wdiff rfc9622v1.txt rfc9622.txt

Internet Engineering Task Force (IETF) B. Trammell, Ed.
Request for Comments: 9622 Google Switzerland GmbH
Category: Standards Track M. Welzl, Ed.
ISSN: 2070-1721 University of Oslo
R. Enghardt
Netflix
G. Fairhurst
University of Aberdeen
M. Kühlewind
Ericsson
C. S. Perkins
University of Glasgow
P.
P.S. Tiesel
SAP SE
T. Pauly
Apple Inc.
November 2024

An Abstract Application-Layer Interface to

The Transport Services Application Programming Interface

Abstract

This document describes an abstract Application Programming Interface
(API) to the transport layer that enables the selection of transport
protocols and network paths dynamically at runtime. This API enables
faster deployment of new protocols and protocol features without
requiring changes to the applications. The specified API follows the
Transport Services architecture by providing asynchronous, atomic
transmission of messages. It is intended to replace the BSD Socket
API as the common interface to the transport layer, in an environment
where endpoints could select from multiple network paths and
potential transport protocols.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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/rfc9622.

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
1.1. Terminology and Notation
1.2. Specification of Requirements
2. Overview of the API Design
3. API Summary
3.1. Usage Examples
3.1.1. Server Example
3.1.2. Client Example
3.1.3. Peer Example
4. Transport Properties
4.1. Transport Property Names
4.2. Transport Property Types
5. Scope of the API Definition
6. Preestablishment Phase
6.1. Specifying Endpoints
6.1.1. Using Multicast Endpoints
6.1.2. Constraining Interfaces for Endpoints
6.1.3. Protocol-Specific Endpoints
6.1.4. Endpoint Examples
6.1.5. Multicast Examples
6.2. Specifying Transport Properties
6.2.1. Reliable Data Transfer (Connection)
6.2.2. Preservation of Message Boundaries
6.2.3. Configure Per-Message Reliability
6.2.4. Preservation of Data Ordering
6.2.5. Use 0-RTT Session Establishment with a Safely
Replayable Message
6.2.6. Multistream Connections in a Group
6.2.7. Full Checksum Coverage on Sending
6.2.8. Full Checksum Coverage on Receiving
6.2.9. Congestion Control
6.2.10. Keep-Alive Packets
6.2.11. Interface Instance or Type
6.2.12. Provisioning Domain Instance or Type
6.2.13. Use Temporary Local Address
6.2.14. Multipath Transport
6.2.15. Advertisement of Alternative Addresses
6.2.16. Direction of Communication
6.2.17. Notification of ICMP Soft Error Message Arrival
6.2.18. Initiating Side Is Not the First to Write
6.3. Specifying Security Parameters and Callbacks
6.3.1. Allowed Security Protocols
6.3.2. Certificate Bundles
6.3.3. Pinned Server Certificate
6.3.4. Application-Layer Protocol Negotiation
6.3.5. Groups, Ciphersuites, and Signature Algorithms
6.3.6. Session Cache Options
6.3.7. Pre-Shared Key
6.3.8. Connection Establishment Callbacks
7. Establishing Connections
7.1. Active Open: Initiate
7.2. Passive Open: Listen
7.3. Peer-to-Peer Establishment: Rendezvous
7.4. Connection Groups
7.5. Adding and Removing Endpoints on a Connection
8. Managing Connections
8.1. Generic Connection Properties
8.1.1. Required Minimum Corruption Protection Coverage for
Receiving
8.1.2. Connection Priority
8.1.3. Timeout for Aborting Connection
8.1.4. Timeout for Keep-Alive Packets
8.1.5. Connection Group Transmission Scheduler
8.1.6. Capacity Profile
8.1.7. Policy for Using Multipath Transports
8.1.8. Bounds on Send or Receive Rate
8.1.9. Group Connection Limit
8.1.10. Isolate Session
8.1.11. Read-Only Connection Properties
8.2. TCP-Specific Properties: User Timeout Option (UTO)
8.2.1. Advertised User Timeout
8.2.2. User Timeout Enabled
8.2.3. Timeout Changeable
8.3. Connection Lifecycle Events
8.3.1. Soft Errors
8.3.2. Path Change
9. Data Transfer
9.1. Messages and Framers
9.1.1. Message Contexts
9.1.2. Message Framers
9.1.3. Message Properties
9.2. Sending Data
9.2.1. Basic Sending
9.2.2. Send Events
9.2.3. Partial Sends
9.2.4. Batching Sends
9.2.5. Send on Active Open: InitiateWithSend
9.2.6. Priority and the Transport Services API
9.3. Receiving Data
9.3.1. Enqueuing Receives
9.3.2. Receive Events
9.3.3. Receive Message Properties
10. Connection Termination
11. Connection State and Ordering of Operations and Events
12. IANA Considerations
13. Privacy and Security Considerations
14. References
14.1. Normative References
14.2. Informative References
Appendix A. Implementation Mapping
A.1. Types
A.2. Events and Errors
A.3. Time Duration
Appendix B. Convenience Functions
B.1. Adding Preference Properties
B.2. Transport Property Profiles
B.2.1. reliable-inorder-stream
B.2.2. reliable-message
B.2.3. unreliable-datagram
Appendix C. Relationship to the Minimal Set of Transport Services
for End Systems
Acknowledgements
Authors' Addresses

1. Introduction

This document specifies an abstract Application Programming Interface
(API) that describes the interface component of the high-level
Transport Services architecture defined in [RFC9621]. A Transport
Services system supports asynchronous, atomic transmission of
messages over transport protocols and network paths dynamically
selected at runtime, in environments where an endpoint selects from
multiple network paths and potential transport protocols.

Applications that adopt this API will benefit from a wide set of
transport features that can evolve over time. This protocol-
independent API ensures that the system providing the API can
optimize its behavior based on the application requirements and
network conditions, without requiring changes to the applications.
This flexibility enables faster deployment of new features and
protocols and can support applications by offering racing and
fallback mechanisms, which otherwise need to be separately
implemented in each application. Transport Services Implementations
are free to take any desired form as long as the API specification in
this document is honored; a non-prescriptive guide to implementing a
Transport Services system is available (see [RFC9623]).

The Transport Services system derives specific path and Protocol
Selection Properties and supported transport features from the
analysis provided in [RFC8095], [RFC8923], and [RFC8922]. The
Transport Services API enables an implementation to dynamically
choose a transport protocol rather than statically binding
applications to a protocol at compile time. The Transport Services
API also provides applications with a way to override transport
selection and instantiate a specific stack, e.g., to support servers
wishing to listen to a specific protocol. However, forcing a choice
to use a specific transport stack is discouraged for general use
because it can reduce portability.

1.1. Terminology and Notation

The Transport Services API is described in terms of:

* Objects with which an application can interact;

* Actions the application can perform on these objects;

* Events, which an object can send to an application to be processed
asynchronously; and

* Parameters associated with these actions and events.

The following notations, which can be combined, are used in this
document:

* An action that creates an object:

Object := Action()

* An action that creates an array of objects:

[]Object := Action()

* An action that is performed on an object:

Object.Action()

* An object sends an event:

Object -> Event<>

* An action takes a set of parameters; an event contains a set of
parameters. Action and event parameters whose names are suffixed
with a question mark are optional:

Action(param0, param1?, ...)
Event<param0, param1?, ...>

Objects that are passed as parameters to actions use call-by-value
behavior. Actions not associated with no an object are actions on the
API; they are equivalent to actions on a per-application global
context.

Events are sent to the application or application-supplied code
(e.g., framers; see Section 9.1.2) for processing; the details of
event interfaces are specific to the platform or implementation and
can be implemented using other forms of asynchronous processing, as
idiomatic for the implementing platform.

We also make use of the following basic types:

Boolean: Instances take the value true or false.

Integer: Instances take integer values.

Numeric: Instances take real number values.

String: Instances are represented in UTF-8.

IP Address: An IPv4 address [RFC791] or IPv6 address [RFC4291].

Enumeration: A family of types in which each instance takes one of a
fixed, predefined set of values specific to a given enumerated
type.

Tuple: An ordered grouping of multiple value types, represented as a
comma-separated list in parentheses, e.g., (Enumeration,
Preference). Instances take a sequence of values, each valid for
the corresponding value type.

Array: Denoted []Type, an instance takes a value for each of zero or
more elements in a sequence of the given Type. An array can be of
fixed or variable length.

Set: An unordered grouping of one or more different values of the
same type.

For guidance on how these abstract concepts can be implemented in
languages in accordance with language-specific design patterns and
platform features, see Appendix A.

1.2. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Overview of the API Design

The design of the API specified in this document is based on a set of
principles, themselves an elaboration on the architectural design
principles defined in [RFC9621]. The API defined in this document
provides:

* A Transport Services system that can offer a variety of transport
protocols, independent of the Protocol Stacks that will be used at
runtime. To the degree possible, all common features of these
Protocol Stacks are made available to the application in a
transport-independent way. This enables applications written to for
a single API to make use of transport protocols in terms of the
features they provide.

* A unified API to datagram and stream-oriented transports, allowing
the use of a common API for Connection establishment and closing.

* Message orientation, Message-orientation, as opposed to stream orientation, stream-orientation, using
application-assisted framing and deframing where the underlying
transport does not itself provide these. the required framing.

* Asynchronous Connection establishment, transmission, and
reception. This allows concurrent operations during establishment
and event-driven application interactions with the transport
layer.

* Selection between alternate network paths, using additional
information about the networks over which a Connection can operate
(e.g., Provisioning Domain (PvD) information [RFC7556]) where
available.

* Explicit support for transport-specific features to be applied,
when that particular transport is part of a chosen Protocol Stack.

* Explicit support for security properties as first-order transport
features.

* Explicit support for configuration of cryptographic identities and
transport security parameters persistent across multiple
Connections.

* Explicit support for multistreaming and multipath transport
protocols, and the grouping of related Connections into Connection
Groups through "cloning" of Connections (see Section 7.4). This
function allows applications to take full advantage of new
transport protocols supporting these features.

3. API Summary

An application primarily interacts with this API through two objects:
Preconnections and Connections. A Preconnection object (Section 6)
represents a set of properties and constraints on the selection and
configuration of paths and protocols to establish a Connection with
an Endpoint. A Connection object represents an instance of a
transport Protocol Stack on which data can be sent to and/or received
from a Remote Endpoint (i.e., a logical connection that, depending on
the kind of transport, can be bidirectional or unidirectional, and
that can use a stream protocol or a datagram protocol). Connections
are presented consistently to the application, irrespective of
whether the underlying transport is connectionless or connection
oriented. Connections can be created from Preconnections in three
ways:

* initiating the Preconnection (i.e., creating a Connection from the
Preconnection, actively opening, as in a client; see Initiate() in
Section 7.1),

* listening on the Preconnection (i.e., creating a Listener based on
the Preconnection, passively opening, as in a server; see Listen()
in Section 7.2), or

* a rendezvous for the Preconnection (i.e., peer-to-peer connection
establishment; see Rendezvous() in Section 7.3).

Once a Connection is established, data can be sent and received on it
in the form of Messages. The API supports the preservation of
message boundaries via both explicit Protocol Stack support and
application support through a Message Framer that finds message
boundaries in a stream. Messages are received asynchronously through
event handlers registered by the application. Errors and other
notifications also happen asynchronously on the Connection. It is
not necessary for an application to handle all events; some events
can have implementation-specific default handlers.

The application SHOULD NOT assume that ignoring events (e.g., errors)
is always safe.

3.1. Usage Examples

The following usage examples illustrate how an application might use
the Transport Services API to act as:

* a server, by listening for incoming Connections, receiving
requests, and sending responses; see Section 3.1.1.

* a client, by connecting to a Remote Endpoint using Initiate,
sending requests, and receiving responses; see Section 3.1.2.

* a peer, by connecting to a Remote Endpoint using Rendezvous while
simultaneously waiting for incoming Connections, sending Messages,
and receiving Messages; see Section 3.1.3.

The examples in this section presume that a transport protocol is
available between the Local and Remote Endpoints and that this
protocol provides reliable data transfer, preservation of data
ordering, and preservation of message boundaries. In this case, the
application can choose to receive only complete Messages.

If none of the available transport protocols provide preservation of
message boundaries, but there is a transport protocol that provides a
reliable ordered byte-stream, an application could receive this byte-
stream as partial Messages and transform it into application-layer
Messages. Alternatively, an application might provide a Message
Framer, which can transform a sequence of Messages into a byte-stream
and vice versa (Section 9.1.2).

3.1.1. Server Example

This is an example of how an application might listen for incoming
Connections using the Transport Services API, receive a request, and
send a response.

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithInterface("any")
LocalSpecifier.WithService("https")

TransportProperties := NewTransportProperties()
TransportProperties.Require(preserveMsgBoundaries)
// Reliable data transfer and preserve order are required by default

SecurityParameters := NewSecurityParameters()
SecurityParameters.Set(serverCertificate, myCertificate)

// Specifying a Remote Endpoint is optional when using Listen
Preconnection := NewPreconnection(LocalSpecifier,
TransportProperties,
SecurityParameters)

Listener := Preconnection.Listen()

Listener -> ConnectionReceived<Connection>

// Only receive complete messages in a Conn.Received handler
Connection.Receive()

Connection -> Received<messageDataRequest, messageContext>

//---- Receive event handler begin ----
Connection.Send(messageDataResponse)
Connection.Close()

// Stop listening for incoming Connections
// (this example supports only one Connection)
Listener.Stop()
//---- Receive event handler end ----

3.1.2. Client Example

This is an example of how an application might open two Connections
to a remote application using the Transport Services API and API, send a
request as well as
request, and receive a response on for each of them. the two Connections. The
code designated with comments as "Ready event handler" could, for
example, be implemented as a callback function. This function would
receive the Connection that it expects to operate on ("Connection"
and "Connection2" in the example) handed over using the variable name
"C".

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostName("example.com")
RemoteSpecifier.WithService("https")

TransportProperties := NewTransportProperties()
TransportProperties.Require(preserve-msg-boundaries)
// Reliable data transfer and preserve order are required by default

SecurityParameters := NewSecurityParameters()
TrustCallback := NewCallback({
// Verify the identity of the Remote Endpoint and return the result
})
SecurityParameters.SetTrustVerificationCallback(TrustCallback)

// Specifying a Local Endpoint is optional when using Initiate
Preconnection := NewPreconnection(RemoteSpecifier,
TransportProperties,
SecurityParameters)

Connection := Preconnection.Initiate()
Connection2 := Connection.Clone()

Connection -> Ready<>
Connection2 -> Ready<>

//---- Ready event handler for any Connection C begin ----
C.Send(messageDataRequest)

// Only receive complete messages
C.Receive()
//---- Ready event handler for any Connection C end ----

Connection -> Received<messageDataResponse, messageContext>
Connection2 -> Received<messageDataResponse, messageContext>

// Close the Connection in a Receive event handler
Connection.Close()
Connection2.Close()

A Preconnection serves as a template for creating a Connection via
initiating, listening, or via rendezvous. Once a Connection has been
created, changes made to the Preconnection that was used to create it
do not affect this Connection. Preconnections are reusable after
being used to create a Connection, whether or not this Connection was
closed. Hence, in the above example, it would be correct for the
client to initiate a third Connection to the example.com server by
continuing as follows:

//.. carry out adjustments to the Preconnection, if desired
Connection3 := Preconnection.Initiate()

3.1.3. Peer Example

This is an example of how an application might establish a Connection
with a peer using Rendezvous, send a Message, and receive a Message.

// Configure local candidates: a port on the Local Endpoint
// and via a Session Traversal Utilities for NAT (STUN) server
HostCandidate := NewLocalEndpoint()
HostCandidate.WithPort(9876)

StunCandidate := NewLocalEndpoint()
StunCandidate.WithStunServer(address, port, credentials)

LocalCandidates = [HostCandidate, StunCandidate]

TransportProperties := // ...Configure transport properties
SecurityParameters := // ...Configure security properties

Preconnection := NewPreconnection(LocalCandidates,
[], // No remote candidates yet
TransportProperties,
SecurityParameters)

// Resolve the LocalCandidates. The Preconnection.Resolve()
// call resolves both local and remote candidates; however,
// because the remote candidates have not yet been specified,
// the ResolvedRemote list returned will be empty and is not
// used.
ResolvedLocal, ResolvedRemote = Preconnection.Resolve()

// Application-specific code goes here to send the ResolvedLocal
// list to the peer via some out-of-band signaling channel (e.g.,
// in a SIP message).
...

// Application-specific code goes here to receive RemoteCandidates
// (type []RemoteEndpoint, a list of RemoteEndpoint objects) from
// the peer via the signaling channel.
...

// Add remote candidates and initiate the rendezvous:
Preconnection.AddRemote(RemoteCandidates)
Preconnection.Rendezvous()

Preconnection -> RendezvousDone<Connection>

//---- RendezvousDone event handler begin ----
Connection.Send(messageDataRequest)
Connection.Receive()
//---- RendezvousDone event handler end ----

Connection -> Received<messageDataResponse, messageContext>

// If new Remote Endpoint candidates are received from the
// peer over the signaling channel -- for example, if using
// Trickle Interactive Connectivity Establishment (ICE) --
// then add them to the Connection:
Connection.AddRemote(NewRemoteCandidates)

// On a PathChange<> event, resolve the Local Endpoint Identifiers to
// see if a new Local Endpoint has become available and, if
// so, send to the peer as a new candidate and add to the
// Connection:
Connection -> PathChange<>

//---- PathChange event handler begin ----
ResolvedLocal, ResolvedRemote = Preconnection.Resolve()
if ResolvedLocal has changed:
// Application-specific code goes here to send the
// ResolvedLocal list to the peer via the signaling channel
...

