wdiff rfc9621.original rfc9621.txt

TAPS Working Group

Internet Engineering Task Force (IETF) T. Pauly, Ed.
Internet-Draft
Request for Comments: 9621 Apple Inc.
Intended status:
Category: Standards Track B. Trammell, Ed.
Expires: 12 May 2024
ISSN: 2070-1721 Google Switzerland GmbH
A. Brunstrom
Karlstad University
G. Fairhurst
University of Aberdeen
C. S. Perkins
University of Glasgow
9 November 2023
December 2024

Architecture and Requirements for Transport Services
draft-ietf-taps-arch-19

Abstract

This document describes an architecture for exposing transport
protocol features to applications for network communication. This
system that exposes transport
protocol features to applications for network communication. The
Transport Services Application Programming Interface (API) is based
on an asynchronous, event-driven interaction pattern. This API uses
messages for representing data transfer to applications, applications and describes
how a Transport Services Implementation can use multiple IP
addresses, multiple protocols, and multiple paths, paths and can provide
multiple application streams. This document provides the
architecture and requirements. It defines common terminology and
concepts to be used in definitions of a Transport Service Services API and a
Transport Services Implementation.

Status of This Memo

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

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum 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 six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 12 May 2024.
https://www.rfc-editor.org/info/rfc9621.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Specification of Requirements . . . . . . . . . . . . . . 5
1.4. Glossary of Key Terms . . . . . . . . . . . . . . . . . . 5
2. API Model . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Event-Driven API . . . . . . . . . . . . . . . . . . . . 10
2.2. Data Transfer Using Messages . . . . . . . . . . . . . . 11
2.3. Flexible Implementation . . . . . . . . . . . . . . . . . 12
2.4. Coexistence . . . . . . . . . . . . . . . . . . . . . . . 13
3. API and Implementation Requirements . . . . . . . . . . . . . 13
3.1. Provide Common APIs for Common Features . . . . . . . . . 14
3.2. Allow Access to Specialized Features . . . . . . . . . . 15
3.3. Select Between Equivalent Protocol Stacks . . . . . . . . 16
3.4. Maintain Interoperability . . . . . . . . . . . . . . . . 17
3.5. Support Monitoring . . . . . . . . . . . . . . . . . . . 17
4. Transport Services Architecture and Concepts . . . . . . . . 18
4.1. Transport Services API Concepts . . . . . . . . . . . . . 20
4.1.1. Endpoint Objects . . . . . . . . . . . . . . . . . . 22
4.1.2. Connections and Related Objects . . . . . . . . . . . 22
4.1.3. Pre-establishment . . . . . . . . . . . . . . . . . . 24 Preestablishment
4.1.4. Establishment Actions . . . . . . . . . . . . . . . . 24
4.1.5. Data Transfer Objects and Actions . . . . . . . . . . 25
4.1.6. Event Handling . . . . . . . . . . . . . . . . . . . 26
4.1.7. Termination Actions . . . . . . . . . . . . . . . . . 27
4.1.8. Connection Groups . . . . . . . . . . . . . . . . . . 28
4.2. Transport Services Implementation . . . . . . . . . . . . 28
4.2.1. Candidate Gathering . . . . . . . . . . . . . . . . . 30
4.2.2. Candidate Racing . . . . . . . . . . . . . . . . . . 30
4.2.3. Separating Connection Contexts . . . . . . . . . . . 30
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
6. Security and Privacy Considerations . . . . . . . . . . . . . 31
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
8.1.
7.1. Normative References . . . . . . . . . . . . . . . . . . 32
8.2.
7.2. Informative References . . . . . . . . . . . . . . . . . 33
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35

1. Introduction

Many application programming interfaces Application Programming Interfaces (APIs) to provide transport
interfaces to networks have been deployed, perhaps the most widely
known and imitated being the BSD Socket [POSIX] interface (Socket
API). API) [POSIX].
The naming of objects and functions across these APIs is not
consistent and varies varies, depending on the protocol being used. For
example, the concept of sending and receiving streams of data is conceptually the
same for both an unencrypted Transmission Control Protocol (TCP)
stream and operating on an encrypted Transport Layer Security (TLS)
[RFC8446]
stream [RFC8446] over TCP, but applications cannot use the same
socket send() and recv() calls on top of both kinds of connections.
Similarly, terminology for the implementation of transport protocols
varies based on the context of the protocols themselves: terms such
as "flow", "stream", "message", and "connection" can take on many
different meanings. This variety can lead to confusion when trying
to understand the similarities and differences between protocols, protocols and
how applications can use them effectively.

