of the
   events and actions taken on a Message Framer are synchronized to
   ensure consistent behavior.  For example, some of the actions defined
   below (such as PrependFramer and StartPassthrough) modify how data
   flows in a Protocol Stack and require synchronization with sending
   and parsing data in the Message Framer.

   When a Connection establishment attempt begins, an event can be
   delivered to notify the Framer implementation that a new Connection
   is being created.  Similarly, a Stop event can be delivered when a
   Connection is being torn down.  The Framer implementation can use the
   Connection object to look up specific Properties of the Connection or
   the network being used that may influence how to frame Messages.

   MessageFramer -> Start<connection>
   MessageFramer -> Stop<connection>

   When a Message Framer generates a Start event, the Framer
   implementation has the opportunity to start writing some data prior
   to the Connection delivering its Ready event.  This allows the
   implementation to communicate control data to the Remote Endpoint
   that can be used to parse Messages.

   Once the Framer implementation has completed its setup or handshake,
   it can indicate to the application that it is ready for handling data
   with this call.

   MessageFramer.MakeConnectionReady(connection)

   Similarly, when a Message Framer generates a Stop event, the Framer
   implementation has the opportunity to write some final data or clear
   up its local state before the Closed event is delivered to the
   application.  The Framer implementation can indicate that it has
   finished with this call.

   MessageFramer.MakeConnectionClosed(connection)

   If the implementation encounters a fatal error at any time, it can
   also cause the Connection to fail and provide an error.

   MessageFramer.FailConnection(connection, error)

   Should the Framer implementation deem the candidate selected during
   racing unsuitable, it can signal this to the Transport Services API
   by failing the Connection prior to marking it as ready.  If there are
   no other candidates available, the Connection will fail.  Otherwise,
   the Connection will select a different candidate and the Message
   Framer will generate a new Start event.

   Before an implementation marks a Message Framer as ready, it can also
   dynamically add a protocol or Framer above it in the stack.  This
   allows protocols that need to add TLS conditionally, like STARTTLS
   [RFC3207], to modify the Protocol Stack based on a handshake result.

   otherFramer := NewMessageFramer()
   MessageFramer.PrependFramer(connection, otherFramer)

   A Message Framer might also choose to go into a passthrough mode once
   an initial exchange or handshake has been completed, such as the
   STARTTLS case mentioned above.  This can also be useful for proxy
   protocols like SOCKS [RFC1928] or HTTP CONNECT [RFC9110].  In such
   cases, a Message Framer implementation can initially intercept
   Messages being sent and received and subsequently indicate that no
   further processing is needed.

   MessageFramer.StartPassthrough()

6.2.  Sender-Side Message Framing

   Message Framers generate an event whenever a Connection sends a new
   Message.  The parameters to the event align with the Send action in
   the API (Section 9.2 of [RFC9622]).

                           MessageFramer
                                 |
                                 V
   NewSentMessage<connection, messageData, messageContext, endOfMessage>

   Upon receiving this event, a Framer implementation is responsible for
   performing any necessary transformations and sending the resulting
   data back to the Message Framer, which, in turn, will send it to the
   next protocol.  To improve performance, implementations should ensure
   that there is a way to pass the original data through without
   copying.

   MessageFramer.Send(connection, messageData)

   To provide an example, a simple protocol that adds the length of the
   Message data as a header would receive the NewSentMessage event,
   create a data representation of the length of the Message data, and
   then send a block of data that is the concatenation of the length
   header and the original Message data.

6.3.  Receiver-Side Message Framing

   In order to parse a received flow of data into Messages, the Message
   Framer notifies the Framer implementation whenever new data is
   available to parse.

   The parameters to the events and calls for receiving data with a
   Framer align with the Receive action in the API (Section 9.3 of
   [RFC9622]).

   MessageFramer -> HandleReceivedData<connection>

   Upon receiving this event, the Framer implementation can inspect the
   inbound data.  The data is parsed from a particular cursor
   representing the unprocessed data.  The application requests a
   specific amount of data it needs to have available in order to parse.
   If the data is not available, the parse fails.

 MessageFramer.Parse(connection, minimumIncompleteLength, maximumLength)
                                  |
                                  V
              (messageData, messageContext, endOfMessage)

   The Framer implementation can directly advance the receive cursor
   once it has parsed data to effectively discard data (for example,
   discard a header once the content has been parsed).

   To deliver a Message to the application, the Framer implementation
   can either directly deliver data that it has allocated or deliver a
   range of data directly from the underlying transport and
   simultaneously advance the receive cursor.

MessageFramer.AdvanceReceiveCursor(connection, length)
MessageFramer.DeliverAndAdvanceReceiveCursor(connection, messageContext,
                                             length, endOfMessage)
MessageFramer.Deliver(connection, messageContext, messageData,
                      endOfMessage)

   Note that MessageFramer.DeliverAndAdvanceReceiveCursor allows the
   Framer implementation to earmark bytes as part of a Message even
   before they are received by the transport.  This allows the delivery
   of very large Messages without requiring the implementation to
   directly inspect all of the bytes.