// Add the new Local Endpoints to the Connection:
Connection.AddLocal(ResolvedLocal)
//---- PathChange event handler end ----

// Close the Connection in a Receive event handler:
Connection.Close()

4. Transport Properties

Each application using the Transport Services API declares its
preferences for how the Transport Services system is to operate.
This is done by using Transport Properties, as defined in [RFC9621],
at each stage of the lifetime of a Connection.

Transport Properties are divided into Selection, Connection, and
Message Properties.

Selection Properties (see Section 6.2) can only be set during
preestablishment. They are only used to specify which paths and
Protocol Stacks can be used and are preferred by the application.
Calling Initiate on a Preconnection creates an outbound Connection,
and the Selection Properties remain readable from the Connection but
become immutable. Selection Properties can be set on Preconnections,
and the effect of Selection Properties can be queried on Connections
and Messages.

Connection Properties (see Section 8.1) are used to inform decisions
made during establishment and to fine-tune the established
Connection. They can be set during preestablishment and can be
changed later. Connection Properties can be set on Connections and
Preconnections; when set on Preconnections, they act as an initial
default for the resulting Connections.

Message Properties (see Section 9.1.3) control the behavior of the
selected Protocol Stack(s) when sending Messages. Message Properties
can be set on Messages, Connections, and Preconnections; when set on
the latter two, they act as an initial default for the Messages sent
over those Connections.

Note that configuring Connection Properties and Message Properties on
Preconnections is preferred over setting them later. Early
specification of Connection Properties allows their use as additional
input to the selection process. Protocol-specific Properties, which
enable configuration of specialized features of a specific protocol
(see Section 3.2 of [RFC9621]), are not used as input to the
selection process; they only support configuration if the respective
protocol has been selected.

4.1. Transport Property Names

Transport Properties are referred to by property names, represented
as case-insensitive strings. These names serve two purposes:

* Allowing different components of a Transport Services
Implementation to pass Transport Properties, e.g., Properties (e.g., between a
language front end frontend and a policy manager manager) or as to enable a representation of Transport
Services Implementation to represent properties retrieved from a
file or other storage. storage to the application.

* Making the code of different Transport Services Implementations
look similar. While individual programming languages might
preclude strict adherence to the aforementioned naming convention of representing
property names as case-insensitive strings (for instance, by
prohibiting the use of hyphens in symbols), users interacting with
multiple implementations will still benefit from the consistency
resulting from the use of visually similar symbols.

Transport Property Names are hierarchically organized in the form
[<Namespace>.]<PropertyName>.

* The optional Namespace component and its trailing dot character .
(".") MUST be omitted for well-known, well-known generic properties, i.e., for
properties that are not specific to a protocol.

* Protocol-specific Properties MUST use the protocol acronym as the
Namespace (e.g., a Connection that uses TCP could support a TCP-
specific Transport Property, such as the TCP User Timeout value,
in a Protocol-specific Property called tcp.userTimeoutValue (see
Section 8.2)).

* Vendor-specific or implementation-specific properties MUST be
placed in a Namespace starting with the underscore _ character and
SHOULD use a string identifying the vendor or implementation.

* For IETF protocols, the name of a Protocol-specific Property MUST
be specified in an IETF document published in the RFC Series after from the IETF Stream (after IETF review. review).
An IETF protocol Namespace does not start with an underscore character.
character ("_").

Namespaces for each of the keywords provided in the "Protocol
Numbers" registry (see <https://www.iana.org/assignments/protocol-
numbers/>) are reserved for Protocol-specific Properties and MUST NOT
be used for vendor-specific or implementation-specific properties.
Terms listed as keywords, as in the "Protocol Numbers" registry,
SHOULD be avoided as any part of a vendor-specific or implementation-
specific property name.

Though Transport Property Names are case insensitive, it is
recommended to use camelCase to improve readability. Implementations
may transpose Transport Property Names into snake_case or PascalCase
to blend into the language environment.

4.2. Transport Property Types

Each Transport Property has one of the basic types described in
Section 1.1.

Most Selection Properties (see Section 6.2) are of the Enumeration
type, and they use the Preference Enumeration, which takes one of
five possible values (Prohibit, Avoid, No Preference, Prefer, or
Require) denoting the level of preference for a given property during
protocol selection.

5. Scope of the API Definition

This document defines a language- and platform-independent API of a
Transport Services system. Given the wide variety of languages and
language conventions used to write applications that use the
transport layer to connect to other applications over the Internet,
this independence makes this API necessarily abstract.

There is no interoperability benefit in tightly defining how the API
is presented to application programmers across diverse platforms.
However, maintaining the "shape" of the abstract API across different
platforms reduces the effort for programmers who learn to use the
Transport Services API to then apply their knowledge to another
platform. That said, implementations have significant freedom in
presenting this API to programmers, balancing the conventions of the
protocol with the shape of the API. We make the following
recommendations:

* Actions, events, and errors in implementations of the Transport
Services API SHOULD use the names given for assigned to them in the this
document, subject to capitalization, punctuation, and other
typographic conventions in the language of the implementation,
unless the implementation itself uses different names for
substantially equivalent objects for networking by convention.

* Transport Services systems SHOULD implement each Selection
Property, Connection Property, and Message Context Property
specified in this document. These features SHOULD be implemented
even when, in a specific implementation, it will always result in
no operation, e.g., there is no action when the API specifies a
Property that is not available in a transport protocol implemented
on a specific platform. For example, if TCP is the only
underlying transport protocol, the Message Property msgOrdered can
be implemented (trivially, as a no-op) as disabling the
requirement for ordering will not have any effect on delivery
order for Connections over TCP. Similarly, the msgLifetime
Message Property can be implemented but ignored, as the
description of this Property (Section 9.1.3.1) states that "it is
not guaranteed that a Message will not be sent when its Lifetime
has expired".

* Implementations can use other representations for Transport
Property Names, e.g., by providing constants, but should provide a
straightforward mapping between their representation and the
property names specified here.

6. Preestablishment Phase

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.

A Preconnection object represents a potential Connection. It is a
passive object (a data structure) that merely maintains the state
that describes the properties of a Connection that might exist in the
future. This state comprises Local Endpoint and Remote Endpoint
objects that denote the endpoints of the potential Connection (see
Section 6.1), the Selection Properties (see Section 6.2), any
preconfigured Connection Properties (Section 8.1), and the security
parameters (see Section 6.3):

Preconnection := NewPreconnection([]LocalEndpoint,
[]RemoteEndpoint,
TransportProperties,
SecurityParameters)

At least one Local Endpoint MUST be specified if the Preconnection is
used to Listen for incoming Connections, but the list of Local
Endpoints MAY be empty if the Preconnection is used to Initiate
connections. If no Local Endpoint is specified, the Transport
Services system will assign an ephemeral local port to the Connection
on the appropriate interface(s). At least one Remote Endpoint MUST
be specified if the Preconnection is used to Initiate Connections,
but the list of Remote Endpoints MAY be empty if the Preconnection is
used to Listen for incoming Connections. At least one Local Endpoint
and one Remote Endpoint MUST be specified if a peer-to-peer
Rendezvous is to occur based on the Preconnection.

If more than one Local Endpoint is specified on a Preconnection, then
the application is indicating that all of the Local Endpoints are
eligible to be used for Connections. For example, their Endpoint
Identifiers might correspond to different interfaces on a multihomed
host or their Endpoint Identifiers might correspond to local
interfaces and a STUN server that can be resolved to a server-
reflexive address for a Preconnection used to make a peer-to-peer
Rendezvous.

If more than one Remote Endpoint is specified on the Preconnection,
the application is indicating that it expects all of the Remote
Endpoints to offer an equivalent service and that the Transport
Services system can choose any of them for a Connection. For
example, a Remote Endpoint might represent various network interfaces
of a host, or a server-reflexive address that can be used to reach a
host, or a set of hosts that provide equivalent local balanced
service.

In most cases, it is expected that a single Remote Endpoint will be
specified by name, and a later call to Initiate on the Preconnection
(see Section 7.1) will internally resolve that name to a list of
concrete Endpoint Identifiers. Specifying multiple Remote Endpoints
on a Preconnection allows applications to override this for more
detailed control.

If Message Framers are used (see Section 9.1.2), they MUST be added
to the Preconnection during preestablishment.

6.1. Specifying Endpoints

The Transport Services API uses the Local Endpoint and Remote
Endpoint objects to refer to the endpoints of a Connection.
Endpoints can be created as either remote or local:

RemoteSpecifier := NewRemoteEndpoint()
LocalSpecifier := NewLocalEndpoint()

A single Endpoint object represents the identity of a network host.
That endpoint can be more or less specific, depending on which
Endpoint Identifiers are set. For example, an Endpoint that only
specifies a hostname can, in fact, finally correspond to several
different IP addresses on different hosts.

An Endpoint object can be configured with the following identifiers:

* HostName (string):

RemoteSpecifier.WithHostName("example.com")

* Port (a 16-bit unsigned Integer):

RemoteSpecifier.WithPort(443)

* Service (an identifier string that maps to a port; either a
service name associated with a port number (from
<https://www.iana.org/assignments/service-names-port-numbers/>) or
a DNS SRV service name to be resolved):

RemoteSpecifier.WithService("https")

* IP address (an IPv4 or IPv6 address type; note that the examples
here show the human-readable form of the IP addresses, but the
functions can take a binary encoding of the addresses):

RemoteSpecifier.WithIPAddress(192.0.2.21)

RemoteSpecifier.WithIPAddress(2001:db8:4920:e29d:a420:7461:7073:a)

* Interface identifier (which can be a string name or other
platform-specific identifier), e.g., to qualify link-local
addresses (see Section 6.1.2 for details):

LocalSpecifier.WithInterface("en0")

The Resolve action on a Preconnection can be used to obtain a list of
available local interfaces.

Note that an IPv6 address specified with a scope zone ID (e.g.,
fe80::2001:db8%en0) is equivalent to WithIPAddress with an unscoped
address and WithInterface together.

Applications creating Endpoint objects using WithHostName SHOULD
provide Fully Qualified Domain Names (FQDNs). Not providing an FQDN
will result in the Transport Services Implementation needing to use
DNS search domains for name resolution, which might lead to
inconsistent or unpredictable behavior.

The design of the API MUST NOT permit an Endpoint object to be
configured with multiple Endpoint Identifiers of the same type. For
example, an Endpoint object cannot specify two IP addresses. Two
separate IP addresses are represented as two Endpoint objects. If a
Preconnection specifies a Remote Endpoint with a specific IP address
set, it will only establish Connections to that IP address. If, on
the other hand, a Remote Endpoint specifies a hostname but no
addresses, the Transport Services Implementation can perform name
resolution and attempt using any address derived from the original
hostname of the Remote Endpoint. Note that multiple Remote Endpoints
can be added to a Preconnection, as discussed in Section 7.5.

The Transport Services system resolves names internally, when the
Initiate, Listen, or Rendezvous method is called to establish a
Connection. Privacy considerations for the timing of this resolution
are given in Section 13.

The Resolve action on a Preconnection can be used by the application
to force early binding when required, for example, with some Network
Address Translator (NAT) traversal protocols (see Section 7.3).

6.1.1. Using Multicast Endpoints

To use multicast, a Preconnection is first created with the Local/ Local or
Remote Endpoint Identifier specifying the Any-Source Multicast (ASM)
or Source-Specific Multicast (SSM) multicast group and destination port number.
This is then followed by a call to either Initiate, Listen, or
Rendezvous, depending on whether the resulting Connection is to be
used to send messages to the multicast group, receive messages from
the group, or both send and receive messages (as is the case for an
ASM group).

Note that the Transport Services API has separate specifier calls for
multicast groups to avoid introducing filter properties for single-
source multicast and seeks to avoid confusion that can be caused by
overloading the unicast specifiers.

Calling Initiate on that Preconnection creates a Connection that can
be used to send Messages to the multicast group. The Connection
object that is created will support Send but not Receive. Any
Connections created this way are send-only and do not join the
multicast group. The resulting Connection will have a Local Endpoint
identifying the local interface to which the Connection is bound and
a Remote Endpoint identifying the multicast group.

The following API calls can be used to configure a Preconnection
before calling Initiate:

RemoteSpecifier.WithMulticastGroupIP(GroupAddress)
RemoteSpecifier.WithPort(PortNumber)
RemoteSpecifier.WithHopLimit(HopLimit)

Calling Listen on a Preconnection with a multicast group specified on
the Remote Endpoint will join the multicast group to receive
Messages. This Listener will create one Connection for each Remote
Endpoint sending to the group, with the Local Endpoint Identifier
specified as a group address. The set of Connection objects created
forms a Connection Group. The receiving interface can be restricted
by passing it as part of the LocalSpecifier or queried through the
Message Context on the Messages received (see Section 9.1.1 for
further details).

Specifying WithHopLimit sets the Time To Live (TTL) field in the
header of IPv4 packets or the Hop Count field in the header of IPv6
packets.

The following API calls can be used to configure a Preconnection
before calling Listen:

LocalSpecifier.WithSingleSourceMulticastGroupIP(GroupAddress,
SourceAddress)
LocalSpecifier.WithAnySourceMulticastGroupIP(GroupAddress)
LocalSpecifier.WithPort(PortNumber)

Calling Rendezvous on a Preconnection with an ASM group address as
the Remote Endpoint Identifier will trigger the Transport Services
Implementation to join the multicast group, group and also indicates that
the resulting Connection can be used to send Messages to the
multicast group. The Rendezvous call will return both:

1. a Connection that can be used to send to the group and that acts
the same as a Connection returned by calling Initiate with a
multicast Remote Endpoint and

2. a Listener that acts as if Listen had been called with a
multicast Remote Endpoint.

Calling Rendezvous on a Preconnection with an SSM group address as
the Local Endpoint Identifier results in an EstablishmentError.

The following API calls can be used to configure a Preconnection
before calling Rendezvous:

RemoteSpecifier.WithMulticastGroupIP(GroupAddress)
RemoteSpecifier.WithPort(PortNumber)
RemoteSpecifier.WithHopLimit(HopLimit)
LocalSpecifier.WithAnySourceMulticastGroupIP(GroupAddress)
LocalSpecifier.WithPort(PortNumber)
LocalSpecifier.WithHopLimit(HopLimit)

See Section 6.1.5 for more examples.

6.1.2. Constraining Interfaces for Endpoints

Note that this API has multiple ways to constrain and prioritize
endpoint candidates based on the network interface:

* Specifying an interface on a Remote Endpoint qualifies the scope
zone of the Remote Endpoint, e.g., for link-local addresses.

* Specifying an interface on a Local Endpoint explicitly binds all
candidates derived from this Endpoint to use the specified
interface.

* Specifying an interface using the interface Selection Property
(Section 6.2.11) or indirectly via the pvd Selection Property
(Section 6.2.12) influences the selection among the available
candidates.

While specifying an Interface on an Endpoint restricts the candidates
available for Connection establishment in the preestablishment phase,
the Selection Properties prioritize and constrain the Connection
establishment.

6.1.3. Protocol-Specific Endpoints

An Endpoint can have an alternative definition when using different
protocols. For example, a server that supports both TLS/TCP and QUIC
could be accessible on two different port numbers, depending on which
protocol is used.

To scope an Endpoint to apply conditionally to a specific transport
protocol (such as defining an alternate port to use when QUIC is
selected, as opposed to TCP), an Endpoint can be associated with a
protocol identifier. Protocol identifiers are objects or enumeration
values provided by the Transport Services API that will vary based on
which protocols are implemented in a particular system.

AlternateRemoteSpecifier.WithProtocol(QUIC)

The following example shows a case where example.com has a server
running on port 443 with an alternate port of 8443 for QUIC. Both
endpoints can be passed when creating a Preconnection.

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostName("example.com")
RemoteSpecifier.WithPort(443)

QUICRemoteSpecifier := NewRemoteEndpoint()
QUICRemoteSpecifier.WithHostName("example.com")
QUICRemoteSpecifier.WithPort(8443)
QUICRemoteSpecifier.WithProtocol(QUIC)

RemoteSpecifiers := [ RemoteSpecifier, QUICRemoteSpecifier ]

6.1.4. Endpoint Examples

The following examples of Endpoints show common usage patterns.

Specify a Remote Endpoint using a hostname "example.com" and a
service name "https", which tells the system to use the default port
for HTTPS (443):

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostName("example.com")
RemoteSpecifier.WithService("https")

Specify a Remote Endpoint using an IPv6 address and remote port:

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPAddress(2001:db8:4920:e29d:a420:7461:7073:a)
RemoteSpecifier.WithPort(443)

Specify a Remote Endpoint using an IPv4 address and remote port:

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPAddress(192.0.2.21)
RemoteSpecifier.WithPort(443)

Specify a Local Endpoint using a local interface name and no local
port to let the system assign an ephemeral local port:

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithInterface("en0")

Specify a Local Endpoint using a local interface name and local port:

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithInterface("en0")
LocalSpecifier.WithPort(443)

As an alternative to specifying an interface name for the Local
Endpoint, an application can express more fine-grained preferences
using the Interface Instance or Type Selection Property; see
Section 6.2.11. However, if the application specifies Selection
Properties that are inconsistent with the Local Endpoint, this will
result in an error once the application attempts to open a
Connection.