The goal of the Transport Services System architecture is to provide
a flexible and reusable system with a common interface for transport
protocols. An application uses the Transport Services System through
an abstract Connection (we use capitalization to distinguish these
from the underlying connections of, e.g., for example, TCP). This provides
flexible connection establishment allowing an application to request
or require a set of properties.

As applications adopt this interface, they will benefit from a wide
set of transport features that can evolve over time, time and will ensure
that the system providing the interface 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.

This architecture can also support applications by offering racing
mechanisms (attempting multiple IP addresses, protocols, or network
paths in parallel), which otherwise need to be implemented in each
application separately (see Section 4.2.2). Racing selects one or
more candidates candidates, each with equivalent protocol stacks Protocol Stacks that are used
to identify an optimal combination of a transport protocol instance
such as TCP, UDP, or another transport, together with configuration
of parameters and interfaces. A Connection represents an object
that, once established, can be used to send and receive messages. A
Connection can also be created from another Connection, by cloning,
and then forms a part of a Connection Group whose Connections share
properties.

This document was developed in parallel with the specification of the
Transport Services API [I-D.ietf-taps-interface] [RFC9622] and implementation guidelines [I-D.ietf-taps-impl].
[RFC9623]. Although following the Transport Services architecture Architecture
does not require all APIs and implementations to be identical, a
common minimal set of features represented in a consistent fashion
will enable applications to be easily ported from one implementation
of the Transport Services System to another.

1.1. Background

The architecture of the Transport Services System is based on the
survey of services provided by IETF transport protocols and
congestion control mechanisms [RFC8095], [RFC8095] and the distilled minimal set
of the features offered by transport protocols [RFC8923]. These
documents identified common features and patterns across all
transport protocols developed thus far in the IETF.

Since transport security is an increasingly relevant aspect of using
transport protocols on the Internet, this document also considers the
impact of transport security protocols on the feature-set feature set exposed by
Transport Services [RFC8922].

One of the key insights to come from identifying the minimal set of
features provided by transport protocols [RFC8923] was that features
either (1) require application interaction and guidance (referred to
in that document as Functional or Optimizing Features), Features) or else (2) can be
handled automatically by an implementation of the Transport Services
System (referred to as Automatable Features). Among the identified
Functional and Optimizing Features, some are common across all or
nearly all transport protocols, while others present features that,
if specified, would only be useful with a subset of protocols, but
would not harm the functionality of other protocols. For example,
some protocols can deliver messages faster more quickly for applications
that do not require messages to arrive in the order in which they
were sent. This functionality needs to be explicitly allowed by the
application, since reordering messages would be undesirable in many
cases.

1.2. Overview

This document describes

The following sections describe the Transport Services System in three
sections: System:

* Section 2 describes how the Transport Services API model differs
from that of traditional socket-based APIs. Specifically, it offers
asynchronous event-driven interaction, the use of messages for
data transfer, and the flexibility to use different transport
protocols and paths without requiring major changes to the
application.

* Section 3 explains the fundamental requirements for a Transport
Services System. These principles are intended to make sure that
transport protocols can continue to be enhanced and evolve without
requiring significant changes by application developers.

* Section 4 presents the Transport Services Implementation and
defines the concepts that are used by the API
[I-D.ietf-taps-interface] [RFC9622] and
described in the implementation guidelines [I-D.ietf-taps-impl]. [RFC9623]. This
introduces the Preconnection, which allows applications to
configure Connection Properties.

1.3. 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.

1.4. Glossary of Key Terms

This subsection provides a glossary of key terms related to the
Transport Services architecture. Architecture. It provides a short description of
key terms that are later defined later in this document.

Application: An entity that uses the transport layer for end-to-
end end-to-end
delivery of data across the network [RFC8095].

Cached State: The state and history that the Transport Services
Implementation keeps for each set of the associated Endpoints that
have been used previously.

Candidate Path: One path that is available to an application and
conforms to the Selection Properties and System Policy during
racing.

Candidate Protocol Stack: One Protocol Stack that can be used by an
application for a Connection during racing.

Client: The peer responsible for initiating a Connection.

Clone: A Connection that was created from another Connection, Connection and
that forms a part of a Connection Group.