   To provide an example, a simple protocol that parses the length of
   the Message data as a header value would receive the
   HandleReceivedData event and call Parse with a minimum and maximum
   set to the length of the header field.  Once the parse succeeded, it
   would call AdvanceReceiveCursor with the length of the header field
   and then call DeliverAndAdvanceReceiveCursor with the length of the
   body that was parsed from the header, marking the new Message as
   complete.

7.  Implementing Connection Management

   Once a Connection is established, the Transport Services API allows
   applications to interact with the Connection by modifying or
   inspecting Connection Properties.  A Connection can also generate
   error events in the form of SoftError events.

   The set of Connection Properties that are supported for setting and
   getting on a Connection are described in [RFC9622].  For any
   Properties that are generic and, thus, could apply to all protocols
   being used by a Connection, the Transport Services Implementation
   should store the Properties in storage common to all protocols and
   notify the Protocol Stack as a whole whenever the Properties have
   been modified by the application.  [RFC8303] and [RFC8304] offer
   guidance on how to do this for TCP, Multipath TCP (MPTCP), SCTP, UDP,
   and UDP-Lite; see Section 10 for a description of a backtracking
   method to find the relevant protocol primitives using these
   documents.  For Protocol-specific Properties, such as the User
   Timeout that applies to TCP, the Transport Services Implementation
   only needs to update the relevant protocol instance.

   Some Connection Properties might apply to multiple protocols within a
   Protocol Stack.  Depending on the specific Property, it might be
   appropriate to apply the Property across multiple protocols
   simultaneously or only apply it to one protocol.  In general, the
   Transport Services Implementation should allow the protocol closest
   to the application to interpret Connection Properties and,
   potentially, modify the set of Connection Properties passed down to
   the next protocol in the stack.  For example, if the application has
   requested to use keep-alives with the keepAlive Property, and the
   Protocol Stack contains both HTTP/2 and TCP, the HTTP/2 protocol can
   choose to enable its own keep-alives to satisfy the application
   request and disable TCP-level keep-alives.  For cases where the
   application needs to have fine-grained per-protocol control, the
   Transport Services Implementation can expose Protocol-specific
   Properties.

   If an error is encountered in setting a Property (for example, if the
   application tries to set a TCP-specific Property on a Connection that
   is not using TCP), the action must fail gracefully.  The application
   must be informed of the error but the Connection itself must not be
   terminated.

   When protocol instances in the Protocol Stack report generic or
   protocol-specific errors, the API will deliver them to the
   application as SoftError events.  These allow the application to be
   informed of ICMP errors and other similar events.

7.1.  Pooled Connection

   For applications that do not need in-order delivery of Messages, the
   Transport Services Implementation may distribute Messages of a single
   Connection across several underlying transport connections or
   multiple streams of multistreaming connections between endpoints, as
   long as all of these satisfy the Selection Properties.  The Transport
   Services Implementation will then hide this connection management and
   only expose a single Connection object, which we call a Pooled
   Connection.  This is in contrast to Connection Groups, which
   explicitly expose combined treatment of Connections, giving the
   application control over multiplexing, for example.

   Pooled Connections can be useful when the application using the
   Transport Services System implements a protocol such as HTTP, which
   employs request/response pairs and does not require in-order delivery
   of responses.  This enables implementations of Transport Services
   Systems to realize transparent connection coalescing and connection
   migration and to perform per-Message endpoint and path selection by
   choosing among multiple underlying connections.

7.2.  Handling Path Changes

   When a path change occurs, e.g., when the IP address of an interface
   changes or a new interface becomes available, the Transport Services
   Implementation is responsible for notifying the protocol instance of
   the change.  The path change may interrupt connectivity on a path for
   an active Connection or provide an opportunity for a transport that
   supports multipath or migration to adapt to the new paths.  Note
   that, in the model of the Transport Services API, migration is
   considered a part of multipath connectivity; it is just a limiting
   policy on multipath usage.  If the multipath Selection Property is
   set to Disabled, migration is disallowed.

   For protocols that do not support multipath or migration, the
   protocol instances should be informed of the path change but should
   not be forcibly disconnected if the previously used path becomes
   unavailable.  There are many common usage scenarios that can lead to
   a path becoming temporarily unavailable and then recovering before
   the transport protocol reaches a timeout error.  These are
   particularly common using mobile devices.  Examples include:

   *  an Ethernet cable becoming unplugged and then plugged back in;

   *  a device losing a Wi-Fi signal while a user is in an elevator and
      reattaching when the user leaves the elevator; and

   *  a user losing the radio signal while riding a train through a
      tunnel.

   If the device is able to rejoin a network with the same IP address, a
   stateful transport connection can generally resume.  Thus, while it
   is useful for a protocol instance to be aware of a temporary loss of
   connectivity, the Transport Services Implementation should not
   aggressively close Connections in these scenarios.