Specify a Local Endpoint using a STUN server:

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithStunServer(address, port, credentials)

6.1.5. Multicast Examples

The following examples show how multicast groups can be used.

Join an ASM group in receive-only mode, bound to a known port on a
named local interface:

RemoteSpecifier := NewRemoteEndpoint()

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithAnySourceMulticastGroupIP(233.252.0.0)
LocalSpecifier.WithPort(5353)
LocalSpecifier.WithInterface("en0")

TransportProperties := ...
SecurityParameters := ...

Preconnection := NewPreconnection(LocalSpecifier,
RemoteSpecifier,
TransportProperties,
SecurityProperties)
Listener := Preconnection.Listen()

Join an SSM group in receive-only mode, bound to a known port on a
named local interface:

RemoteSpecifier := NewRemoteEndpoint()

LocalSpecifier := NewLocalEndpoint()

LocalSpecifier.WithSingleSourceMulticastGroupIP(233.252.0.0,
198.51.100.10)
LocalSpecifier.WithPort(5353)
LocalSpecifier.WithInterface("en0")

TransportProperties := ...
SecurityParameters := ...

Preconnection := NewPreconnection(LocalSpecifier,
RemoteSpecifier,
TransportProperties,
SecurityProperties)
Listener := Preconnection.Listen()

Create an SSM group as a sender:

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithMulticastGroupIP(233.251.240.1)
RemoteSpecifier.WithPort(5353)
RemoteSpecifier.WithHopLimit(8)

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithIPAddress(192.0.2.22)
LocalSpecifier.WithInterface("en0")

TransportProperties := ...
SecurityParameters := ...

Preconnection := NewPreconnection(LocalSpecifier,
RemoteSpecifier,
TransportProperties,
SecurityProperties)
Connection := Preconnection.Initiate()

Join an ASM group as both a sender and a receiver:

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithMulticastGroupIP(233.252.0.0)
RemoteSpecifier.WithPort(5353)
RemoteSpecifier.WithHopLimit(8)

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithAnySourceMulticastGroupIP(233.252.0.0)
LocalSpecifier.WithIPAddress(192.0.2.22)
LocalSpecifier.WithPort(5353)
LocalSpecifier.WithInterface("en0")

TransportProperties := ...
SecurityParameters := ...

Preconnection := NewPreconnection(LocalSpecifier,
RemoteSpecifier,
TransportProperties,
SecurityProperties)
Connection, Listener := Preconnection.Rendezvous()

6.2. Specifying Transport Properties

A Preconnection object holds properties reflecting the application's
requirements and preferences for the transport. These include
Selection Properties for selecting Protocol Stacks and paths, as well
as Connection Properties and Message Properties for configuration of
the detailed operation of the selected Protocol Stacks on a per-
Connection and Message per-Message level.

The protocol(s) and path(s) selected as candidates during
establishment are determined and configured using these properties.
Since there could be paths over which some transport protocols are
unable to operate, or Remote Endpoints that support only specific
network addresses or transports, transport protocol selection is
necessarily tied to path selection. This could involve choosing
between multiple local interfaces that are connected to different
access networks.

When additional information (such as PvD information [RFC7556]) is
available about the networks over which an endpoint can operate, this
can inform the selection between alternate network paths. Path
information can include the Path MTU (PMTU), the set of supported
Differentiated Services Code Points (DSCPs), expected usage, cost,
etc. The usage of this information by the Transport Services System
is generally independent of the specific mechanism/protocol mechanism or protocol used
to receive the information (e.g., zero-conf, DHCP, or IPv6 Router
Advertisements (RAs)).

Most Selection Properties are represented as Preferences, which can
take one of five values:

Table 1: Selection Property Preference Levels

The implementation MUST ensure an outcome that is consistent with all
application requirements expressed using Require and Prohibit. While
preferences expressed using Prefer and Avoid influence protocol and
path selection as well, outcomes can vary, even given the same
Selection Properties, because the available protocols and paths can
differ across systems and contexts. However, implementations are
RECOMMENDED to seek to provide a consistent outcome to an
application, when provided with the same set of Selection Properties.

Note that application preferences can conflict with each other. For
example, if an application indicates a preference for a specific path
by specifying an interface, but also a preference for a protocol, a
situation might occur in which the preferred protocol is not
available on the preferred path. In such cases, applications can
expect properties that determine path selection to be prioritized
over properties that determine protocol selection. The transport
system SHOULD determine the preferred path first, regardless of
protocol preferences. This ordering is chosen to provide consistency
across implementations; this is based on the fact that it is more
common for the use of a given network path to determine cost to the
user (i.e., an interface type preference might be based on a user's
preference to avoid being charged more for a cellular data plan).

Selection and Connection Properties, as well as defaults for Message
Properties, can be added to a Preconnection to configure the
selection process and to further configure the eventually selected
Protocol Stack(s). They are collected into a TransportProperties
object to be passed into a Preconnection object:

TransportProperties := NewTransportProperties()

Individual properties are then set on the TransportProperties object.
Setting a Transport Property to a value overrides the previous value
of this Transport Property.

TransportProperties.Set(property, value)

To aid readability, implementations MAY provide additional
convenience functions to simplify the use of Selection Properties:
see Appendix B.1 for examples. In addition, implementations MAY
provide a mechanism to create TransportProperties objects that are
preconfigured for common use cases, as outlined in Appendix B.2.

Transport Properties for an established Connection can be queried via
the Connection object, as outlined in Section 8.

A Connection gets its Transport Properties by either being explicitly
configured via a Preconnection, being configured after establishment,
or inheriting them from an antecedent via cloning; see Section 7.4
for more details.

Section 8.1 provides a list of Connection Properties, while Selection
Properties are listed in the subsections below. Selection Properties
are only considered during establishment and cannot be changed after
a Connection is established. At which point, Selection Properties
can only be read to check the properties used by the Connection.
Upon reading, the Preference type of a Selection Property changes
into Boolean, where:

* true means that the selected Protocol Stack supports the feature
or uses the path associated with the Selection Property, and

* false means that the Protocol Stack does not support the feature
or use the path.

Implementations of Transport Services systems could alternatively use
the Require and Prohibit Preference values to represent true and
false, respectively. Other types of Selection Properties remain
unchanged when they are made available for reading after a Connection
is established.

An implementation of the Transport Services API needs to provide
sensible defaults for Selection Properties. The default values for
each property below represent a configuration that can be implemented
over TCP. If these default values are used and TCP is not supported
by a Transport Services system, then an application using the default
set of Properties might not succeed in establishing a Connection.
Using the same default values for independent Transport Services
systems can be beneficial when applications are ported between
different implementations/platforms, even if this default could lead
to a Connection failure when TCP is not available. If default values
other than those suggested below are used, it is RECOMMENDED to
clearly document any differences.

6.2.1. Reliable Data Transfer (Connection)

Name: reliability

Type: Preference

Default: Require

This property specifies whether the application needs to use a
transport protocol that ensures that all data is received at the
Remote Endpoint in order, without loss or duplication. When reliable
data transfer is enabled, this also entails being notified when a
Connection is closed or aborted.

6.2.2. Preservation of Message Boundaries

Name: preserveMsgBoundaries

Type: Preference

Default: No Preference

This property specifies whether the application needs or prefers to
use a transport protocol that preserves message boundaries.

6.2.3. Configure Per-Message Reliability

Name: perMsgReliability

Type: Preference

Default: No Preference

This property specifies whether an application considers it useful to
specify different reliability requirements for individual Messages in
a Connection.

6.2.4. Preservation of Data Ordering

Name: preserveOrder

Type: Preference

Default: Require

This property specifies whether the application wishes to use a
transport protocol that can ensure that data is received by the
application at the Remote Endpoint in the same order as it was sent.

6.2.5. Use 0-RTT Session Establishment with a Safely Replayable Message

Name: zeroRttMsg

Type: Preference

Default: No Preference

This property specifies whether an application would like to supply a
Message to the transport protocol before connection establishment,
which will then be reliably transferred to the Remote Endpoint before
or during connection establishment. This Message can potentially be
received multiple times (i.e., multiple copies of the Message data
could be passed to the Remote Endpoint). See also Section 9.1.3.4.

6.2.6. Multistream Connections in a Group

Name: multistreaming

Type: Preference

Default: Prefer

This property specifies whether the application would prefer multiple
Connections within a Connection Group to be provided by streams of a
single underlying transport connection, where possible.

6.2.7. Full Checksum Coverage on Sending

Name: fullChecksumSend

Type: Preference

Default: Require

This property specifies the application's need for protection against
corruption for all data transmitted on this Connection. Disabling
this property could enable the application to influence the sender
checksum coverage after Connection establishment (see
Section 9.1.3.6).

6.2.8. Full Checksum Coverage on Receiving

Name: fullChecksumRecv

Type: Preference

Default: Require

This property specifies the application's need for protection against
corruption for all data received on this Connection. Disabling this
property could enable the application to influence the required
minimum receiver checksum coverage after Connection establishment
(see Section 8.1.1).

6.2.9. Congestion Control

Name: congestionControl

Type: Preference

Default: Require

This property specifies whether or not the application would like the
Connection to be congestion controlled. Note that if a Connection is
not congestion controlled, an application using such a Connection
SHOULD itself perform congestion control in accordance with [RFC2914]
or use a circuit breaker in accordance with [RFC8084], whichever is
appropriate. Also note that reliability is usually combined with
congestion control in protocol implementations rendering "reliable
but not congestion controlled", a request that is unlikely to
succeed. If the Connection is congestion controlled, performing
additional congestion control in the application can have negative
performance implications.

6.2.10. Keep-Alive Packets

Name: keepAlive

Type: Preference

Default: No Preference

This property specifies whether or not the application would like the
Connection to send keep-alive packets. Note that if a Connection
determines that keep-alive packets are being sent, the application
itself SHOULD avoid generating additional keep-alive messages. Note
that, when supported, the system will use the default period for
generation of the keep-alive packets. (See also Section 8.1.4.)

6.2.11. Interface Instance or Type

Name: interface

Type: Set of (Preference, Enumeration)

Default: Empty (not setting a preference for any interface)

This property allows the application to select any specific network
interfaces or categories of interfaces it wants to Require, Prohibit,
Prefer, or Avoid. Note that marking a specific interface as Require
strictly limits path selection to that single interface, and often
leads to less flexible and resilient connection establishment.

In contrast to other Selection Properties, this property is a set of
tuples of (Enumerated) interface identifier and preference. It can
either be implemented directly as such, such or for making be implemented to make one
preference available for each interface and interface type available
on the system.

The set of valid interface types is specific to the implementation or
system. For example, on a mobile device, there could be Wi-Fi and
Cellular interface types available; whereas, on a desktop computer,
Wi-Fi and Wired Ethernet interface types might be available. An
implementation should provide all types that are supported on the
local system to allow applications to be written generically. For
example, if a single implementation is used on both mobile devices
and desktop devices, it ought to define the Cellular interface type
for both systems, since an application might wish to always prohibit
cellular.

The set of interface types is expected to change over time as new
access technologies become available. The taxonomy of interface
types on a given Transport Services system is implementation
specific.

Interface types SHOULD NOT be treated as a proxy for properties of
interfaces, such as metered or unmetered network access. If an
application needs to prohibit metered interfaces, this should be
specified via Provisioning Domain attributes (see Section 6.2.12) or
another specific property.

Note that this property is not used to specify an interface scope
zone for a particular Endpoint. Section 6.1.2 provides details about
how to qualify endpoint candidates on a per-interface basis.

6.2.12. Provisioning Domain Instance or Type

Name: pvd

Type: Set of (Preference, Enumeration)

Default: Empty (not setting a preference for any PvD)

Similar to interface (see Section 6.2.11), this property allows the
application to control path selection by selecting which specific PvD
or categories of PvDs it wants to Require, Prohibit, Prefer, or
Avoid. Provisioning Domains define consistent sets of network
properties that might be more specific than network interfaces
[RFC7556].

As with interface instances and types, this property is a set of
tuples of (Enumerated) PvD identifier and preference. It can either
be implemented directly as such, such or for making be implemented to make one
preference available for each interface and interface type available
on the system.

The identification of a specific PvD is specific to the
implementation or system. [RFC8801] defines how to use an FQDN to
identify a PvD when advertised by a network, but systems might also
use other locally relevant identifiers such as string names or
Integers to identify PvDs. As with requiring specific interfaces,
requiring a specific PvD strictly limits the path selection.

Categories or types of PvDs are also defined to be specific to the
implementation or system. These can be useful to identify a service
that is provided by a PvD. For example, if an application wants to
use a PvD that provides a Voice-Over-IP (VoIP) service on a Cellular
network, it can use the relevant PvD type to require a PvD that
provides this service, without needing to look up a particular
instance. While this does restrict path selection, it is broader
than requiring specific PvD instances or interface instances and
should be preferred over these options.

6.2.13. Use Temporary Local Address

Name: useTemporaryLocalAddress

Type: Preference

Default: Avoid for Listeners and Rendezvous Connections; Prefer for
other Connections

This property allows the application to express a preference for the
use of temporary local addresses, sometimes called "privacy"
addresses [RFC8981]. Temporary addresses are generally used to
prevent linking connections over time when a stable address,
sometimes called a "permanent" address, is not needed. There are
some caveats to note when specifying this property. First, if an
application Requires the use of temporary addresses, the resulting
Connection cannot use IPv4 because temporary addresses do not exist
in IPv4. Second, temporary local addresses might involve trading off
privacy for performance. For instance, temporary addresses (e.g.,
[RFC8981]) can interfere with resumption mechanisms that some
protocols rely on to reduce initial latency.

6.2.14. Multipath Transport

Name: multipath

Type: Enumeration

Default: Disabled for Connections created through initiate and
rendezvous; Passive for Listeners

This property specifies whether, and how, applications want to take
advantage of transferring data across multiple paths between the same
end hosts. Using multiple paths allows Connections to migrate
between interfaces or aggregate bandwidth as availability and
performance properties change. Possible values are as follows:

Disabled: The Connection will not use multiple paths once
established, even if the chosen transport supports using multiple
paths.

Active: The Connection will negotiate the use of multiple paths if
the chosen transport supports it.

Passive: The Connection will support the use of multiple paths if
the Remote Endpoint requests it.

The policy for using multiple paths is specified using the separate
multipathPolicy property; see Section 8.1.7. To enable the peer
endpoint to initiate additional paths toward a local address other
than the one initially used, it is necessary to set the
advertisesAltaddr property (see Section 6.2.15).

Setting this property to Active can have privacy implications. It
enables the transport to establish connectivity using alternate paths
that might result in users being linkable across the multiple paths,
even if the advertisesAltaddr property (see Section 6.2.15) is set to
false.

Note that Multipath Transport has no corresponding Selection Property
of type Preference. Enumeration values other than Disabled are
interpreted as a preference for choosing protocols that can make use
of multiple paths. The Disabled value implies a requirement not to
use multiple paths in parallel but does not prevent choosing a
protocol that is capable of using multiple paths, e.g., it does not
prevent choosing TCP but prevents sending the MP_CAPABLE option in
the TCP handshake.

6.2.15. Advertisement of Alternative Addresses

Name: advertisesAltaddr

Type: Boolean

Default: false

This property specifies whether alternative addresses, e.g., of other
interfaces, ought to be advertised to the peer endpoint by the
Protocol Stack. Advertising these addresses enables the peer
endpoint to establish additional connectivity, e.g., for Connection
migration or using multiple paths.

Note that this can have privacy implications because it might result
in users being linkable across the multiple paths. Also, note that
setting this to false does not prevent the local Transport Services
system from _establishing_ connectivity using alternate paths (see
Section 6.2.14); it only prevents _proactive advertisement_ of
addresses.

6.2.16. Direction of Communication

Name: direction

Type: Enumeration

Default: Bidirectional

This property specifies whether an application wants to use the
Connection for sending and/or receiving data. Possible values are as
follows:

Bidirectional: The Connection must support sending and receiving
data.

Unidirectional send: The Connection must support sending data, and
the application cannot use the Connection to receive any data.

Unidirectional receive: The Connection must support receiving data,
and the application cannot use the Connection to send any data.

Since unidirectional communication can be supported by transports
offering bidirectional communication, specifying unidirectional
communication might cause a transport stack that supports
bidirectional communication to be selected.

6.2.17. Notification of ICMP Soft Error Message Arrival

Name: softErrorNotify

Type: Preference

Default: No Preference

This property specifies whether an application considers it useful to
be informed when an ICMP error message arrives that does not force
termination of a connection. When set to true, received ICMP errors
are available as SoftError events; see Section 8.3.1. Note that even
if a protocol supporting this property is selected, not all ICMP
errors will necessarily be delivered, so applications cannot rely
upon receiving them [RFC8085].

6.2.18. Initiating Side Is Not the First to Write

Name: activeReadBeforeSend

Type: Preference

Default: No Preference

The most common client-server communication pattern involves the
client actively opening a Connection, then sending data to the
server. The server listens (passive open), reads, and then answers.
This property specifies whether an application wants to diverge from
this pattern by either:

1. actively opening with Initiate, immediately followed by reading
or

2. passively opening with Listen, immediately followed by writing.

This property is ignored when establishing connections using
Rendezvous. Requiring this property limits the choice of mappings to
underlying protocols, which can reduce efficiency. For example, it
prevents the Transport Services system from mapping Connections to
Stream Control Transmission Protocol (SCTP) streams, where the first
transmitted data takes the role of an active open signal.