Connection: Shared state of two or more Endpoints that persists
across Messages that are transmitted and received between these
Endpoints [RFC8303]. When this document (and and other Transport
Services documents) documents use the capitalized "Connection" term, it
refers to a Connection object that is being offered by the
Transport Services system, as opposed to more generic uses of the
word "connection".

Connection Context: A set of stored properties across Connections,
such as cached protocol state, cached path state, and heuristics,
which can include one or more Connection Groups.

Connection Group: A set of Connections that share properties and
caches.

Connection Property: A Transport Property that controls per-
Connection behavior of a Transport Services implementation.

* Implementation.

Endpoint: An entity that communicates with one or more other
endpoints using a transport protocol.

Endpoint Identifier: An identifier that specifies one side of a
Connection (local or remote), such as a hostname or URL.

Equivalent Protocol Stacks: Protocol Stacks that can be safely
swapped or raced in parallel during establishment of a Connection.

Event: A primitive that is invoked by an Endpoint [RFC8303].

Framer: A data translation layer that can be added to a Connection
to define how application-layer Messages are transmitted over a
Protocol Stack.

Local Endpoint: The local Endpoint.

Local Endpoint Identifier: A representation of the application's
identifier for itself that it uses for a Connection.

Message: A unit of data that can be transferred between two
Endpoints over a Connection.

Message Property: A property that can be used to specify details
about Message transmission, transmission or obtain details about the
transmission after receiving a Message.

Parameter: A value passed between an application and a transport
protocol by a primitive [RFC8303].

Path: A representation of an available set of properties that a
Local Endpoint can use to communicate with a Remote Endpoint.

Peer: An Endpoint application party to a Connection.

Preconnection: an An object that represents a Connection that has not
yet been established.

Preference: A preference to prohibit, avoid, ignore, prefer, for prohibiting, avoiding, ignoring,
preferring, or
require requiring a specific Transport Feature.

* transport feature.

Primitive: A function call that is used to locally communicate
between an application and an Endpoint, which is related to one or
more Transport Features transport features [RFC8303].

Protocol Instance: A single instance of one protocol, including any
state necessary to establish connectivity or send and receive
Messages.

Protocol Stack: A set of Protocol Instances that are used together
to establish connectivity or send and receive Messages.

Racing: The attempt to select between multiple Protocol Stacks based
on the Selection and Connection Properties communicated by the
application, along with any Security Parameters.

Remote Endpoint: The peer that a local Endpoint can communicate with
when a Connection is established.

Remote Endpoint Identifier: A representation of the application's
identifier for a peer that can participate in establishing a
Connection.

Rendezvous: The action of establishing a peer-to-peer Connection
with a Remote Endpoint.

Security Parameters: Parameters that define an application's
requirements for authentication and encryption on a Connection.

Selection Property: A Transport Property that can be set to
influence the selection of paths between the Local and Remote
Endpoints.

Server: The peer responsible for responding to a Connection
initiation.

Socket: The combination of a destination IP address and a
destination port number [RFC8303].

System Policy: The input from an operating system or other global
preferences that can constrain or influence how an implementation
will gather Candidate Paths and Candidate Protocol Stacks and race
the candidates during establishment of a Connection.

* Selection Property: A Transport Property that can be set to
influence the selection of paths between the Local and Remote
Endpoints.

Transport Feature: A specific end-to-end feature that the transport
layer provides to an application.

Transport Property: A property that expresses requirements,
prohibitions of a transport protocol and preferences the
services it provides [RFC8095].

Transport Service: A set of transport features, without an
association to not associated with
any given framing protocol, that provides a complete service to an
application.

Transport Services API: The abstract interface [RFC9622] to a
Transport Services Implementation [RFC9623].

Transport Services Implementation: This consists of all All objects and protocol
instances used internally to a system or library to implement the
functionality needed to provide a transport service across a
network, as required by the abstract interface.

Transport Services System: The Transport Services Implementation and
the Transport Services API.

2. API Model

The traditional model of using sockets can be represented as follows (see figure
Figure 1):

* Applications create connections and transfer data using the Socket
API.

* The Socket API provides the interface to the implementations of
TCP and UDP (typically implemented in the system's kernel).

* TCP and UDP in the kernel send and receive data over the available
network-layer interfaces.

* Sockets are bound directly to transport-layer and network-layer
addresses, obtained via a separate resolution step, usually
performed by a system-provided DNS stub resolver.