   If the Protocol Stack includes a transport protocol that supports
   multipath connectivity, the Transport Services Implementation should
   also inform the protocol instance about potentially new paths that
   become permissible based on the multipath Selection Property and the
   multipathPolicy Connection Property choices made by the application.
   A protocol can then establish new subflows over new paths while an
   active path is still available or after a break has been detected,
   and it should attempt to tear down subflows over paths that are no
   longer used.  The Connection Property multipathPolicy of the
   Transport Services API allows an application to indicate when and how
   different paths should be used.  However, detailed handling of these
   policies is implementation specific.  For example, if the multipath
   Selection Property is set to Active, the decision about when to
   create a new path or to announce a new path or set of paths to the
   Remote Endpoint, e.g., in the form of additional IP addresses, is
   implementation specific.  If the Protocol Stack includes a transport
   protocol that does not support multipath but does support migrating
   between paths, the update to the set of available paths can trigger
   the connection to be migrated.

   In the case of a Pooled Connection (Section 7.1), the Transport
   Services Implementation may add connections over new paths to the
   pool if permissible based on the multipathPolicy and Selection
   Properties.  If a previously used path becomes unavailable, the
   Transport Services System may disconnect all connections that require
   this path, but it should not disconnect the Pooled Connection object
   exposed to the application.  The strategy to do so is implementation
   specific, but it should be consistent with the behavior of multipath
   transports.

8.  Implementing Connection Termination

   For Close (which leads to a Closed event) and Abort (which leads to a
   ConnectionError event), the application might find it useful 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.  When an underlying transport connection supports
   multistreaming (such as SCTP), the Transport Services System can use
   a stream reset procedure to cause a Finish event upon a Close action
   from the peer [NEAT-flow-mapping].

9.  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 reusing 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; thus, they should be cached based on a connection's
   hostname Endpoint Identifier (if applicable).  However, performance
   characteristics of a path are more likely tied to the IP address and
   subnet being used.

9.1.  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:

   *  The DNS protocol can cache resolved addresses (such as those
      retrieved from A and AAAA queries) associated with a Time To Live
      (TTL) to be used for future hostname resolutions without requiring
      asking the DNS resolver again.

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

   *  TCP can cache cookies for use in TFO.

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

   Applications can use the Transport Services API to request that a
   Connection Group maintain a separate cache for protocol state.
   Connections in the group will not use Cached State from Connections
   outside the group, and Connections outside the group will not use
   state cached from Connections inside the group.  This may be
   necessary, for example, if application-layer identifiers rotate and
   clients wish to avoid linkability via trackable TLS tickets or TFO
   cookies.

9.2.  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:

   *  Observed RTT

   *  Connection establishment latency

   *  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
   influence 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 4.2) to ensure better performance for the
   application.

   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, the RTT
   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.

   [RFC9040] provides guidance about sharing of TCP Control Block
   information between connections on initialization.

10.  Specific Transport Protocol Considerations

   Each protocol that is supported by a Transport Services
   Implementation should have a well-defined API mapping.  API mappings
   for a protocol are important for Connections in which a given
   protocol is the "top" of the Protocol Stack.  For example, the
   mapping of the Send action for TCP applies to Connections in which
   the application directly sends over TCP.

   Each protocol has a notion of "Connectedness".  Possible definitions
   of Connectedness for various types of protocols are:

   Connectionless:  Connectionless protocols do not establish explicit
      state between endpoints and do not perform a handshake during
      connection establishment.

   Connected:  Connected (also called "connection-oriented") 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.

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

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

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

   Datagram:  Datagram protocols define message boundaries at the same
      level of transmission, such that only complete (not partial)
      messages are supported.

   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.

   Below, terms in capitals with a dot character (".") (e.g.,
   "CONNECT.SCTP") refer to the primitives with the same name in
   Section 4 of [RFC8303].  For further implementation details, the
   description of these primitives in [RFC8303] points to Section 3 of
   [RFC8303] and Section 3 of [RFC8304], which refers back to the
   relevant specifications for each protocol.  This applies to all
   elements of [RFC8923] (see Appendix C of [RFC9622]): they are listed
   in Appendix A of [RFC8923] with an implementation hint in the same
   style, pointing back to Section 4 of [RFC8303].

   This document presents the protocol mappings defined in [RFC8923].
   Other protocol mappings can be provided as separate documents,
   following the mapping template in Appendix A.

10.1.  TCP

   Connectedness:  Connected

   Data Unit:  Byte-stream

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

   Initiate:  CONNECT.TCP.  Calling Initiate on a TCP connection causes
      it to reserve a local port and send a SYN to the Remote Endpoint.

   InitiateWithSend:  CONNECT.TCP with parameter user message.  Early
      safely replayable data is sent on a TCP connection in the SYN, as
      TFO data.

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

   EstablishmentError:  Failure of CONNECT.TCP.  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 receiving a RST from
      the peer.