6.3. Specifying Security Parameters and Callbacks

Most security parameters, e.g., TLS ciphersuites, local identity and
private key, etc., can be configured statically. Others are
dynamically configured during Connection establishment. Security
parameters and callbacks are partitioned based on their place in the
lifetime of Connection establishment. Similar to Transport
Properties, both parameters and callbacks are inherited during
cloning (see Section 7.4).

This document specifies an abstract API, which could appear to
conflict with the need for security parameters to be unambiguous.
The Transport Services System SHOULD provide reasonable, secure
defaults for each enumerated security parameter, such that users of
the system only need to specify parameters required to establish a
secure connection (e.g., serverCertificate or clientCertificate).
Specifying security parameters from enumerated values (e.g., specific
ciphersuites) might constrain which transport protocols can be
selected during Connection establishment.

Security configuration parameters are specified in the
preestablishment phase and are created as follows:

SecurityParameters := NewSecurityParameters()

Specific parameters are added using a call to Set() on the
SecurityParameters.

As with the rest of the Transport Services API, the exact names of
parameters and/or values of enumerations (e.g., ciphersuites) used in
the security parameters are specific to the system or implementation
and ought to be chosen to follow the principle of least surprise for
users of the platform/language environment in question.

For security parameters that are enumerations of known values, such
as TLS ciphersuites, implementations are responsible for exposing the
set of values they support. For security parameters that are not
simple value types, such as certificates and keys, implementations
are responsible for exposing types appropriate for the platform/
language environment.

Applications SHOULD use common safe defaults for values such as TLS
ciphersuites whenever possible. However, as discussed in [RFC8922],
many transport security protocols require specific security
parameters and constraints from the client at the time of
configuration and actively during a handshake.

The set of security parameters defined here is not exhaustive, but
illustrative. Implementations SHOULD expose an equivalent to the
parameters listed below to allow for sufficient configuration of
security parameters, but the details are expected to vary based on
platform and implementation constraints. Applications MUST be able
to constrain the security protocols and versions that the Transport
Services System will use.

Representation of security parameters in implementations ought to
parallel that chosen for Transport Property names as suggested in
Section 5.

Connections that use Transport Services SHOULD use security in
general. However, for compatibility with endpoints that do not
support transport security protocols (such as a TCP endpoint that
does not support TLS), applications can initialize their security
parameters to indicate that security can be disabled or
opportunistic. If security is disabled, the Transport Services
system will not attempt to add transport security automatically. If
security is opportunistic, it will allow Connections without
transport security, but it will still attempt to use unauthenticated
security if available.

SecurityParameters := NewDisabledSecurityParameters()

SecurityParameters := NewOpportunisticSecurityParameters()

6.3.1. Allowed Security Protocols

Name: allowedSecurityProtocols

Type: Implementation-specific enumeration of security protocol names
and/or versions

Default: Implementation-specific best available security protocols

This property allows applications to restrict which security
protocols and security protocol versions can be used in the Protocol
Stack. Applications MUST be able to constrain the security protocols
used by this or an equivalent mechanism, in order to prevent the use
of security protocols with unknown or weak security properties.

SecurityParameters.Set(allowedSecurityProtocols, [ tls_1_2, tls_1_3 ])

6.3.2. Certificate Bundles

Names: serverCertificate, clientCertificate

Type: Array of certificate objects

Default: Empty array

One or more certificate bundles identifying the Local Endpoint as a
server certificate or a client certificate. Multiple bundles may be
provided to allow selection among different Protocol Stacks that may
require differently formatted bundles. The form and format of the
certificate bundle are implementation specific. Note that if the
private keys associated with a bundle are not available, e.g., since
they are stored in Hardware Security Modules (HSMs), handshake
callbacks are necessary. See below for details.

SecurityParameters.Set(serverCertificate, myCertificateBundle[])
SecurityParameters.Set(clientCertificate, myCertificateBundle[])

6.3.3. Pinned Server Certificate

Name: pinnedServerCertificate

Type: Array of certificate chain objects

Default: Empty array

Zero or more certificate chains to use as pinned server certificates,
such that connecting will fail if the presented server certificate
does not match one of the supplied pinned certificates. The form and
format of the certificate chain are implementation specific.

SecurityParameters.Set(pinnedServerCertificate, yourCertificateChain[])

6.3.4. Application-Layer Protocol Negotiation

Name: alpn

Type: Array of Strings strings

Default: Automatic selection

Application-Layer Protocol Negotiation (ALPN) values: used to
indicate which application-layer protocols are negotiated by the
security protocol layer. See [ALPN] for a definition of the ALPN
field. Note that the Transport Services System can provide ALPN
values automatically based on the protocols being used, if not
explicitly specified by the application.

SecurityParameters.Set(alpn, ["h2"])

6.3.5. Groups, Ciphersuites, and Signature Algorithms

Names: supportedGroup, ciphersuite, signatureAlgorithm

Types: Arrays of implementation-specific enumerations

Default: Automatic selection

These are used to restrict what cryptographic parameters are used by
underlying transport security protocols. When not specified, these
algorithms should use known and safe defaults for the system.

SecurityParameters.Set(supportedGroup, secp256r1)
SecurityParameters.Set(ciphersuite, TLS_AES_128_GCM_SHA256)
SecurityParameters.Set(signatureAlgorithm, ecdsa_secp256r1_sha256)

6.3.6. Session Cache Options

Names: maxCachedSessions, cachedSessionLifetimeSeconds

Type: Integer

Default: Automatic selection

These values are used to tune session cache capacity and lifetime and
can be extended to include other policies.

SecurityParameters.Set(maxCachedSessions, 16)
SecurityParameters.Set(cachedSessionLifetimeSeconds, 3600)

6.3.7. Pre-Shared Key

Name: preSharedKey

Type: Key and identity (platform specific)

Default: None

Used to install pre-shared keying material established out of band.
Each instance of pre-shared keying material is associated with some
identity that typically identifies its use or has some protocol-
specific meaning to the Remote Endpoint. Note that the use of a pre-
shared key will tend to select a single security protocol and,
therefore, directly select a single underlying Protocol Stack. A
Transport Services API could express None in an environment-typical
way, e.g., as a Union type or special value.

SecurityParameters.Set(preSharedKey, key, myIdentity)

6.3.8. Connection Establishment Callbacks

Security decisions, especially pertaining to trust, are not static.
Once configured, parameters can also be supplied during Connection
establishment. These are best handled as client-provided callbacks.
Callbacks block the progress of the Connection establishment, which
distinguishes them from other events in the transport system. How
callbacks and events are implemented is specific to each
implementation. Security handshake callbacks that could be invoked
during Connection establishment include:

* Trust verification callback: Invoked when a Remote Endpoint's
trust must be verified before the handshake protocol can continue.
For example, the application could verify an X.509 certificate as
described in [RFC5280].

TrustCallback := NewCallback({
// Handle the trust and return the result
})
SecurityParameters.SetTrustVerificationCallback(TrustCallback)

* Identity challenge callback: Invoked when a private key operation
is required, e.g., when local authentication is requested by a
Remote Endpoint.

ChallengeCallback := NewCallback({
// Handle the challenge
})
SecurityParameters.SetIdentityChallengeCallback(ChallengeCallback)

7. Establishing Connections

Before a Connection can be used for data transfer, it needs to be
established. Establishment ends the preestablishment phase; all
transport properties and cryptographic parameter specification must
be complete before establishment, as these will be used to select
candidate Paths and Protocol Stacks for the Connection.
Establishment can be active, using the Initiate action; passive,
using the Listen action; or simultaneous for peer-to-peer
connections, using the Rendezvous action. These actions are
described in the subsections below.

7.1. Active Open: Initiate

Active open is the action of establishing a Connection to a Remote
Endpoint presumed to be listening for incoming Connection requests.
Active open is used by clients in client-server interactions. Active
open is supported by the Transport Services API through the Initiate
action:

Connection := Preconnection.Initiate(timeout?)

The timeout parameter specifies how long to wait before aborting
Active open. Before calling Initiate, the caller must have populated
a Preconnection object with a Remote Endpoint object to identify the
endpoint, optionally a Local Endpoint object (if not specified, the
system will attempt to determine a suitable Local Endpoint), as well
as all properties necessary for candidate selection.

The Initiate action returns a Connection object. Once Initiate has
been called, any changes to the Preconnection MUST NOT have any
effect on the Connection. However, the Preconnection can be reused,
e.g., to Initiate another Connection.

Once Initiate is called, the candidate Protocol Stack(s) can cause
one or more candidate transport-layer connections to be created to
the specified Remote Endpoint. The caller could immediately begin
sending Messages on the Connection (see Section 9.2) after calling
Initiate; note that any data marked as "safely replayable" that is
sent while the Connection is being established could be sent multiple
times or on using multiple candidates.

The following events can be sent by the Connection after Initiate is
called:

Connection -> Ready<>

The Ready event occurs after Initiate has established a transport-
layer connection on at least one usable candidate Protocol Stack over
at least one candidate Path. No Receive events (see Section 9.3)
will occur before the Ready event for Connections established using
Initiate.

Connection -> EstablishmentError<reason?>

An EstablishmentError occurs when:

* the set of transport properties and security parameters cannot be
fulfilled on a Connection for initiation (e.g., the set of
available Paths and/or Protocol Stacks meeting the constraints is
empty) or reconciled with the Local and/or Remote Endpoints,

* a Remote Endpoint Identifier cannot be resolved, or

* no transport-layer connection can be established to the Remote
Endpoint (e.g., because the Remote Endpoint is not accepting
connections, the application is prohibited from opening a
Connection by the operating system, or the establishment attempt
has timed out for any other reason).

Connection establishment and transmission of the first Message can be
combined in a single action (Section 9.2.5).

7.2. Passive Open: Listen

Passive open is the action of waiting for Connections from Remote
Endpoints, commonly used by servers in client-server interactions.
Passive open is supported by the Transport Services API through the
Listen action and returns a Listener object:

Listener := Preconnection.Listen()

Before calling Listen, the caller must have initialized the
Preconnection during the preestablishment phase with a Local Endpoint
object, as well as all properties necessary for Protocol Stack
selection. A Remote Endpoint can optionally be specified, to
constrain what Connections are accepted.

The Listen action returns a Listener object. Once Listen has been
called, any changes to the Preconnection MUST NOT have any effect on
the Listener. The Preconnection can be disposed of or reused, e.g.,
to create another Listener.

Listener.Stop()

Listening continues until the global context shuts down or until the
Stop action is performed on the Listener object.

Listener -> ConnectionReceived<Connection>

The ConnectionReceived event occurs when:

* a Remote Endpoint has established or cloned (e.g., by creating a
new stream in a multi-stream transport; see Section 7.4) a
transport-layer connection to this Listener (for Connection-
oriented transport protocols), or

* the first Message has been received from the Remote Endpoint (for
Connectionless protocols or streams of a multi-streaming
transport) causing a new Connection to be created.

The resulting Connection is contained within the ConnectionReceived
event and is ready to use as soon as it is passed to the application
via the event.

Listener.SetNewConnectionLimit(value)

If the caller wants to rate-limit the number of inbound Connections
that will be delivered, it can set a cap using SetNewConnectionLimit.
This mechanism allows a server to protect itself from being drained
of resources. Each time a new Connection is delivered by the
ConnectionReceived event, the value is automatically decremented.
Once the value reaches zero, no further Connections will be delivered
until the caller sets the limit to a higher value. By default, this
value is Infinite. The caller is also able to reset the value to
Infinite at any point.

Listener -> EstablishmentError<reason?>

An EstablishmentError occurs when:

* the Properties and security parameters of the Preconnection cannot
be fulfilled for listening or cannot be reconciled with the Local
Endpoint (and/or Remote Endpoint, if specified),

* the Local Endpoint (or Remote Endpoint, if specified) cannot be
resolved, or

* the application is prohibited from listening by policy.

Listener -> Stopped<>

A Stopped event occurs after the Listener has stopped listening.

7.3. Peer-to-Peer Establishment: Rendezvous

Simultaneous peer-to-peer Connection establishment is supported by
the Rendezvous action:

Preconnection.Rendezvous()

A Preconnection object used in a Rendezvous MUST have both the Local
Endpoint candidates and the Remote Endpoint candidates specified,
along with the Transport Properties and security parameters needed
for Protocol Stack selection before the Rendezvous action is
initiated.

The Rendezvous action listens on the Local Endpoint candidates for an
incoming Connection from the Remote Endpoint candidates, while also
simultaneously trying to establish a Connection from the Local
Endpoint candidates to the Remote Endpoint candidates.

If there are multiple Local Endpoints or Remote Endpoints configured,
then initiating a Rendezvous action will cause the Transport Services
Implementation to systematically probe the reachability of those
endpoint candidates following an approach such as that used in
Interactive Connectivity Establishment (ICE) [RFC8445].

If the endpoints are suspected to be behind a NAT, and the Local
Endpoint supports a method of discovering NAT bindings, such as STUN
[RFC8489] or Traversal Using Relays around NAT (TURN) [RFC8656], then
the Resolve action on the Preconnection can be used to discover such
bindings:

[]LocalEndpoint, []RemoteEndpoint := Preconnection.Resolve()

The Resolve call returns lists of Local Endpoints and Remote
Endpoints that represent the concrete addresses, local and server
reflexive, on which a Rendezvous for the Preconnection will listen
for incoming Connections and to which it will attempt to establish
Connections.

Note that the set of Local Endpoints returned by Resolve might or
might not contain information about all possible local interfaces,
depending on how the Preconnection is configured. The set of
available local interfaces can also change over time, so care needs
to be taken when using stored interface names.

An application that uses Rendezvous to establish a peer-to-peer
Connection in the presence of NATs will configure the Preconnection
object with at least one Local Endpoint that supports NAT binding
discovery. It will then Resolve the Preconnection and pass the
resulting list of Local Endpoint candidates to the peer via a
signaling protocol, for example, as part of an ICE exchange [RFC8445]
within SIP [RFC3261] or WebRTC [RFC7478]. The peer will then, via
the same signaling channel, return the Remote Endpoint candidates.
The set of Remote Endpoint candidates is then configured on the
Preconnection:

Preconnection.AddRemote([]RemoteEndpoint)

Once the application has added both the Local Endpoint candidates and
the Remote Endpoint candidates retrieved from the peer via the
signaling channel to the Preconnection, the Rendezvous action is
initiated and causes the Transport Services Implementation to begin
connectivity checks.

If successful, the Rendezvous action returns a Connection object via
a RendezvousDone<> event:

Preconnection -> RendezvousDone<Connection>

The RendezvousDone<> event occurs when a Connection is established
with the Remote Endpoint. For Connection-oriented transports, this
occurs when the transport-layer connection is established; for
Connectionless transports, it occurs when the first Message is
received from the Remote Endpoint. The resulting Connection is
contained within the RendezvousDone<> event and is ready to use as
soon as it is passed to the application via the event. Changes made
to a Preconnection after Rendezvous has been called MUST NOT have any
effect on existing Connections.

An EstablishmentError occurs when:

* the Properties and Security Parameters of the Preconnection cannot
be fulfilled for rendezvous or cannot be reconciled with the Local
and/or Remote Endpoints,

* the Local Endpoint or Remote Endpoint cannot be resolved,

* no transport-layer connection can be established to the Remote
Endpoint, or

* the application is prohibited from rendezvous by policy.

Preconnection -> EstablishmentError<reason?>

7.4. Connection Groups

Connection Groups can be created using the Clone action:

Connection := Connection.Clone(framer?, connectionProperties?)

Calling Clone on a Connection yields a Connection Group containing
two Connections: the parent Connection on which Clone was called and
a resulting cloned Connection. The new Connection is actively
opened, and it will locally send a Ready event or an
EstablishmentError event. Calling Clone on any of these Connections
adds another Connection to the Connection Group. Connections in a
Connection Group share all Connection Properties except connPriority
(see Section 8.1.2), and these Connection Properties are entangled:
changing one of the Connection Properties on one Connection in the
Connection Group automatically changes the Connection Property for
all others. For example, changing connTimeout (see Section 8.1.3) on
one Connection in a Connection Group will automatically make the same
change to this Connection Property for all other Connections in the
Connection Group. Like all other Properties, connPriority is copied
to the new Connection when calling Clone, but, in this case, a later
change to the connPriority on one Connection does not change it on
the other Connections in the same Connection Group.

The optional connectionProperties parameter allows passing Transport
Properties that control the behavior of the underlying stream or
connection to be created, e.g., Protocol-specific Properties to
request specific stream IDs for SCTP or QUIC.

Message Properties set on a Connection also apply only to that
Connection.

A new Connection created by Clone can have a Message Framer assigned
via the optional framer parameter of the Clone action. If this
parameter is not supplied, the stack of Message Framers associated
with a Connection is copied to the cloned Connection when calling
Clone. Then, a cloned Connection has the same stack of Message
Framers as the Connection from which they are cloned, but these
Framers can internally maintain per-Connection state.

It is also possible to check which Connections belong to the same
Connection Group. Calling GroupedConnections on a specific
Connection returns a set of all Connections in the same group.

[]Connection := Connection.GroupedConnections()

Connections will belong to the same group if the application
previously called Clone. Passive Connections can also be added to
the same group, e.g., when a Listener receives a new Connection that
is just a new stream of an already-active multi-streaming protocol
instance.

If the underlying protocol supports multi-streaming, it is natural to
use this functionality to implement Clone. In that case, Connections
in a Connection Group are multiplexed together, giving them similar
treatment not only inside Endpoints, but also across the end-to-end
Internet path.

Note that calling Clone can result in on-the-wire signaling, e.g., to
open a new transport connection, depending on the underlying Protocol
Stack. When Clone leads to the opening of multiple such connections,
the Transport Services system will ensure consistency of Connection
Properties by uniformly applying them to all underlying connections
in a group. Even in such a case, it is possible for a Transport
Services system to implement prioritization within a Connection Group
(see [TCP-COUPLING] and [RFC8699]).

Attempts to clone a Connection can result in a CloneError:

Connection -> CloneError<reason?>

A CloneError can also occur later, after Clone was successfully
called. In this case, it informs the application that the Connection
that sends the CloneError is no longer a part of any Connection
Group. For example, this can occur when the Transport Services
system is unable to implement entanglement (a Connection Property was
changed on a different Connection in the Connection Group, but this
change could not be successfully applied to the Connection that sends
the CloneError).

The connPriority Connection Property operates on Connections in a
Connection Group using the same approach as that used in
Section 9.1.3.2: when allocating available network capacity among
Connections in a Connection Group, sends on Connections with
numerically lower Priority values will be prioritized over sends on
Connections that have numerically higher Priority values. Capacity
will be shared among these Connections according to the connScheduler
property (Section 8.1.5). See Section 9.2.6 for more details.

7.5. Adding and Removing Endpoints on a Connection

Transport protocols that are explicitly multipath aware are expected
to automatically manage the set of Remote Endpoints that they are
communicating with and the paths to those endpoints. A PathChange<>
event, described in Section 8.3.2, will be generated when the path
changes.

However, in some cases, it is necessary to explicitly indicate to a
Connection that a new Remote Endpoint has become available for use or
indicate that a Remote Endpoint is no longer available. This is most
common in the case of peer-to-peer connections using Trickle ICE
[RFC8838].

The AddRemote action can be used to add one or more new Remote
Endpoints to a Connection:

Connection.AddRemote([]RemoteEndpoint)

Endpoints that are already known to the Connection are ignored. A
call to AddRemote makes the new Remote Endpoints available to the
Connection, but whether the Connection makes use of those Endpoints
will depend on the underlying transport protocol.

Similarly, the RemoveRemote action can be used to tell a Connection
to stop using one or more Remote Endpoints:

Connection.RemoveRemote([]RemoteEndpoint)

Removing all known Remote Endpoints can have the effect of aborting
the connection. The effect of removing the active Remote Endpoint(s)
depends on the underlying transport: multipath-aware transports might
be able to switch to a new path if other reachable Remote Endpoints
exist or the connection might abort.

Similarly, the AddLocal and RemoveLocal actions can be used to add
and remove Local Endpoints to or from a Connection.

8. Managing Connections

During preestablishment and after establishment, Preconnections or
Connections can be configured and queried using Connection
Properties, and asynchronous information could be available about the
state of the Connection via SoftError events.

Connection Properties represent the configuration and state of the
selected Protocol Stack(s) backing a Connection. These Connection
Properties can be generic (applying regardless of transport protocol)
or specific (applicable to a single implementation of a single
transport Protocol Stack). Generic Connection Properties are defined
in Section 8.1.

Protocol-specific Properties are defined in a way that is specific to
the transport or implementation to permit more specialized protocol
features to be used. Too much reliance by an application on
Protocol-specific Properties can significantly reduce the flexibility
of a Transport Services system to make appropriate selection and
configuration choices. Therefore, it is RECOMMENDED that Generic
Connection Properties be used for properties common across different
protocols and that Protocol-specific Connection Properties are only
used where specific protocols or properties are necessary.

The application can set and query Connection Properties on a per-
Connection basis. Connection Properties that are not read-only can
be set during preestablishment (see Section 6.2), as well as on
Connections directly using the SetProperty action:

ErrorCode := Connection.SetProperty(property, value)

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 application MUST be informed about this error
via the ErrorCode Object. Such errors MUST NOT cause the Connection
to be terminated. Note that changing one of the Connection
Properties on one Connection in a Connection Group will also change
it for all other Connections of that group; see Section 7.4.

At any point, the application can query Connection Properties.

ConnectionProperties := Connection.GetProperties()
value := ConnectionProperties.Get(property)
if ConnectionProperties.Has(boolean_or_preference_property) then...

Depending on the status of the Connection, the queried Connection
Properties will include different information:

* The Connection state, which can be one of the following:
Establishing, Established, Closing, or Closed (see
Section 8.1.11.1).

* Whether the Connection can be used to send data (see
Section 8.1.11.2). A Connection cannot be used for sending if the
Connection was created with the Selection Property direction set
to unidirectional receive or if a Message marked as Final was sent
over this Connection. See also Section 9.1.3.5.

* Whether the Connection can be used to receive data (see
Section 8.1.11.3). A Connection cannot be used for receiving if
the Connection was created with the Selection Property direction
set to unidirectional send or if a Message marked as Final was
received (see Section 9.3.3.3). The latter is only supported by
certain transport protocols, e.g., by TCP as a half-closed
connection.

* For Connections that are Established, Closing, or Closed:
Connection Properties (Section 8.1) of the actual protocols that
were selected and instantiated, and Selection Properties that the
application specified on the Preconnection. Selection Properties
of type Preference will be exposed as boolean Boolean values indicating
whether or not the property applies to the selected transport.
Note that the instantiated Protocol Stack might not match all
Protocol Selection Properties that the application specified on
the Preconnection.

* For Connections that are Established: Transport Services system
implementations ought to provide information concerning the
path(s) used by the Protocol Stack. This can be derived from
local PvD information, measurements by the Protocol Stack, or
other sources. For example, a transport system that is configured
to receive and process PvD information [RFC7556] could also
provide network configuration information for the chosen path(s).

8.1. Generic Connection Properties

Generic Connection Properties are defined independently of the chosen
Protocol Stack; therefore, they are available on all Connections.

Many Connection Properties have a corresponding Selection Property
that enables applications to express their preference for protocols
providing a supporting transport feature.

8.1.1. Required Minimum Corruption Protection Coverage for Receiving

Name: recvChecksumLen

Type: Integer (non-negative) or Full Coverage

Default: Full Coverage

If this property is an Integer, it specifies the minimum number of
bytes in a received Message that need to be covered by a checksum. A
receiving endpoint will not forward Messages that have less coverage
to the application. The application is responsible for handling any
corruption within the non-protected part of the Message [RFC8085]. A
special value of 0 means that a received packet might also have a
zero checksum field, and the enumerated value Full Coverage means
that the entire Message needs to be protected by a checksum. An
implementation is supposed to express Full Coverage in an
environment-typical way, e.g., as a Union type or special value.

8.1.2. Connection Priority

Name: connPriority

Type: Integer (non-negative)

Default: 100

This property is a non-negative Integer representing the priority of
this Connection relative to other Connections in the same Connection
Group. A numerically lower value reflects a higher priority. It has
no effect on Connections not part of a Connection Group. As noted in
Section 7.4, this property is not entangled when Connections are
cloned, i.e., changing the Priority on one Connection in a Connection
Group does not change it on the other Connections in the same
Connection Group. No guarantees of a specific behavior regarding
Connection Priority are given; a Transport Services system could
ignore this property. See Section 9.2.6 for more details.

8.1.3. Timeout for Aborting Connection

Name: connTimeout

Type: Numeric (positive) or Disabled

Default: Disabled

If this property is Numeric, it specifies how long to wait before
deciding that an active Connection has failed when trying to reliably
deliver data to the Remote Endpoint. Adjusting Adjustments to this property
will only take effect when if the underlying stack supports reliability.
If this property has the enumerated value Disabled, it means that no
timeout is scheduled. A Transport Services API could express
Disabled in an environment-typical way, e.g., as a Union type or
special value.

8.1.4. Timeout for Keep-Alive Packets

Name: keepAliveTimeout

Type: Numeric (positive) or Disabled

Default: Disabled

A Transport Services API can request a protocol that supports sending
keep-alive packets (Section 6.2.10). If this property is Numeric, it
specifies the maximum length of time an idle Connection (one for
which no transport packets have been sent) ought to wait before the
Local Endpoint sends a keep-alive packet to the Remote Endpoint.
Adjusting
Adjustments to this property will only take effect when if the underlying
stack supports sending keep-alive packets. Guidance on setting this
value for connectionless transports is provided in [RFC8085]. A
value greater than the Connection timeout (Section 8.1.3) or the
enumerated value Disabled will disable the sending of keep-alive
packets. A Transport Services API could express Disabled in an
environment-typical way, e.g., as a Union type or special value.

8.1.5. Connection Group Transmission Scheduler

Name: connScheduler

Type: Enumeration

Default: Weighted Fair Queueing (see Section 3.6 of [RFC8260])

This property specifies which scheduler is used among Connections
within a Connection Group to apportion the available capacity
according to Connection priorities (see Sections 7.4 and 8.1.2). A
set of schedulers is described in [RFC8260].

8.1.6. Capacity Profile

Name: connCapacityProfile

Type: Enumeration

Default: Default Profile (Best Effort)

This property specifies the desired network treatment for traffic
sent by the application and the trade-offs the application is
prepared to make in path and protocol selection to receive that
desired treatment. When the capacity profile is set to a value other
than Default, the Transport Services system SHOULD select paths and
configure protocols to optimize the trade-off between delay, delay
variation, and efficient use of the available capacity based on the
capacity profile specified. How this is realized is implementation
specific. The capacity profile MAY also be used to set markings on
the wire for Protocol Stacks supporting this action. Recommendations
for use with DSCPs are provided below for each profile; note that
when a Connection is multiplexed, the guidelines in Section 6 of
[RFC7657] apply.

The following values are valid for the capacity profile:

Default: The application provides no information about its expected
capacity profile. Transport Services systems that map the
requested capacity profile to per-connection DSCP signaling SHOULD
assign the DSCP Default Forwarding Per Hop Behavior (PHB)
[RFC2474].

Scavenger: The application is not interactive. It expects to send
and/or receive data without any urgency. This can, for example,
be used to select Protocol Stacks with scavenger transmission
control and/or to assign the traffic to a lower-effort service.
Transport Services systems that map the requested capacity profile
to per-connection DSCP signaling SHOULD assign the DSCP "Less than
best effort" PHB [RFC8622].

Low Latency/Interactive: The application is interactive and prefers
loss to latency. Response time SHOULD be optimized at the expense
of delay variation and efficient use of the available capacity
when sending on this Connection. This can be used by the system
to disable the coalescing of multiple small Messages into larger
packets (Nagle's algorithm); (Nagle algorithm (see Section 4.2.3.4 of [RFC1122])); to
prefer immediate acknowledgement from the peer endpoint when
supported by the underlying transport; and so on. Transport
Services systems that map the requested capacity profile to per-connection per-
connection DSCP signaling without multiplexing SHOULD assign a
DSCP Assured Forwarding (AF41,AF42,AF43,AF44) PHB [RFC2597].
Inelastic traffic that is expected to conform to the configured
network service rate could be mapped to the DSCP Expedited
Forwarding PHBs [RFC3246] or PHBs as discussed in [RFC5865].

Low Latency/Non-Interactive: The application prefers loss to latency
but is not interactive. Response time SHOULD be optimized at the
expense of delay variation and efficient use of the available
capacity when sending on this Connection. Transport system
implementations that map the requested capacity profile to per-
connection DSCP signaling without multiplexing SHOULD assign a
DSCP Assured Forwarding (AF21,AF22,AF23,AF24) PHB [RFC2597].

Constant-Rate Streaming: The application expects to send/receive
data at a constant rate after Connection establishment. Delay and
delay variation SHOULD be minimized at the expense of efficient
use of the available capacity. This implies that the Connection
might fail if the Path is unable to maintain the desired rate. A
transport can interpret this capacity profile as preferring a
circuit breaker [RFC8084] to a rate-adaptive congestion
controller. Transport system implementations that map the
requested capacity profile to per-connection DSCP signaling
without multiplexing SHOULD assign a DSCP Assured Forwarding
(AF31,AF32,AF33,AF34) PHB [RFC2597].

Capacity-Seeking: The application expects to send/receive data at
the maximum rate allowed by its congestion controller over a
relatively long period of time. Transport Services systems that
map the requested capacity profile to per-connection DSCP
signaling without multiplexing SHOULD assign a DSCP Assured
Forwarding (AF11,AF12,AF13,AF14) PHB [RFC2597] per Section 4.8 of
[RFC4594].

The capacity profile for a selected Protocol Stack may be modified on
a per-Message basis using the Transmission Profile Message Property;
see Section 9.1.3.8.

8.1.7. Policy for Using Multipath Transports

Name: multipathPolicy

Type: Enumeration

Default: Handover

This property specifies the local policy for transferring data across
multiple paths between the same end hosts if Multipath Transport is
not set to Disabled (see Section 6.2.14). Possible values are as
follows:

Handover: The Connection ought only to attempt to migrate between
different paths when the original path is lost or becomes
unusable. The thresholds used to declare a path unusable are
implementation specific.

Interactive: The Connection ought only to attempt to minimize the
latency for interactive traffic patterns by transmitting data
across multiple paths when this is beneficial. The goal of
minimizing the latency will be balanced against the cost of each
of these paths. Depending on the cost of the lower-latency path,
the scheduling might choose to use a higher-latency path. Traffic
can be scheduled such that data may be transmitted on multiple
paths in parallel to achieve a lower latency. The specific
scheduling algorithm is implementation specific.

Aggregate: The Connection ought to attempt to use multiple paths in
parallel to maximize available capacity and possibly overcome the
capacity limitations of the individual paths. The actual strategy
is implementation specific.

Note that this is a local choice: the Remote Endpoint can choose a
different policy.

8.1.8. Bounds on Send or Receive Rate

Name: minSendRate / minRecvRate / maxSendRate / maxRecvRate

Type: Numeric (positive) or Unlimited / Numeric (positive) or
Unlimited / Numeric (positive) or Unlimited / Numeric (positive)
or Unlimited

Default: Unlimited / Unlimited / Unlimited / Unlimited

Numeric values of these properties specify an upper-bound rate that a
transfer is not expected to exceed (even if flow control and
congestion control allow higher rates) and/or a lower-bound
application-layer rate below which the application does not deem it
will be useful. These rate values are measured at the application
layer, i.e., do not consider the header overhead from protocols used
by the Transport Services system. The values are specified in bits
per second and assumed to be measured over one-second time intervals.
For example, specifying a maxSendRate of X bits per second means
that, from the moment at which the property value is chosen, not more
than X bits will be sent in any following second. The enumerated
value Unlimited indicates that no bound is specified. A Transport
Services API could express Unlimited in an environment-typical way,
e.g., as a Union type or special value.

8.1.9. Group Connection Limit

Name: groupConnLimit

Type: Numeric (positive) or Unlimited

Default: Unlimited

If this property is Numeric, it controls the number of Connections
that can be accepted from a peer as new members of the Connection's
group. Similar to SetNewConnectionLimit, this limits the number of
ConnectionReceived events that will occur, but constrained to the
group of the Connection associated with this property. For a multi-
streaming transport, this limits the number of allowed streams. A
Transport Services API could express Unlimited in an environment-
typical way, e.g., as a Union type or special value.

8.1.10. Isolate Session

Name: isolateSession

Type: Boolean

Default: false

When set to true, this property will initiate new Connections using
as little cached information (such as session tickets or cookies) as
possible from previous Connections that are not in the same
Connection Group. Any state generated by this Connection will only
be shared with Connections in the same Connection Group. Cloned
Connections will use saved state from within the Connection Group.
This is used for separating Connection Contexts as specified in
Section 4.2.3 of [RFC9621].

Note that this does not guarantee no leakage of information, as that information will not leak
because implementations might not be able to fully isolate all caches
(e.g., RTT estimates). Note that this property could degrade
Connection performance.

8.1.11. Read-Only Connection Properties

The following generic Connection Properties are read-only, i.e., they
cannot be changed by an application.

8.1.11.1. Connection State

Name: connState

Type: Enumeration

This property provides information about the current state of the
Connection. Possible values are Establishing, Established, Closing,
or Closed. For more details on Connection state, see Section 11.

8.1.11.2. Can Send Data

Name: canSend

Type: Boolean

This property can be queried to learn whether the Connection can be
used to send data.

8.1.11.3. Can Receive Data

Name: canReceive

Type: Boolean

This property can be queried to learn whether the Connection can be
used to receive data.

8.1.11.4. Maximum Message Size Before Fragmentation or Segmentation

Name: singularTransmissionMsgMaxLen

Type: Integer (non-negative) or Not applicable

This property, if applicable, represents the maximum Message size
that can be sent without incurring network-layer fragmentation at the
sender. It is specified as a number of bytes and is less than or
equal to the Maximum Message Size on Send. It exposes a readable
value to the application based on the Maximum Packet Size (MPS). The
value of this property can change over time (and can be updated via
Datagram Packetization Layer Path MTU Discovery (DPLPMTUD)
[RFC8899]). This value allows a sending stack to avoid unwanted
fragmentation at the network layer or segmentation by the transport
layer before choosing the message size and/or after a SendError
occurs indicating an attempt to send a Message that is too large. A
Transport Services API could express Not applicable in an
environment-typical way, e.g., as a Union type or special value
(e.g., 0).

8.1.11.5. Maximum Message Size on Send

Name: sendMsgMaxLen

Type: Integer (non-negative)

This property represents the maximum Message size that an application
can send. It is specified as the number of bytes. A value of 0
indicates that sending is not possible.

8.1.11.6. Maximum Message Size on Receive

Name: recvMsgMaxLen

Type: Integer (non-negative)

This property represents the maximum Message size that an application
can receive. It is specified as the number of bytes. A value of 0
indicates that receiving is not possible.