   Listen:  LISTEN.TCP.  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 TCP
      connection with equivalent parameters.  The two associated
      Connection objects, and Connections generated via later calls to
      Clone on an Established Connection, form a Connection Group.  To
      realize entanglement for these Connections, with the exception of
      connPriority, changing a Connection Property on one of them must
      affect the Connection Properties of the others too.  No guarantees
      of honoring the connPriority Connection Property are given; thus,
      it is safe for an implementation of a Transport Services System to
      ignore this Property.  When it is reasonable to assume that
      Connections traverse the same path (e.g., when they share the same
      encapsulation), support for it can also experimentally be
      implemented using a congestion control coupling mechanism (for
      example, see [TCP-COUPLING] or [RFC3124]).

   Send:  SEND.TCP.  On its own, TCP does not 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, by calling CLOSE.TCP immediately upon
      successful termination of SEND.TCP.  Note that transmitting a
      Message marked as Final should not cause the Closed event to be
      delivered to the application as it will still be possible to
      receive data until the peer closes or aborts the TCP connection.

   Receive:  With RECEIVE.TCP, 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 Message Framer is used).  The value
      of the endOfMessage Property will be delivered when the TCP
      connection has received a FIN (CLOSE-EVENT.TCP) from the peer.
      Note that reception of a FIN should not cause the Closed event to
      be delivered to the application, as it will still be possible for
      the application to send data.

   Close:  Calling Close on a TCP connection indicates that the TCP
      connection should be gracefully closed (CLOSE.TCP) by sending a
      FIN to the peer.  It will then still be possible to receive data
      until the peer closes or aborts the TCP connection.  The Closed
      event will be issued upon reception of a FIN.

   Abort:  Calling Abort on a TCP connection indicates that the TCP
      connection should be immediately closed by sending a RST to the
      peer (ABORT.TCP).

   CloseGroup:  Calling CloseGroup on a TCP connection (CLOSE.TCP) is
      identical to calling Close on its Connection object and on all
      Connections in the same ConnectionGroup.

   AbortGroup:  Calling AbortGroup on a TCP connection (ABORT.TCP) is
      identical to calling Abort on its Connection object and on all
      Connections in the same ConnectionGroup.

10.2.  MPTCP

   Connectedness:  Connected

   Data Unit:  Byte-stream

   The Transport Services API mappings for MPTCP are identical to TCP.
   MPTCP adds support for multipath Properties, such as multipath and
   multipathPolicy, and actions for managing paths, such as AddRemote
   and RemoveRemote.

10.3.  UDP

   Connectedness:  Connectionless

   Data Unit:  Datagram

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

   Initiate:  CONNECT.UDP.  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.

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

   ConnectionError:  UDP connections can only generate Connection errors
      in response to Abort actions.  (Once in use, UDP connections can
      also generate SoftError events (ERROR.UDP) upon receiving ICMP
      notifications indicating failures in the network.)

   Listen:  LISTEN.UDP.  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 UDP
      connection with equivalent parameters.  The two associated
      Connection objects are otherwise independent.

   Send:  SEND.UDP.  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.  Upon sending
      a UDP datagram, some relevant fields and flags in the IP header
      can be controlled: DSCP (SET_DSCP.UDP), DF in IPv4 (SET_DF.UDP),
      and ECN flag (SET_ECN.UDP).

   Receive:  RECEIVE.UDP.  UDP only delivers complete Messages to
      Received, each of which represents a single datagram received in a
      UDP packet.  Upon receiving a UDP datagram, the ECN flag from the
      IP header can be obtained (GET_ECN.UDP).

   Close:  Calling Close on a UDP connection (ABORT.UDP) releases the
      local port reservation.  A Closed event is then issued.

   Abort:  Calling Abort on a UDP connection (ABORT.UDP) is identical to
      calling Close except that a ConnectionError event rather than a
      Closed event is issued.

   CloseGroup:  Calling CloseGroup on a UDP connection (ABORT.UDP) is
      identical to calling Close on its Connection object and on all
      Connections in the same ConnectionGroup.

   AbortGroup:  Calling AbortGroup on a UDP connection (ABORT.UDP) is
      identical to calling Close on its Connection object and on all
      Connections in the same ConnectionGroup.

10.4.  UDP-Lite

   Connectedness:  Connectionless

   Data Unit:  Datagram

   The Transport Services API mappings for UDP-Lite are identical to
   UDP.  In addition, UDP-Lite supports the msgChecksumLen and
   recvChecksumLen Properties that allow an application to specify the
   minimum number of bytes in a Message that need to be covered by a
   checksum.

   This includes: CONNECT.UDP-Lite; LISTEN.UDP-Lite; SEND.UDP-Lite;
   RECEIVE.UDP-Lite; ABORT.UDP-Lite; ERROR.UDP-Lite; SET_DSCP.UDP-Lite;
   SET_DF.UDP-Lite; SET_ECN.UDP-Lite; GET_ECN.UDP-Lite.