8.2. TCP-Specific Properties: User Timeout Option (UTO)

These properties specify configurations for the TCP User Timeout
Option (UTO). This is a TCP-specific property that is only used in
the case that TCP becomes the chosen transport protocol. It is
useful only if TCP is implemented in the Transport Services system.
Protocol-specific options could also be defined for other transport
protocols.

These properties are included here because the feature Suggest
timeout to the peer is part of the minimal set of Transport Services
[RFC8923], where this feature was categorized as "functional". This
means that when a Transport Services system offers this feature, the
Transport Services API has to expose an interface to the application.
Otherwise, the implementation might violate assumptions by the
application, which could cause the application to fail.

All of the below properties are optional (e.g., it is possible to
specify User Timeout Enabled as true but not specify an Advertised
User Timeout value; in this case, the TCP default will be used).
These properties reflect the API extension specified in Section 3 of
[RFC5482].

8.2.1. Advertised User Timeout

Name: tcp.userTimeoutValue

Type: Integer (positive)

Default: the TCP default

This time value is advertised via the TCP User Timeout Option (UTO)
[RFC5482] to the Remote Endpoint, which can use it to adapt its own
Timeout for aborting the Connection (see Section 8.1.3) value.

8.2.2. User Timeout Enabled

Name: tcp.userTimeoutEnabled

Type: Boolean

Default: false

This property controls whether the TCP UTO is enabled for a
connection. This applies to both sending and receiving.

8.2.3. Timeout Changeable

Name: tcp.userTimeoutChangeable

Type: Boolean

Default: true

This property controls whether the TCP connTimeout (see
Section 8.1.3) can be changed based on a UTO received from the remote
peer. This boolean Boolean becomes false when connTimeout (see
Section 8.1.3) is used.

8.3. Connection Lifecycle Events

During the lifetime of a Connection there are events that can occur
when configured.

8.3.1. Soft Errors

Asynchronous introspection is also possible, via the SoftError event.
This event informs the application about the receipt and contents of
an ICMP error message related to the Connection. This will only
happen if the underlying Protocol Stack supports access to soft
errors; however, even if the underlying stack supports it, there is
no guarantee that a soft error will be signaled.

Connection -> SoftError<>

8.3.2. Path Change

This event notifies the application when at least one of the paths
underlying a Connection has changed. Changes occur on a single path
when the PMTU changes as well as when multiple paths are used and
paths are added or removed, the set of local endpoints changes, or a
handover has been performed.

Connection -> PathChange<>

9. Data Transfer

Data is sent and received as Messages, which allows the application
to communicate the boundaries of the data being transferred.

9.1. Messages and Framers

Each Message has an optional Message Context, which allows adding
Message Properties, to identify Send events related to a specific
Message or to inspect metadata related to the Message sent. Framers
can be used to extend or modify the Message data with additional
information that can be processed at the receiver to detect message
boundaries.

9.1.1. Message Contexts

Using the MessageContext object, the application can set and retrieve
metadata of the Message, including Message Properties (see
Section 9.1.3) and framing metadata (see Section 9.1.2.2).
Therefore, a MessageContext object can be passed to the Send action
and is returned by each event related to Send and Receive.

Message Properties can be set and queried using the Message Context:

MessageContext.add(property, value)
PropertyValue := MessageContext.get(property)

These Message Properties can be generic properties or Protocol-
specific Properties.

For MessageContexts returned by Send events (see Section 9.2.2) and
Receive events (see Section 9.3.2), the application can query
information about the Local and Remote Endpoint:

RemoteEndpoint := MessageContext.GetRemoteEndpoint()
LocalEndpoint := MessageContext.GetLocalEndpoint()

9.1.2. Message Framers

Although most applications communicate over a network using well-
formed Messages, the boundaries and metadata of the Messages are
often not directly communicated by the transport protocol itself.
For example, HTTP applications send and receive HTTP messages over a
byte-stream transport, requiring that the boundaries of HTTP messages
be parsed from the stream of bytes.

Message Framers allow extending a Connection's Protocol Stack to
define how to encapsulate or encode outbound Messages and how to
decapsulate or decode inbound data into Messages. Message Framers
allow message boundaries to be preserved when using a Connection
object, even when using byte-stream transports. This is designed
based on the fact that many of the application protocols in use at
the time of writing evolved over TCP, which does not provide message
boundary preservation; because many of these protocols require
message boundaries to function, each application-layer protocol has
defined its own framing.

To use a Message Framer, the application adds it to its Preconnection
object. Then, the Message Framer can intercept all calls to Send or
Receive on a Connection to add Message semantics, in addition to
interacting with the setup and teardown of the Connection. A Framer
can start sending data before the application sends data if the
framing protocol requires a prefix or handshake (see [RFC9329] for an
example of such a framing protocol).

Initiate() Send() Receive() Close()
| | ^ |
| | | |
+----v----------v---------+----------v-----+
| Connection |
+----+----------+---------^----------+-----+
| | | |
| +-----------------+ |
| | Messages | |
| +-----------------+ |
| | | |
+----v----------v---------+----------v-----+
| Framer(s) |
+----+----------+---------^----------+-----+
| | | |
| +-----------------+ |
| | Byte-stream | |
| +-----------------+ |
| | | |
+----v----------v---------+----------v-----+
| Transport Protocol Stack |
+------------------------------------------+

Figure 1: Protocol Stack Showing a Message Framer

Note that while Message Framers add the most value when placed above
a protocol that otherwise does not preserve message boundaries, they
can also be used with datagram- or message-based protocols. In these
cases, they add a transformation to further encode or encapsulate and
can potentially support packing multiple application-layer Messages
into individual transport datagrams.

The API to implement a Message Framer can vary, depending on the
implementation; guidance on implementing Message Framers can be found
in [RFC9623].

9.1.2.1. Adding Message Framers to Preconnections

The Message Framer object can be added to one or more Preconnections
to run on top of transport protocols. Multiple Framers can be added
to a Preconnection; in this case, the Framers operate as a framing
stack, i.e., the last one added runs first when framing outbound
Messages, and last when parsing inbound data.

The following example adds a basic HTTP Message Framer to a
Preconnection:

framer := NewHTTPMessageFramer()
Preconnection.AddFramer(framer)

Since Message Framers pass from Preconnection to Listener or
Connection, addition of Framers must happen before any operation that
might result in the creation of a Connection.

9.1.2.2. Framing Metadata

When sending Messages, applications can add Framer-specific
properties to a MessageContext (Section 9.1.1) with the add action.
To avoid naming conflicts, the property names SHOULD be prefixed with
a namespace referencing the framer implementation or the protocol it
implements as described in Section 4.1.

This mechanism can be used, for example, to set the type of a Message
for a TLV format. The namespace of values is custom for each unique
Message Framer.

messageContext := NewMessageContext()
messageContext.add(framer, key, value)
Connection.Send(messageData, messageContext)

When an application receives a MessageContext in a Receive event, it
can also look to see if a value was set by a specific Message Framer.

messageContext.get(framer, key) -> value

For example, if an HTTP Message Framer is used, the values could
correspond to HTTP headers:

httpFramer := NewHTTPMessageFramer()
...
messageContext := NewMessageContext()
messageContext.add(httpFramer, "accept", "text/html")

9.1.3. Message Properties

Applications needing to annotate the Messages they send with extra
information (for example, to control how data is scheduled and
processed by the transport protocols supporting the Connection) can
include this information in the Message Context passed to the Send
action. For other uses of the Message Context, see Section 9.1.1.

Message Properties are per-Message, not per-Send, if partial Messages
are sent (Section 9.2.3). All data blocks associated with a single
Message share properties specified in the Message Contexts. For
example, it would not make sense to have the beginning of a Message
expire but and then allow the end of the Message to still be sent.

A MessageContext object contains metadata for the Messages to be sent
or received.

messageData := "hello"
messageContext := NewMessageContext()
messageContext.add(parameter, value)
Connection.Send(messageData, messageContext)

The simpler form of Send, which does not take any MessageContext, is
equivalent to passing a default MessageContext without adding any
Message Properties.

If an application wants to override Message Properties for a specific
Message, it can acquire an empty MessageContext object and add all
desired Message Properties to that object. It can then reuse the
same MessageContext object for sending multiple Messages with the
same properties.

Properties can be added to a MessageContext object only before the
context is used for sending. Once a MessageContext has been used
with a Send action, further modifications to the MessageContext
object do not have any effect on this Send call. Message Properties
that are not added to a MessageContext object before using the
context for sending will either take a specific default value or be
configured based on Selection or Connection Properties of the
Connection that is associated with the Send call. This
initialization behavior is defined per Message Property below.

The Message Properties could be inconsistent with the properties of
the Protocol Stacks underlying the Connection on which a given
Message is sent. For example, a Protocol Stack must be able to
provide ordering if the msgOrdered property of a Message is enabled.
Sending a Message with Message Properties inconsistent with the
Selection Properties of the Connection yields an error.

If a Message Property contradicts a Connection Property, and if this
per-Message behavior can be supported, it overrides the Connection
Property for the specific Message. For example, if reliability is
set to Require and a protocol with configurable per-Message
reliability is used, setting msgReliable to false for a particular
Message will allow this Message to be sent without any reliability
guarantees. Changing the msgReliable Message Property is only
possible for Connections that were established enabling the Selection
Property perMsgReliability. If the contradicting Message Property
cannot be supported by the Connection (such as requiring reliability
on a Connection that uses an unreliable protocol), the Send action
will result in a SendError event.

The Message Properties in the following subsections are supported.

9.1.3.1. Lifetime

Name: msgLifetime

Type: Numeric (positive)

Default: infinite

The Lifetime specifies how long a particular Message can wait in the
Transport Services system before it is sent to the Remote Endpoint.
After this time, it is irrelevant and no longer needs to be
(re-)transmitted. This is a hint to the Transport Services system --
it is not guaranteed that a Message will not be sent when its
Lifetime has expired.

Setting a Message's Lifetime to infinite indicates that the
application does not wish to apply a time constraint on the
transmission of the Message, but it does not express a need for
reliable delivery; reliability is adjustable per Message via the
perMsgReliability property (see Section 9.1.3.7). The type and units
of Lifetime are implementation specific.

9.1.3.2. Priority

Name: msgPriority

Type: Integer (non-negative)

Default: 100

This property specifies the priority of a Message, relative to other
Messages sent over the same Connection. A numerically lower value
represents a higher priority.

A Message with Priority 2 will yield to a Message with Priority 1,
which will yield to a Message with Priority 0, and so on. Priorities
can be used as a sender-side scheduling construct only or be used to
specify priorities on the wire for Protocol Stacks supporting
prioritization.

Note that this property is not a per-Message override of
connPriority; see Section 8.1.2. The priority properties might
interact, but they can be used independently and be realized by
different mechanisms; see Section 9.2.6.

9.1.3.3. Ordered

Name: msgOrdered

Type: Boolean

Default: the queried Boolean value of the Selection Property
preserveOrder (Section 6.2.4)

The order in which Messages were submitted for transmission via the
Send action will be preserved on delivery via Receive events for all
Messages on a Connection that have this Message Property set to true.

If false, the Message is delivered to the receiving application
without preserving the ordering. This property is used for protocols
that support preservation of data ordering (see Section 6.2.4) but
allow out-of-order delivery for certain Messages, e.g., by
multiplexing independent Messages onto different streams.

If it is not configured by the application before sending, this
property's default value will be based on the Selection Property
preserveOrder of the Connection associated with the Send action.

9.1.3.4. Safely Replayable

Name: safelyReplayable

Type: Boolean

Default: false

If true, safelyReplayable specifies that a Message is safe to send to
the Remote Endpoint more than once for a single Send action. It
marks the data as safe for certain 0-RTT establishment techniques,
where retransmission of the 0-RTT data could cause the remote
application to receive the Message multiple times.

For protocols that do not protect against duplicated Messages, e.g.,
UDP, all Messages need to be marked as "safely replayable" by
enabling this property. To enable protocol selection to choose such
a protocol, safelyReplayable needs to be added to the
TransportProperties passed to the Preconnection. If such a protocol
was chosen, disabling safelyReplayable on individual Messages MUST
result in a SendError.

9.1.3.5. Final

Name: final

Type: Boolean

Default: false

If true, this indicates a Message is the last that the application
will send on a Connection. This allows underlying protocols to
indicate to the Remote Endpoint that the Connection has been
effectively closed in the sending direction. For example, TCP-based
Connections can send a FIN once a Message marked as Final has been
completely sent, indicated by marking endOfMessage. Protocols that
do not support signaling the end of a Connection in a given direction
will ignore this property.

A Final Message must always be sorted to the end of a list of
Messages. The Final property overrides Priority and any other
property that would reorder Messages. If another Message is sent
after a Message marked as Final has already been sent on a
Connection, the Send action for the new Message will cause a
SendError event.

9.1.3.6. Sending Corruption Protection Length

Name: msgChecksumLen

Type: Integer (non-negative) or Full Coverage

Default: Full Coverage

If this property is an Integer, it specifies the minimum length of
the section of a sent Message, starting from byte 0, that the
application requires to be delivered without corruption due to lower-
layer errors. It is used to specify options for simple integrity
protection via checksums. A value of 0 means that no checksum needs
to be calculated, and the enumerated value Full Coverage means that
the entire Message needs to be protected by a checksum. Only Full
Coverage is guaranteed: any other requests are advisory, which may
result in Full Coverage being applied.

9.1.3.7. Reliable Data Transfer (Message)

Name: msgReliable

Type: Boolean

Default: the queried Boolean value of the Selection Property
reliability (Section 6.2.1)

When true, this property specifies that a Message should be sent in
such a way that the transport protocol ensures that all data is
received by the Remote Endpoint. Changing the msgReliable property
on Messages is only possible for Connections that were established
enabling the Selection Property perMsgReliability. When this is not
the case, changing msgReliable will generate an error.

Disabling this property indicates that the Transport Services system
could disable retransmissions or other reliability mechanisms for
this particular Message, but such disabling is not guaranteed.

If it is not configured by the application before sending, this
property's default value will be based on the Selection Property
reliability of the Connection associated with the Send action.

9.1.3.8. Message Capacity Profile Override

Name: msgCapacityProfile

Type: Enumeration

Default: inherited from the Connection Property connCapacityProfile
(Section 8.1.6)

This enumerated property specifies the application's preferred trade-
offs for sending this Message; it is a per-Message override of the
connCapacityProfile Connection Property (see Section 8.1.6). If it
is not configured by the application before sending, this property's
default value will be based on the Connection Property
connCapacityProfile of the Connection associated with the Send
action.

9.1.3.9. No Network-Layer Fragmentation

Name: noFragmentation

Type: Boolean

Default: false

This property specifies that a Message should be sent and received
without network-layer fragmentation, if possible. It can be used to
avoid network-layer fragmentation when transport segmentation is
preferred.

This only takes effect when the transport uses a network layer that
supports this functionality. When it does take effect, setting this
property to true will cause the sender to avoid network-layer source
fragmentation. When using IPv4, this will result in the Don't
Fragment (DF) bit being set in the IP header.

Attempts to send a Message with this property that result in a size
greater than the transport's current estimate of its maximum packet
size (singularTransmissionMsgMaxLen) can result in transport
segmentation when permitted or in a SendError.

| Note: noSegmentation is used when it is desired to only sending send a
| Message within a single network packet.

9.1.3.10. No Segmentation

Name: noSegmentation

Type: Boolean

Default: false

When set to true, this property requests that the transport layer not
provide segmentation of Messages larger than the maximum size
permitted by the network layer and that it avoid network-layer source
fragmentation of Messages. When running over IPv4, setting this
property to true will result in a sending endpoint setting the Don't
Fragment bit in the IPv4 header of packets generated by the transport
layer.

An attempt to send a Message that results in a size greater than the
transport's current estimate of its maximum packet size
(singularTransmissionMsgMaxLen) will result in a SendError. This
only takes effect when the transport and network layers support this
functionality.

9.2. Sending Data

Once a Connection has been established, it can be used for sending
Messages. By default, Send enqueues a complete Message and takes
optional per-Message properties (see Section 9.2.1). All Send
actions are asynchronous and deliver events (see Section 9.2.2).
Sending partial Messages for streaming large data is also supported
(see Section 9.2.3).

Messages are sent on a Connection using the Send action:

Connection.Send(messageData, messageContext?, endOfMessage?)

where messageData is the data object to send and messageContext
allows adding Message Properties, identifying Send events related to
a specific Message or inspecting metadata related to the Message sent
(see Section 9.1.1).

The optional endOfMessage parameter supports partial sending and is
described in Section 9.2.3.

9.2.1. Basic Sending

The most basic form of sending on a Connection involves enqueuing a
single Data block as a complete Message with default Message
Properties.

messageData := "hello"
Connection.Send(messageData)

The interpretation of a Message to be sent is dependent on the
implementation and on the constraints on the Protocol Stacks implied
by the Connection's transport properties. For example, a Message
could be the payload of a single datagram for a UDP Connection; or Connection.
Another example would be an HTTP Request for an HTTP Connection.

Some transport protocols can deliver arbitrarily sized Messages, but
other protocols constrain the maximum Message size. Applications can
query the Connection Property sendMsgMaxLen (Section 8.1.11.5) to
determine the maximum size allowed for a single Message. If a
Message is too large to fit in the Maximum Message Size for the
Connection, the Send will fail with a SendError event
(Section 9.2.2.3). For example, it is invalid to send a Message over
a UDP connection that is larger than the available datagram sending
size.