10.5.  UDP Multicast Receive

   Connectedness:  Connectionless

   Data Unit:  Datagram

   Connection Object:  Established UDP Multicast Receive connections
      represent a pair of specific IP addresses and ports.  The
      direction Selection Property must be set to Unidirectional
      receive, and the Local Endpoint must be configured with a group IP
      address and a port.

   Initiate:  Calling Initiate on a UDP Multicast Receive connection
      causes an immediate EstablishmentError.  This is an unsupported
      operation.

   InitiateWithSend:  Calling InitiateWithSend on a UDP Multicast
      Receive connection causes an immediate EstablishmentError.  This
      is an unsupported operation.

   Ready:  A UDP Multicast Receive connection is ready once the system
      has received traffic for the appropriate group and port.

   EstablishmentError:  UDP Multicast Receive connections cause an
      EstablishmentError indicating that joining a multicast group
      failed if Initiate is called.

   ConnectionError:  The only ConnectionError generated by a UDP
      Multicast Receive connection is in response to an Abort action.

   Listen:  LISTEN.UDP.  Calling Listen for UDP Multicast Receive binds
      a local port, prepares it to receive inbound UDP datagrams from
      peers, and issues a multicast host join.  If a Remote Endpoint
      Identifier with an address is supplied, the join is Source-
      Specific Multicast, and the path selection is based on the route
      to the Remote Endpoint.  If a Remote Endpoint Identifier is not
      supplied, the join is Any-Source Multicast, and the path selection
      is based on the outbound route to the group supplied in the Local
      Endpoint.

   There are cases where it is required to open multiple connections for
   the same address(es).  For example, one Connection might be opened
   for a multicast group used for a shared control bus, and another
   application later opens a separate Connection to the same group to
   send signals to and/or receive signals from the common bus.  In such
   cases, the Transport Services System needs to explicitly enable reuse
   of the same set of addresses (equivalent to setting SO_REUSEADDR in
   the Socket API).

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

   Clone:  Calling Clone on a UDP Multicast Receive connection creates a
      new UDP Multicast Receive connection with equivalent parameters.
      The two associated Connection objects are otherwise independent.

   Send:  SEND.UDP.  Calling Send on a UDP Multicast Receive connection
      causes an immediate SendError.  This is an unsupported operation.

   Receive:  RECEIVE.UDP.  UDP Multicast Receive only delivers complete
      Messages to Received, each of which represents a single datagram
      received in a UDP packet.  Upon receiving a UDP datagram, the ECN
      flag from the IP header can be obtained (GET_ECN.UDP).

   Close:  Calling Close on a UDP Multicast Receive connection
      (ABORT.UDP) releases the local port reservation and leaves the
      group.  A Closed event is then issued.

   Abort:  Calling Abort on a UDP Multicast Receive connection
      (ABORT.UDP) is identical to calling Close except that a
      ConnectionError event rather than a Closed event is issued.

   CloseGroup:  Calling CloseGroup on a UDP Multicast Receive connection
      (ABORT.UDP) is identical to calling Close on its Connection object
      and on all Connections in the same ConnectionGroup.

   AbortGroup:  Calling AbortGroup on a UDP Multicast Receive connection
      (ABORT.UDP) is identical to calling Close on its Connection object
      and on all Connections in the same ConnectionGroup.

10.6.  SCTP

   Connectedness:  Connected

   Data Unit:  Message

   Connection Object:  Connection objects can be mapped to an SCTP
      association or a stream in an SCTP association.  Mapping
      Connection objects to SCTP streams is called "stream mapping" and
      has additional requirements as follows.  The following explanation
      assumes a client-server communication model.

      Stream mapping requires an association to already be in place
      between the client and the server, and it requires the server to
      understand that a new incoming stream should be represented as a
      new Connection object by the Transport Services System.  A new
      SCTP stream is created by sending an SCTP message with a new
      stream id.  Thus, to implement stream mapping, the Transport
      Services API must provide a newly created Connection object to the
      application upon the reception of such a message.  The necessary
      semantics to implement a Transport Services System's Close and
      Abort primitives are provided by the stream reconfiguration
      (reset) procedure described in [RFC6525].  This also allows a
      stream id to be reused after resetting ("closing") the stream.  To
      implement this functionality, SCTP stream reconfiguration
      [RFC6525] must be supported by both the client and the server
      side.

      To avoid head-of-line blocking, stream mapping should only be
      implemented when both sides support message interleaving
      [RFC8260].  This allows a sender to schedule transmissions between
      multiple streams without risking that transmission of a large
      message on one stream will block transmissions on other streams
      for a long time.

      To avoid conflicts between stream ids, the following procedure is
      recommended: the first Connection, for which the SCTP association
      has been created, must always use stream id zero.  All additional
      Connections are assigned to unused stream ids in ascending order.
      To avoid a conflict when both endpoints map new Connections
      simultaneously, the peer that initiated association must use even
      stream ids whereas the remote side must map its Connections to odd
      stream ids.  Both sides maintain a status map of the assigned
      stream ids.  Generally, new streams should consume the lowest
      available (even or odd, depending on the side) stream id; this
      rule is relevant when lower stream ids become available because
      Connection objects associated with the streams are closed.