9.2.2. Send Events

Like all actions in the Transport Services API, the Send action is
asynchronous. There are several events that can be delivered in
response to sending a Message. Exactly one event (Sent, Expired, or
SendError) will be delivered in response to each call to Send.

Note that, if partial Send calls are used (Section 9.2.3), there will
still be exactly one Send event delivered for each call to Send. For
example, if a Message expired while two requests to Send data for
that Message are outstanding, there will be two Expired events
delivered.

The Transport Services API should allow the application to correlate
which Send action resulted in
a Send event to the particular call to Send that triggered the event.
The manner in which this correlation is indicated is implementation
specific.

9.2.2.1. Sent

Connection -> Sent<messageContext>

The Sent event occurs when a previous Send call has completed, i.e.,
when the data derived from the Message has been passed down or
through the underlying Protocol Stack and is no longer the
responsibility of the Transport Services API. The exact disposition
of the Message (i.e., whether it has actually been transmitted, moved
into a buffer on the network interface, moved into a kernel buffer,
and so on) when the Sent event occurs is implementation specific.
The Sent event contains a reference to the Message Context of the
Message to which it applies.

Sent events allow an application to obtain an understanding of the
amount of buffering it creates. That is, if an application calls the
Send action multiple times without waiting for a Sent event, it has
created more buffer inside the Transport Services system than an
application that always waits for the Sent event before calling the
next Send action.

9.2.2.2. Expired

Connection -> Expired<messageContext>

The Expired event occurs when a previous Send action expired before
completion, i.e., when the Message was not sent before its Lifetime
(see Section 9.1.3.1) expired. This is separate from SendError, as
it is an expected behavior for partially reliable transports. The
Expired event contains a reference to the Message Context of the
Message to which it applies.

9.2.2.3. SendError

Connection -> SendError<messageContext, reason?>

A SendError occurs when a Message was not sent due to an error
condition: an attempt to send a Message that is too large for the
system and Protocol Stack to handle, some failure of the underlying
Protocol Stack, or a set of Message Properties not consistent with
the Connection's transport properties. The SendError contains a
reference to the Message Context of the Message to which it applies.

9.2.3. Partial Sends

It is not always possible for an application to send all data
associated with a Message in a single Send action. The Message data
might be too large for the application to hold in memory at one time
or the length of the Message might be unknown or unbounded.

Partial Message sending is supported by passing an endOfMessage
Boolean parameter to the Send action. This value is always true by
default, and the simpler forms of Send are equivalent to passing true
for endOfMessage.

The following example sends a Message in two separate calls to Send:

messageContext := NewMessageContext()
messageContext.add(parameter, value)

messageData := "hel"
endOfMessage := false
Connection.Send(messageData, messageContext, endOfMessage)

messageData := "lo"
endOfMessage := true
Connection.Send(messageData, messageContext, endOfMessage)

All data sent with the same MessageContext object will be treated as
belonging to the same Message and will constitute an in-order series
until the endOfMessage is marked.

9.2.4. Batching Sends

To reduce the overhead of sending multiple small Messages on a
Connection, the application could batch several Send actions
together. This provides a hint to the system that the sending of
these Messages ought to be coalesced when possible and that sending
any of the batched Messages can be delayed until the last Message in
the batch is enqueued.

The semantics for starting and ending a batch can be implementation
specific but need to allow multiple Send actions to be enqueued.

Connection.StartBatch()
Connection.Send(messageData)
Connection.Send(messageData)
Connection.EndBatch()

9.2.5. Send on Active Open: InitiateWithSend

For application-layer protocols where the Connection initiator also
sends the first Message, the InitiateWithSend action combines
Connection initiation with a first Message sent:

Connection := Preconnection.InitiateWithSend(messageData,
messageContext?,
timeout?)

Whenever possible, a MessageContext should be provided to declare the
Message passed to InitiateWithSend as "safely replayable" using the
safelyReplayable property. This allows the Transport Services system
to make use of 0-RTT establishment in case this is supported by the
available Protocol Stacks. When the selected stack or stacks do not
support transmitting data upon connection establishment,
InitiateWithSend is identical to Initiate followed by Send.

Neither partial sends nor send batching are supported by
InitiateWithSend.

The events that are sent after InitiateWithSend are equivalent to
those that would be sent by an invocation of Initiate followed
immediately by an invocation of Send, with the caveat that a send
failure that occurs because the Connection could not be established
will not result in a SendError separate from the EstablishmentError
signaling the failure of Connection establishment.

9.2.6. Priority and the Transport Services API

The Transport Services API provides two properties to allow a sender
to signal the relative priority of data transmission: msgPriority
(see Section 9.1.3.2) and connPriority (see Section 8.1.2). These
properties are designed to allow the expression and implementation of
a wide variety of approaches to transmission priority in the
transport and application layers, including those that do not appear
on the wire (affecting only sender-side transmission scheduling) as
well as those that do (e.g., [RFC9218]). A Transport Services system
gives no guarantees about how its expression of relative priorities
will be realized.

The Transport Services API does order connPriority over msgPriority.
In the absence of other externalities (e.g., transport-layer flow
control), a priority 1 Message on a priority 0 Connection will be
sent before a priority 0 Message on a priority 1 Connection in the
same group.

9.3. Receiving Data

Once a Connection is established, it can be used for receiving data
(unless the direction property is set to unidirectional send). As
with sending, the data is received in Messages. Receiving is an
asynchronous operation in which each call to Receive enqueues a
request to receive new data from the Connection. Once data has been
received, or an error is encountered, an event will be delivered to
complete any pending Receive requests (see Section 9.3.2). If
Messages arrive at the Transport Services system before Receive
requests are issued, ensuing Receive requests will first operate on
these Messages before awaiting any further Messages.

9.3.1. Enqueuing Receives

Receive takes two parameters to specify the length of data that an
application is willing to receive, both of which are optional and
have default values if not specified.

Connection.Receive(minIncompleteLength?, maxLength?)

By default, Receive will try to deliver complete Messages in a single
event (Section 9.3.2.1).

The application can set a minIncompleteLength value to indicate the
smallest partial Message data size in bytes to be delivered in
response to this Receive. By default, this value is infinite, which
means that only complete Messages should be delivered. See
Sections 9.3.2.2 and 9.1.2 for more information on how this is
accomplished. If this value is set to some smaller value, the
associated receive event will be triggered only:

1. when at least that many bytes are available,

2. the Message is complete with fewer bytes, or

3. the system needs to free up memory.

Applications SHOULD always check the length of the data delivered to
the receive event and not assume it will be as long as
minIncompleteLength in the case of shorter complete Messages or
memory issues.

The maxLength argument indicates the maximum size of a Message in
bytes that the application is currently prepared to receive. The
default value for maxLength is infinite. If an incoming Message is
larger than the minimum of this size and the maximum Message size on
receive for the Connection's Protocol Stack, it will be delivered via
ReceivedPartial events (Section 9.3.2.2).

Note that maxLength does not guarantee that the application will
receive that many bytes if they are available; the Transport Services
API could return ReceivedPartial events with less data than maxLength
according to implementation constraints. Note also that maxLength
and minIncompleteLength are intended only to manage buffering and are
not interpreted as a receiver preference for Message reordering.

9.3.2. Receive Events

Each call to Receive will be paired with a single Receive event.
This allows an application to provide backpressure to the transport
stack when it is temporarily not ready to receive Messages. For
example, an application that will later be able to handle multiple
receive events at the same time can make multiple calls to Receive
without waiting for, or processing, any receive events. An
application that is temporarily unable to process received events for
a connection could refrain from calling Receive or could delay
calling it. This would lead to a buildup of unread data, which, in
turn, could result in backpressure to the sender via a transport
protocol's flow control.

The Transport Services API should allow the application to correlate
which call to Receive resulted in
a Receive event to the particular call to Receive that triggered the
event. The manner in which this correlation is indicated is
implementation specific.

9.3.2.1. Received

Connection -> Received<messageData, messageContext>

A Received event indicates the delivery of a complete Message. It
contains two objects: the received bytes as messageData and the
metadata and properties of the received Message as messageContext.

The messageData value provides access to the bytes that were received
for this Message, along with the length of the byte array. The
messageContext value is provided to enable retrieving metadata about
the Message and referring to the Message. The MessageContext object
is described in Section 9.1.1.

See Section 9.1.2 regarding how to handle Message framing in
situations where the Protocol Stack only provides a byte-stream
transport.

9.3.2.2. ReceivedPartial

Connection -> ReceivedPartial<messageData, messageContext,
endOfMessage>

If a complete Message cannot be delivered in one event, one part of
the Message can be delivered with a ReceivedPartial event. To
continue to receive more of the same Message, the application must
invoke Receive again.

Multiple invocations of ReceivedPartial deliver data for the same
Message by passing the same MessageContext until the endOfMessage
flag is delivered or a ReceiveError occurs. All partial blocks of a
single Message are delivered in order without gaps. This event does
not support delivering non-contiguous partial Messages. For example,
if Message A is divided into three pieces (A1, A2, A3), Message B is
divided into three pieces (B1, B2, B3), and preserveOrder is not
Required, the ReceivedPartial could deliver them in a sequence like
this: A1, B1, B2, A2, A3, B3. This is because the MessageContext
allows the application to identify the pieces as belonging to Message
A and B, respectively. However, a sequence like A1, A3 will never
occur.

If the minIncompleteLength in the Receive request was set to be
infinite (indicating a request to receive only complete Messages),
the ReceivedPartial event could still be delivered if one of the
following conditions is true:

* the underlying Protocol Stack supports message boundary
preservation and the size of the Message is larger than the
buffers available for a single Message;

* the underlying Protocol Stack does not support message boundary
preservation and the Message Framer (see Section 9.1.2) cannot
determine the end of the Message using the buffer space it has
available; or

* the underlying Protocol Stack does not support message boundary
preservation and no Message Framer was supplied by the
application.

Note that, in the absence of message boundary preservation or a
Message Framer, all bytes received on the Connection will be
represented as one large Message of indeterminate length.

In the following example, an application only wants to receive up to
1000 bytes at a time from a Connection. If a 1500-byte Message
arrives, it would receive the Message in two separate ReceivedPartial
events.

Connection.Receive(1, 1000)

// Receive the first 1000 bytes; message is incomplete
Connection -> ReceivedPartial<messageData(1000 bytes),
messageContext, false>

Connection.Receive(1, 1000)

// Receive the last 500 bytes; message is now complete
Connection -> ReceivedPartial<messageData(500 bytes),
messageContext, true>

9.3.2.3. ReceiveError

Connection -> ReceiveError<messageContext, reason?>

A ReceiveError occurs when:

* data is received by the underlying Protocol Stack that cannot be
fully retrieved or parsed, and

* it is useful for the application to be notified of such errors.

For example, a ReceiveError can indicate that a Message (identified
via the messageContext value) that was being partially received
previously, but had not completed, encountered an error and will not
be completed. This can be useful for an application, which might
wish to use this error as a hint to remove previously received
Message parts from memory. As another example, if an incoming
Message does not fulfill the recvChecksumLen property (see
Section 8.1.1), an application can use this error as a hint to inform
the peer application to adjust the msgChecksumLen property (see
Section 9.1.3.6).

In contrast, internal protocol reception errors (e.g., loss causing
retransmissions in TCP) are not signaled by this event. Conditions
that irrevocably lead to the termination of the Connection are
signaled using ConnectionError (see Section 10).

9.3.3. Receive Message Properties

Each Message Context could contain metadata from protocols in the
Protocol Stack; which metadata is available is Protocol Stack
dependent. These are exposed through additional read-only Message
Properties that can be queried from the MessageContext object (see
Section 9.1.1) passed by the receive event. The metadata values in
the following subsections are supported.

9.3.3.1. Property Specific to UDP and UDP-Lite: ECN

When available, Message metadata carries the value of the Explicit
Congestion Notification (ECN) field. This information can be used
for logging and debugging as well as building applications that need
access to information about the transport internals for their own
operation. This property is specific to UDP and UDP-Lite, because
these protocols do not implement congestion control; hence, they
expose this functionality to the application (see [RFC8293],
following the guidance in [RFC8085]).

9.3.3.2. Early Data

In some cases, it can be valuable to know whether data was read as
part of early data transfer (before Connection establishment has
finished). This is useful if applications need to treat early data
separately, e.g., if early data has different security properties
than data sent after connection establishment. In the case of TLS
1.3, client early data can be replayed maliciously (see [RFC8446]).
Thus, receivers might wish to perform additional checks for early
data to ensure that it is safely replayable. If TLS 1.3 is available
and the recipient Message was sent as part of early data, the
corresponding metadata carries a flag indicating as such. If early
data is enabled, applications should check this metadata field for
Messages received during Connection establishment and respond
accordingly.

9.3.3.3. Receiving Final Messages

The Message Context can indicate whether or not this Message is the
Final Message on a Connection. For any Message that is marked as
Final, the application can assume that there will be no more Messages
received on the Connection once the Message has been completely
delivered. This corresponds to the final property that can be marked
on a sent Message; see Section 9.1.3.5.

Some transport protocols and peers do not support signaling of the
final property. Therefore, applications SHOULD NOT rely on receiving
a Message marked Final to know that the sending endpoint is done
sending on a Connection.

Any calls to Receive once the Final Message has been delivered will
result in errors.

10. Connection Termination

A Connection can be terminated:

1. by the Local Endpoint (i.e., the application calls the Close,
CloseGroup, Abort, or AbortGroup action),

2. by the Remote Endpoint (i.e., the remote application calls the
Close, CloseGroup, Abort, or AbortGroup action), or

3. because of an error (e.g., a timeout).

A local call of the Close action will cause the Connection to send
either a Closed event or a ConnectionError event; a local call of the
CloseGroup action will cause all of the Connections in the group to
send either a Closed event or a ConnectionError event. A local call
of the Abort action will cause the Connection to send a
ConnectionError event, indicating local Abort as a reason; a local
call of the AbortGroup action will cause all of the Connections in
the group to send a ConnectionError event, indicating local Abort as
a reason.

Remote action calls map to events similar to local calls (e.g., a
remote Close causes the Connection to send either a Closed event or a
ConnectionError event), but in contrast to local action calls, it is
not guaranteed that such events will indeed be invoked. When an
application needs to free resources associated with a Connection, it
ought not rely on the invocation of such events due to termination
calls from the Remote Endpoint; instead, it should use the local
termination actions.

Close terminates a Connection after satisfying all the requirements
that were specified regarding the delivery of Messages that the
application has already given to the Transport Services system. Upon
successfully satisfying all these requirements, the Connection will
send the Closed event. For example, if reliable delivery was
requested for a Message handed over before calling Close, the Closed
event will signify that this Message has indeed been delivered. This
action does not affect any other Connection in the same Connection
Group.

An application MUST NOT assume that it can receive any further data
on a Connection for which it has called Close, even if such data is
already in flight.

Connection.Close()

The Closed event informs the application that a Close action has
successfully completed or that the Remote Endpoint has closed the
Connection. There is no guarantee that a remote Close will be
signaled.

Connection -> Closed<>

Abort terminates a Connection without delivering any remaining
Messages. This action does not affect any other Connection that is
entangled with this one in a Connection Group. When the Abort action
has finished, the Connection will send a ConnectionError event,
indicating local Abort as a reason.

Connection.Abort()

CloseGroup gracefully terminates a Connection and any other
Connections in the same Connection Group. For example, all of the
Connections in a group might be streams of a single session for a
multistreaming protocol; closing the entire group will close the
underlying session. See also Section 7.4. All Connections in the
group will send a Closed event when the CloseGroup action was
successful. As with Close, any Messages remaining to be processed on
a Connection will be handled prior to closing.

Connection.CloseGroup()

AbortGroup terminates a Connection and any other Connections that are
in the same Connection Group without delivering any remaining
Messages. When the AbortGroup action has finished, all Connections
in the group will send a ConnectionError event, indicating local
Abort as a reason.

Connection.AbortGroup()

A ConnectionError informs the application that:

1. data could not be delivered to the peer after a timeout or

2. the Connection has been aborted (e.g., because the peer has
called Abort).

There is no guarantee that an Abort from the peer will be signaled.

Connection -> ConnectionError<reason?>

11. Connection State and Ordering of Operations and Events

This Transport Services API is designed to be independent of an
implementation's concurrency model. The exact details regarding how
actions are handled, and how events are dispatched, are
implementation dependent.

Some transitions of Connection states are associated with events:

* Ready<> occurs when a Connection created with Initiate or
InitiateWithSend transitions to Established state.

* ConnectionReceived<> occurs when a Connection created with Listen
transitions to Established state.

* RendezvousDone<> occurs when a Connection created with Rendezvous
transitions to Established state.

* Closed<> occurs when a Connection transitions to Closed state
without error.

* EstablishmentError<> occurs when a Connection created with
Initiate transitions from Establishing state to Closed state due
to an error.

* ConnectionError<> occurs when a Connection transitions to Closed
state due to an error in all other circumstances.

The following diagram shows the possible states of a Connection and
the events that occur upon a transition from one state to another.

(*) (**)
Establishing -----> Established -----> Closing ------> Closed
| ^
| |
+---------------------------------------------------+
EstablishmentError<>

(*) Ready<>, ConnectionReceived<>, RendezvousDone<>
(**) Closed<>, ConnectionError<>

Figure 2: Connection State Diagram

The Transport Services API provides the following guarantees about
the ordering of operations:

* Sent<> events will occur on a Connection in the order in which the
Messages were sent (i.e., delivered to the kernel or to the
network interface, depending on the implementation).

* Received<> will never occur on a Connection before it is
Established, i.e., before a Ready<> event on that Connection or a
ConnectionReceived<> or RendezvousDone<> containing that
Connection.

* No events will occur on a Connection after it is closed, i.e.,
after a Closed<> event, an EstablishmentError<> or
ConnectionError<> will not occur on that Connection. To ensure
this ordering, Closed<> will not occur on a Connection while other
events on the Connection are still locally outstanding (i.e.,
known to the Transport Services API and waiting to be dealt with
by the application).

12. IANA Considerations

This document has no IANA actions.

Future works might create IANA registries for generic transport
property names and transport property namespaces (see Section 4.1).

13. Privacy and Security Considerations

This document describes a generic API for interacting with a
Transport Services system. Part of this API includes configuration
details for transport security protocols, as discussed in
Section 6.3. It does not recommend use (or disuse) of specific
algorithms or protocols. Any API-compatible transport security
protocol ought to work in a Transport Services system. Security
considerations for these protocols are discussed in the respective
specifications.

[RFC9621] provides general security considerations and requirements
for any system that implements the Transport Services architecture.
These include recommendations of relevance to the API, e.g.,
regarding the use of keying material.

The described API is used to exchange information between an
application and the Transport Services system. While it is not
necessarily expected that both systems are implemented by the same
authority, it is expected that the Transport Services Implementation
is provided as a library either that is selected by the application
from a trusted party or that it is part of the operating system that
the application also relies on for other tasks.

In either case, the Transport Services API is an internal interface
that is used to exchange information locally between two systems.
However, as the Transport Services system is responsible for network
communication, it is in the position to potentially share any
information provided by the application with the network or another
communication peer. Most of the information provided over the
Transport Services API is useful to configure and select protocols
and paths and is not necessarily privacy sensitive. Still, some
information could be privacy sensitive because it might reveal usage
characteristics and habits of the user of an application.

Of course, any communication over a network reveals usage
characteristics, because all packets, as well as their timing and
size, are part of the network-visible wire image [RFC8546]. However,
the selection of a protocol and its configuration also impacts which
information is visible, potentially in clear text, and which other
entities can access it. How Transport Services systems ought to
choose protocols -- depending on the security properties required --
is out of scope for this specification, as it is limited to transport
protocols. The choice of a security protocol can be informed by the
survey provided in [RFC8922].

In most cases, information provided for protocol and path selection
does not directly translate to information that can be observed by
network devices on the path. However, there might be specific
configuration information that is intended for path exposure, e.g., a
Diffserv codepoint setting that is either provided directly by the
application or indirectly configured for a traffic profile.

Applications should be aware that a single communication attempt can
lead to more than one connection establishment procedure. For
example, this is the case when:

* the Transport Services system also executes name resolution,

* support mechanisms such as TURN or ICE are used to establish
connectivity if protocols or paths are raced or if a path fails
and fallback or re-establishment is supported in the Transport
Services system.

Applications should take special care when using 0-RTT session
resumption (see Section 6.2.5), as early data sent across multiple
paths during connection establishment could reveal information that
can be used to correlate endpoints on these paths.

Applications should also take care to not assume that all data
received using the Transport Services API is always complete or well-
formed. Specifically, Messages that are received partially (see
Section 9.3.2.2 )could be a source of truncation attacks if
applications do not distinguish between partial Messages and complete
Messages.

The Transport Services API explicitly does not require the
application to resolve names, though there is a trade-off between
early and late binding of addresses to names. Early binding allows
the Transport Services Implementation to reduce Connection setup
latency. This is at the cost of potentially limited scope for
alternate path discovery during Connection establishment as well as
potential additional information leakage about application interest
when used with a resolution method (such as DNS without TLS) that
does not protect query confidentiality. Names used with the
Transport Services API SHOULD be FQDNs; not providing an FQDN will
result in the Transport Services Implementation needing to use DNS
search domains for name resolution, which might lead to inconsistent
or unpredictable behavior.

These communication activities are not different from what is used at
the time of writing. However, the goal of a Transport Services
system is to support such mechanisms as a generic service within the
transport layer. This enables applications to more dynamically
benefit from innovations and new protocols in the transport, although
it reduces transparency of the underlying communication actions to
the application itself. The Transport Services API is designed such
that protocol and path selection can be limited to a small and
controlled set, if the application requires this, or to implement a
security policy can be limited to a small and controlled set if required by the application to perform a function
or to provide security. Further, introspection on the properties of
Connection objects allows an application to determine which
protocol(s) and path(s) are in use. A Transport Services system
SHOULD provide a facility logging the communication events of each
Connection.

14. References

14.1. Normative References

[ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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, November 2024,
<https://www.rfc-editor.org/info/RFC9621>.

14.2. Informative References

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.

[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.

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

[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
RFC 5482, DOI 10.17487/RFC5482, March 2009,
<https://www.rfc-editor.org/info/rfc5482>.

[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.

[RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use Cases and Requirements", RFC 7478,
DOI 10.17487/RFC7478, March 2015,
<https://www.rfc-editor.org/info/rfc7478>.

[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>.

[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>.

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.

[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>.

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

[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>.

[RFC8293] Ghanwani, A., Dunbar, L., McBride, M., Bannai, V., and R.
Krishnan, "A Framework for Multicast in Network
Virtualization over Layer 3", RFC 8293,
DOI 10.17487/RFC8293, January 2018,
<https://www.rfc-editor.org/info/rfc8293>.

[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>.

[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>.

[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>.

[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>.

[RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.

[RFC8622] Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
June 2019, <https://www.rfc-editor.org/info/rfc8622>.

[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>.

[RFC8699] Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion
Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699,
January 2020, <https://www.rfc-editor.org/info/rfc8699>.

[RFC8801] Pfister, P., Vyncke, É., Pauly, T., Schinazi, D., and W.
Shao, "Discovering Provisioning Domain Names and Data",
RFC 8801, DOI 10.17487/RFC8801, July 2020,
<https://www.rfc-editor.org/info/rfc8801>.

[RFC8838] Ivov, E., Uberti, J., and P. Saint-Andre, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol", RFC 8838,
DOI 10.17487/RFC8838, January 2021,
<https://www.rfc-editor.org/info/rfc8838>.

[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.

[RFC8922] Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
Wood, "A Survey of the Interaction between Security
Protocols and Transport Services", RFC 8922,
DOI 10.17487/RFC8922, October 2020,
<https://www.rfc-editor.org/info/rfc8922>.

[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>.

[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.

[RFC9218] Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,
<https://www.rfc-editor.org/info/rfc9218>.

[RFC9329] Pauly, T. and V. Smyslov, "TCP Encapsulation of Internet
Key Exchange Protocol (IKE) and IPsec Packets", RFC 9329,
DOI 10.17487/RFC9329, November 2022,
<https://www.rfc-editor.org/info/rfc9329>.

[RFC9623] Brunstrom, A., Ed., Pauly, T., Ed., Enghardt, R., Tiesel,
P. S., and M. Welzl, "Implementing Interfaces to Transport
Services", RFC 9623, DOI 10.17487/RFC9623, November 2024,
<https://www.rfc-editor.org/info/rfc9623>.

[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. Implementation Mapping

The way the concepts from this abstract API map to concrete APIs in a
given language on a given platform largely depends on the features
and norms of the language and the platform. For instance, actions Actions could be
implemented as either functions or method calls and events calls. For instance,
actions could be implemented via event queues, handler functions or
classes, communicating sequential processes, or other asynchronous
calling conventions.

A.1. Types

The basic types mentioned in Section 1.1 typically have natural
correspondences in practical programming languages, perhaps
constrained by implementation-specific limitations. For example:

* Typically, an Integer can be represented in C by an int or long;
this is subject to the underlying platform's ranges for each.

* In C, a Tuple may be represented as a struct with one member for
each of the value types in the ordered grouping. However, in
Python, a Tuple may be represented as a tuple, which is a sequence
of dynamically typed elements.

* A Set may be represented as a std::set in C++ or as a set in
Python. In C, it may be represented as an array or as a higher-
level data structure with appropriate accessors defined.

The objects described in Section 1.1 can also be represented in
different ways, depending on which programming language is used.
Objects like Preconnections, Connections, and Listeners can be long-
lived and benefit from using object-oriented constructs. Note that,
in C, these objects may need to provide a way to release or free
their underlying memory when the application is done using them. For
example, since a Preconnection can be used to initiate multiple
Connections, it is the responsibility of the application to clean up
the Preconnection memory if necessary.

A.2. Events and Errors

This specification treats events and errors similarly. Errors, just
as any other events, may occur asynchronously in network
applications. However, implementations of this API may report errors
synchronously. This is done according to the error-handling idioms
of the implementation platform, where they can be immediately
detected. An example of this being generating is to generate an exception when
attempting to initiate a Connection with inconsistent Transport
Properties. An error can provide an optional reason to the
application with further details about why the error occurred.

A.3. Time Duration

Time duration types are implementation specific. For instance, it
could be a number of seconds, a number of milliseconds, or a struct
timeval in C; in C++, it could be a user-defined Duration class.

Appendix B. Convenience Functions

B.1. Adding Preference Properties

TransportProperties will frequently need to set Selection Properties
of type Preference; therefore, implementations can provide special
actions for adding each preference level, i.e.,
TransportProperties.Set(some_property, avoid) is equivalent to
TransportProperties.Avoid(some_property):

TransportProperties.Require(property)
TransportProperties.Prefer(property)
TransportProperties.NoPreference(property)
TransportProperties.Avoid(property)
TransportProperties.Prohibit(property)

B.2. Transport Property Profiles

To ease the use of the Transport Services API, implementations can
provide a mechanism to create Transport Property objects (see
Section 6.2) that are preconfigured with frequently used sets of
properties; the following subsections list those that are in common
use in applications at the time of writing.

B.2.1. reliable-inorder-stream

This profile provides reliable, in-order transport service with
congestion control. TCP is an example of a protocol that provides
this service. It should consist of the following properties:

Table 2: reliable-inorder-stream
Preferences

B.2.2. reliable-message

This profile provides message-preserving, reliable, in-order
transport service with congestion control. SCTP is an example of a
protocol that provides this service. It should consist of the
following properties:

Table 3: reliable-message
Preferences

B.2.3. unreliable-datagram

This profile provides a datagram transport service without any
reliability guarantee. An example of a protocol that provides this
service is UDP. It consists of the following properties:

Table 4: unreliable-datagram Preferences

Applications that choose this Transport Property Profile would avoid
the additional latency that could be introduced by retransmission or
reordering in a transport protocol.

Applications that choose this Transport Property Profile to reduce
latency should also consider setting an appropriate capacity profile
Property (see Section 8.1.6) and might benefit from controlling
checksum coverage (see Sections 6.2.7 and 6.2.8).

Appendix C. Relationship to the Minimal Set of Transport Services for
End Systems

[RFC8923] identifies a minimal set of Transport Services that end
systems should offer. These services make all non-security-related
transport features of TCP, Multipath TCP (MPTCP), UDP, UDP-Lite,
SCTP, and Low Extra Delay Background Transport (LEDBAT) available
that:

1. require interaction with the application and

2. do not get in the way of a possible implementation over TCP (or,
with limitations, UDP).

The following text explains how this minimal set is reflected in the
present API. For brevity, it is based on the list in Section 4.1 of
[RFC8923] and updated according to the discussion in Section 5 of
[RFC8923]. The present API covers all elements of this section.
This list is a subset of the transport features in Appendix A of
[RFC8923], which refers to the primitives in "pass 2" (see 2". See Section 4
of [RFC8303]) for 1) further details on the implementation with TCP,
MPTCP, UDP, UDP-Lite, SCTP, and LEDBAT. This facilitates LEDBAT and 2) how to facilitate
finding the specifications for implementing the services listed below
with these protocols.

* Connect: Initiate action (Section 7.1).

* Listen: Listen action (Section 7.2).

* Specify number of attempts and/or timeout for the first
establishment Message: timeout parameter of Initiate (Section 7.1)
or InitiateWithSend action (Section 9.2.5).

* Disable MPTCP: multipath property (Section 6.2.14).

* Hand over a Message to reliably transfer (possibly multiple times)
before connection establishment: InitiateWithSend action
(Section 9.2.5).

* Change timeout for aborting connection (using retransmit limit or
time value): connTimeout property, using a time value
(Section 8.1.3).

* Timeout event when data could not be delivered for too long:
ConnectionError event (Section 10).

* Suggest timeout to the peer: See "TCP-Specific Properties: User
Timeout Option (UTO)" (Section 8.2).

* Notification of ICMP error message arrival: softErrorNotify
(Section 6.2.17) and SoftError event (Section 8.3.1).

* Choose a scheduler to operate between streams of an association:
connScheduler property (Section 8.1.5).

* Configure priority or weight for a scheduler: connPriority
property (Section 8.1.2).

* "Specify checksum coverage used by the sender" and "Disable
checksum when sending": msgChecksumLen property (Section 9.1.3.6)
and fullChecksumSend property (Section 6.2.7).

* "Specify minimum checksum coverage required by receiver" and
"Disable checksum requirement when receiving": recvChecksumLen
property (Section 8.1.1) and fullChecksumRecv property
(Section 6.2.8).

* "Specify Specify DF field": field: noFragmentation property (Section 9.1.3.9).

* Get maximum transport-message size that may be sent using a non-
fragmented IP packet from the configured interface:
singularTransmissionMsgMaxLen property (Section 8.1.11.4).

* Get maximum transport-message size that may be received from the
configured interface: recvMsgMaxLen property (Section 8.1.11.6).

* Obtain ECN field: This is a read-only Message Property of the
MessageContext object (see "Property Specific to UDP and UDP-Lite:
ECN" (Section 9.3.3.1)).

* "Specify DSCP field", "Disable Nagle algorithm", and "Enable and
configure a Low Extra Delay Background Transfer": as suggested in
Section 5.5 of [RFC8923], these transport features are
collectively offered via the connCapacityProfile property
(Section 8.1.6). Per-Message control ("Request not to bundle
messages") is offered via the msgCapacityProfile property
(Section 9.1.3.8).

* Close after reliably delivering all remaining data, causing an
event informing the application on the other side: this is offered
by the Close action with slightly changed semantics in line with
the discussion in Section 5.2 of [RFC8923] (see also Section 10).

* "Abort without delivering remaining data, causing an event
informing the application on the other side" and "Abort without
delivering remaining data, not causing an event informing the
application on the other side": this is these are offered by the Abort
action without promising that this is these are signaled to the other
side. If it is, they are, a ConnectionError event will be invoked at the
peer (Section 10).

* "Reliably transfer data, with congestion control", "Reliably
transfer a message, with congestion control", and "Unreliably
transfer a message": data is transferred via the Send action
(Section 9.2). Reliability is controlled via the reliability
(Section 6.2.1) property and the msgReliable Message Property
(Section 9.1.3.7). Transmitting data as a Message or without
delimiters is controlled via Message Framers (Section 9.1.2). The
choice of congestion control is provided via the congestionControl
property (Section 6.2.9).

* Configurable Message Reliability: the msgLifetime Message Property
implements a time-based way to configure message reliability
(Section 9.1.3.1).

* "Ordered message delivery (potentially slower than unordered)" and
"Unordered message delivery (potentially faster than ordered)":
these two transport features are controlled via the Message
Property msgOrdered (Section 9.1.3.3).

* Request not to delay the acknowledgement (SACK) of a message:
should the protocol support it, this is one of the transport
features the Transport Services system can apply when an
application uses the connCapacityProfile Property (Section 8.1.6)
or the msgCapacityProfile Message Property (Section 9.1.3.8) with
value Low Latency/Interactive.

* Receive data (with no message delimiting): Receive action
(Section 9.3.1) and Received event (Section 9.3.2.1).

* Receive a message: Receive action (Section 9.3.1) and Received
event (Section 9.3.2.1) using Message Framers (Section 9.1.2).

* Information about partial message arrival: Receive action
(Section 9.3.1) and ReceivedPartial event (Section 9.3.2.2).

* Notification of send failures: Expired event (Section 9.2.2.2) and
SendError event (Section 9.2.2.3).

* Notification that the stack has no more user data to send:
applications can obtain this information via the Sent event
(Section 9.2.2.1).

* Notification to a receiver that a partial message delivery has
been aborted: ReceiveError event (Section 9.3.2.3).

* Notification of Excessive Retransmissions (early warning below
abortion threshold): SoftError event (Section 8.3.1).

Acknowledgements

This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreements No. 644334
(NEAT) and No. 688421 (MAMI).

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 Stuart Cheshire, Josh Graessley, David Schinazi, and Eric
Kinnear for their implementation and design efforts, including Happy
Eyeballs, that heavily influenced this work. Thanks to Laurent Chuat
and Jason Lee for initial work on the Post Sockets interface, from
which this work has evolved. Thanks to Maximilian Franke for asking
good questions based on implementation experience and for
contributing text, e.g., on multicast.

Authors' Addresses

Brian Trammell (editor)
Google Switzerland GmbH
Gustav-Gull-Platz 1
CH-8004 Zurich
Switzerland
Email: ietf@trammell.ch

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

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

Godred Fairhurst
University of Aberdeen
Fraser Noble Building
Aberdeen, AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
URI: https://erg.abdn.ac.uk/

Mirja Kühlewind
Ericsson
Ericsson-Allee 1
Herzogenrath
Germany
Email: mirja.kuehlewind@ericsson.com

Colin S. Perkins
University of Glasgow
School of Computing Science
Glasgow
G12 8QQ
United Kingdom
Email: csp@csperkins.org

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

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