      SCTP stream mapping as described here has been implemented in a
      research prototype; a description of this implementation is given
      in [NEAT-flow-mapping].

   Initiate:  If this is the only Connection object that is assigned to
      the SCTP association or stream mapping is not used, CONNECT.SCTP
      is called.  Else, unless the Selection Property
      activeReadBeforeSend is preferred or required, a new stream is
      used: if there are enough streams available, Initiate is a local
      operation that assigns a new stream id to the Connection object.
      The number of streams is negotiated as a parameter of the prior
      CONNECT.SCTP call, and it represents a trade-off between local
      resource usage and the number of Connection objects that can be
      mapped without requiring a reconfiguration signal.  When running
      out of streams, ADD_STREAM.SCTP must be called.

   InitiateWithSend:  If this is the only Connection object that is
      assigned to the SCTP association or stream mapping is not used,
      CONNECT.SCTP is called with the user message parameter.  Else, a
      new stream is used (see Initiate for how to handle running out of
      streams), and this just sends the first message on a new stream.

   Ready:  Initiate or InitiateWithSend returns without an error, i.e.,
      SCTP's four-way handshake has completed.  If an association with
      the peer already exists, stream mapping is used, and enough
      streams are available, a Connection object instantly becomes Ready
      after calling Initiate or InitiateWithSend.

   EstablishmentError:  Failure of CONNECT.SCTP.

   ConnectionError:  TIMEOUT.SCTP or ABORT-EVENT.SCTP.

   Listen:  LISTEN.SCTP.  If an association with the peer already exists
      and stream mapping is used, Listen just expects to receive a new
      message with a new stream id (chosen in accordance with the stream
      id assignment procedure described above).

   ConnectionReceived:  LISTEN.SCTP returns without an error (a result
      of successful CONNECT.SCTP from the peer) or, in the case of
      stream mapping, the first message has arrived on a new stream (in
      this case, Receive is also invoked).

   Clone:  Calling Clone on an SCTP association creates a new Connection
      object and assigns it a new stream id in accordance with the
      stream id assignment procedure described above.  If there are not
      enough streams available, ADD_STREAM.SCTP must be called.

   Send:  SEND.SCTP.  Message Properties such as msgLifetime and
      msgOrdered map to parameters of this primitive.

   Receive:  RECEIVE.SCTP.  The "partial flag" of RECEIVE.SCTP invokes a
      ReceivedPartial event.

   Close:  If this is the only Connection object that is assigned to the
      SCTP association, CLOSE.SCTP is called and the Closed event will
      be delivered to the application upon the ensuing CLOSE-EVENT.SCTP.
      Else, the Connection object is one out of several Connection
      objects that are assigned to the same SCTP association, and
      RESET_STREAM.SCTP must be called, which informs the peer that the
      stream will no longer be used for mapping and can be used by a
      future Initiate, InitiateWithSend, or Listen action.  At the peer,
      the event RESET_STREAM-EVENT.SCTP will be initiated, which the
      peer must answer by issuing RESET_STREAM.SCTP too.  The resulting
      local RESET_STREAM-EVENT.SCTP informs the Transport Services
      System that the stream id can now be reused by the next Initiate,
      InitiateWithSend, or Listen action, and invokes a Closed event
      toward the application.

   Abort:  If this is the only Connection object that is assigned to the
      SCTP association, ABORT.SCTP is called.  Else, the Connection
      object is one out of several Connection objects that are assigned
      to the same SCTP association, and shutdown proceeds as described
      under Close.

   CloseGroup:  Calling CloseGroup calls CLOSE.SCTP, which closes all
      Connections in the SCTP association.

   AbortGroup:  Calling AbortGroup calls ABORT.SCTP, which immediately
      closes all Connections in the SCTP association.

   In addition to the API mappings described above, when there are
   multiple Connection objects assigned to the same SCTP association,
   SCTP can support Connection Properties such as connPriority and
   connScheduler where CONFIGURE_STREAM_SCHEDULER.SCTP can be called to
   adjust the priorities of streams in the SCTP association.

11.  IANA Considerations

   This document has no IANA actions.

12.  Security Considerations

   [RFC9621] outlines general security considerations and requirements
   for any system that implements the Transport Services Architecture.
   [RFC9622] provides further discussion on security and privacy
   implications of the Transport Services API.  This document provides
   additional guidance on implementation specifics for the Transport
   Services API; as such, the security considerations in both of these
   documents apply.  The next two subsections discuss further
   considerations that are specific to mechanisms specified in this
   document.

12.1.  Considerations for Candidate Gathering

   As discussed in Sections 3 and 6 of [RFC9621], gathering and racing
   with Protocol Stacks that do not have equivalent security properties
   ought not be attempted.  Therefore, implementations need to 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 5.3 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.

13.  References

13.1.  Normative References

   [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>.

   [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>.

   [RFC8421]  Martinsen, P., Reddy, T., and P. Patil, "Guidelines for
              Multihomed and IPv4/IPv6 Dual-Stack Interactive
              Connectivity Establishment (ICE)", BCP 217, RFC 8421,
              DOI 10.17487/RFC8421, July 2018,
              <https://www.rfc-editor.org/info/rfc8421>.

   [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>.

   [RFC8923]  Welzl, M. and S. Gjessing, "A Minimal Set of Transport
              Services for End Systems", RFC 8923, DOI 10.17487/RFC8923,
              October 2020, <https://www.rfc-editor.org/info/rfc8923>.

   [RFC9113]  Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/info/rfc9113>.

   [RFC9621]  Pauly, T., Ed., Trammell, B., Ed., Brunstrom, A.,
              Fairhurst, G., and C. S. Perkins, "Architecture and
              Requirements for Transport Services", RFC 9621,
              DOI 10.17487/RFC9621, January 2025,
              <https://www.rfc-editor.org/info/rfc9621>.

   [RFC9622]  Trammell, B., Ed., Welzl, M., Ed., Enghardt, R.,
              Fairhurst, G., Kühlewind, M., Perkins, C. S., Tiesel, P.
              S., and T. Pauly, "An Abstract Application Programming
              Interface (API) for Transport Services", RFC 9622,
              DOI 10.17487/RFC9622, January 2025,
              <https://www.rfc-editor.org/info/rfc9622>.

13.2.  Informative References

   [NEAT-flow-mapping]
              Weinrank, F. and M. Tuxen, "Transparent flow mapping for
              NEAT", 2017 IFIP Networking Conference (IFIP Networking)
              and Workshops, DOI 10.23919/IFIPNetworking.2017.8264876,
              June 2017, <https://ieeexplore.ieee.org/document/8264876>.

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,
              <https://www.rfc-editor.org/info/rfc1928>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3124]  Balakrishnan, H. and S. Seshan, "The Congestion Manager",
              RFC 3124, DOI 10.17487/RFC3124, June 2001,
              <https://www.rfc-editor.org/info/rfc3124>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <https://www.rfc-editor.org/info/rfc3207>.

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

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,
              <https://www.rfc-editor.org/info/rfc7657>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [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>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8489]  Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
              D., Mahy, R., and P. Matthews, "Session Traversal
              Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
              February 2020, <https://www.rfc-editor.org/info/rfc8489>.

   [RFC8656]  Reddy, T., Ed., Johnston, A., Ed., Matthews, P., and J.
              Rosenberg, "Traversal Using Relays around NAT (TURN):
              Relay Extensions to Session Traversal Utilities for NAT
              (STUN)", RFC 8656, DOI 10.17487/RFC8656, February 2020,
              <https://www.rfc-editor.org/info/rfc8656>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9040]  Touch, J., Welzl, M., and S. Islam, "TCP Control Block
              Interdependence", RFC 9040, DOI 10.17487/RFC9040, July
              2021, <https://www.rfc-editor.org/info/rfc9040>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/info/rfc9460>.

   [TCP-COUPLING]
              Islam, S., Welzl, M., Hiorth, K., Hayes, D., Armitage, G.,
              and S. Gjessing, "ctrlTCP: Reducing latency through
              coupled, heterogeneous multi-flow TCP congestion control",
              IEEE INFOCOM 2018 - IEEE Conference on Computer
              Communications Workshops (INFOCOM WKSHPS),
              DOI 10.1109/INFCOMW.2018.8406887, 2018,
              <https://ieeexplore.ieee.org/document/8406887>.

Appendix A.  API Mapping Template

   Any protocol mapping for the Transport Services API should follow a
   common template.

   Connectedness: (Connectionless/Connected/Multiplexing Connected)

   Data Unit: (Byte-stream/Datagram/Message)

   Connection Object:

   Initiate:

   InitiateWithSend:

   Ready:

   EstablishmentError:

   ConnectionError:

   Listen:

   ConnectionReceived:

   Clone:

   Send:

   Receive:

   Close:

   Abort:

   CloseGroup:

   AbortGroup:

Appendix B.  Reasons for Errors

   The Transport Services API [RFC9622] allows for several generic error
   types to specify a more detailed reason about why an error occurred.
   This appendix lists some of the possible reasons.

   InvalidConfiguration:  The Properties and Endpoint Identifiers
      provided by the application are either contradictory or
      incomplete.  Examples include the lack of a Remote Endpoint
      Identifier on an active open or using a multicast group address
      while not requesting a Unidirectional receive.

   NoCandidates:  The configuration is valid, but none of the available
      transport protocols can satisfy the Properties provided by the
      application.

   ResolutionFailed:  The remote or local specifier provided by the
      application cannot be resolved.

   EstablishmentFailed:  The Transport Services System was unable to
      establish a transport-layer connection to the Remote Endpoint
      specified by the application.

   PolicyProhibited:  The System Policy prevents the Transport Services
      System from performing the action requested by the application.

   NotCloneable:  The Protocol Stack is not capable of being cloned.

   MessageTooLarge:  The Message is too big for the Transport Services
      System to handle.

   ProtocolFailed:  The underlying Protocol Stack failed.

   InvalidMessageProperties:  The Message Properties either contradict
      the Transport Properties or cannot be satisfied by the Transport
      Services System.

   DeframingFailed:  The data that was received by the underlying
      Protocol Stack could not be processed by the Message Framer.

   ConnectionAborted:  The connection was aborted by the peer.

   Timeout:  Delivery of a Message was not possible after a timeout.

Appendix C.  Existing Implementations

   This appendix gives an overview of existing implementations, at the
   time of writing, of Transport Services Systems that are (to some
   degree) in line with this document.

   *  Apple's Network.framework:

      -  Network.framework is a transport-level API built for C,
         Objective-C, and Swift.  It is a connect-by-name API that
         supports transport security protocols.  It provides user-space
         implementations of TCP, UDP, TLS, DTLS, and proxy protocols,
         and it allows extension via custom Framers.

      -  Documentation: https://developer.apple.com/documentation/
         network

   *  NEAT and NEATPy:

      -  NEAT is the output of the European H2020 research project
         "NEAT"; it is a user-space library for protocol-independent
         communication on top of TCP, UDP, and SCTP, with many more
         features, such as a policy manager.

      -  Code: https://github.com/NEAT-project/neat

      -  Code at the Software Heritage Archive:
         https://archive.softwareheritage.org/swh:1:dir:737820840f83c4ec
         9493a8c0cc89b3159e2e1a57;origin=https://github.com/NEAT-
         project/neat;visit=swh:1:snp:bbb611b04e355439d47e426e8ad5d07cdb
         f647e0;anchor=swh:1:rev:652ee991043ce3560a6e5715fa2a5c211139d15
         c

      -  NEATPy is a Python shim over NEAT that updates the NEAT API to
         be in line with version 6 of the Transport Services API
         [RFC9622].

      -  Code: https://github.com/theagilepadawan/NEATPy

      -  Code at the Software Heritage Archive:
         https://archive.softwareheritage.org/swh:1:dir:295ccd148cf918cc
         b9ed7ad14b5ae968a8d2c370;origin=https://github.com/
         theagilepadawan/NEATPy;visit=swh:1:snp:6e1a3a9dd4c532ba6c0f52c8
         f734c1256a06cedc;anchor=swh:1:rev:cd0788d7f7f34a0e9b8654516da7c
         002c44d2e95

   *  PyTAPS:

      -  A Transport Services (TAPS) implementation based on Python
         asyncio, offering protocol-independent communication to
         applications on top of TCP, UDP, and TLS, with support for
         multicast.

      -  Code: https://github.com/fg-inet/python-asyncio-taps

      -  Code at the Software Heritage Archive:
         https://archive.softwareheritage.org/swh:1:dir:a7151096d91352b4
         39b092ef116d04f38e52e556;origin=https://github.com/fg-inet/
         python-asyncio-taps;visit=swh:1:snp:4841e59b53b28bb385726e7d3a5
         69bee0fea7fc4;anchor=swh:1:rev:63571fd7545da25142bc1a6371b8f130
         97cba38e

Acknowledgements

   This work has received funding from the European Union's Horizon 2020
   research and innovation programme under grant agreement No. 644334
   (NEAT) and No. 815178 (5GENESIS).

   This work has been supported by:

   *  Leibniz Prize project funds from the DFG - German Research
      Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ FE 570/4-1).

   *  the UK Engineering and Physical Sciences Research Council under
      grant EP/R04144X/1.

   *  the Research Council of Norway under its "Toppforsk" programme
      through the "OCARINA" project.

   Thanks to Colin S. Perkins, Tom Jones, Karl-Johan Grinnemo, and Gorry
   Fairhurst for their contributions to the design of this
   specification.  Thanks also to Stuart Cheshire, Josh Graessley, David
   Schinazi, and Eric Kinnear for their implementation and design
   efforts, including Happy Eyeballs, that heavily influenced this work.

Authors' Addresses

   Anna Brunstrom (editor)
   Karlstad University
   Universitetsgatan 2
   651 88 Karlstad
   Sweden
   Email: anna.brunstrom@kau.se


   Tommy Pauly (editor)
   Apple Inc.
   One Apple Park Way
   Cupertino, CA 95014
   United States of America
   Email: tpauly@apple.com


   Reese Enghardt
   Netflix
   121 Albright Way
   Los Gatos, CA 95032
   United States of America
   Email: ietf@tenghardt.net


   Philipp S. Tiesel
   SAP SE
   George-Stephenson-Str. 7-13
   10557 Berlin
   Germany
   Email: philipp@tiesel.net


   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   0316 Oslo
   Norway
   Email: michawe@ifi.uio.no