+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
| | |
+------------+ +------------+ +--------------+
| DNS stub Stub | | Stream API | | Datagram API |
| resolver Resolver | +------------+ +--------------+
+------------+ | |
+---------------------------------+
| TCP UDP |
| Kernel Networking Stack |
+---------------------------------+
|
+-----------------------------------------------------+
| Network Layer Network-Layer Interface |
+-----------------------------------------------------+

Figure 1: Socket API Model

The architecture of the Transport Services System is an evolution of
this general model of interaction. It both modernizes the API
presented to applications by the transport layer and enriches the
capabilities of the Transport Services Implementation below this API.

+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services API |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services Implementation |
| (Using: DNS, UDP, TCP, SCTP, DCCP, TLS, QUIC, etc) |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Network Layer Interface |
+-----------------------------------------------------+

Figure 2: Transport Services API Model

The Transport Services API [I-D.ietf-taps-interface] [RFC9622] defines the interface for an
application to create Connections and transfer data. It combines
interfaces for multiple interaction patterns into a unified whole
(see figure Figure 2). This offers generic functions and also the protocol-specific protocol-
specific mappings for TCP, UDP, UDP-Lite, and other protocol layers.
These mapping mappings are extensible. Future documents could define similar
mappings for new layers and for other transport protocols, such as
QUIC [RFC9000].

+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services API |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services Implementation |
| (Using DNS, UDP, TCP, SCTP, DCCP, TLS, QUIC, etc.) |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Network-Layer Interface |
+-----------------------------------------------------+

Figure 2: Transport Services API Model

By combining name resolution with connection establishment and data
transfer in a single API, it allows for more flexible implementations
to provide path and transport protocol agility on the application's
behalf.

The Transport Services Implementation [I-D.ietf-taps-impl] [RFC9623] is the component of
the Transport Services System that implements the
transport layer transport-layer
protocols and other functions needed to send and receive data. It is
responsible for mapping the API to a specific available transport
Protocol Stack and managing the available network interfaces and
paths.

There are key differences between the architecture of the Transport
Services System and the architecture of the Socket API: the API. The API of
the Transport Services System System:

* is asynchronous and event-driven; it

* uses messages for representing data transfer to applications; and it

* describes how a Transport Services Implementation can resolve
Endpoint Identifiers to use multiple IP addresses, multiple
protocols, and multiple paths, paths and to provide multiple application
streams.

2.1. Event-Driven API

Originally, the Socket API presented a blocking interface for
establishing connections and transferring data. However, most modern
applications interact with the network asynchronously. Emulation of
an asynchronous interface using the Socket API can use a try-and-fail
model: If if the application wants to read, read but data has not yet been
received from the peer, the call to read will fail. The application
then waits and can try again later.

In contrast to the Socket API, all interactions using the Transport
Services API are expected to be asynchronous. The API is defined
around an event-driven model (see Section 4.1.6), which models this
asynchronous interaction. Other forms of asynchronous communication
could also be available to applications, depending on the platform
implementing the interface.

For example, when an application that uses the Transport Services API
wants to receive data, it issues an asynchronous call to receive new
data from the Connection. When delivered data becomes available,
this data is delivered to the application using asynchronous events
that contain the data. Error handling is also asynchronous,
resulting in asynchronous error events.

This API also delivers events regarding the lifetime of a connection
and changes in the available network links, which were not previously
made explicit in the Socket API.

Using asynchronous events allows for a more natural interaction model
when establishing connections and transferring data. Events in time
more closely reflect the nature of interactions over networks, as
opposed to how the Socket API represents network resources as file
system objects that may be temporarily unavailable.

Separate from events, callbacks are also provided for asynchronous
interactions with the Transport Services API that are not directly
related to events on the network or network interfaces.

2.2. Data Transfer Using Messages

The Socket API provides a message interface for datagram protocols
like UDP, UDP but provides an unstructured stream abstraction for TCP.
While TCP has the ability to send and receive data as a byte-stream,
most applications need to interpret structure within this byte-
stream. For example, HTTP/1.1 uses character delimiters to segment
messages over a byte-stream [RFC9112]; TLS record headers carry a
version, content type, and length [RFC8446]; and HTTP/2 uses frames
to segment its headers and bodies [RFC9113].

The Transport Services API represents data as messages, so that it
more closely matches the way applications use the network. A
message-based abstraction provides many benefits, such as:

* providing additional information to the Protocol Stack;

* the ability to associate deadlines with messages, for applications
that care about timing;

* the ability to control reliability, which messages to retransmit
when there is packet loss, and how best to make use of the data
that arrived;

* the ability to automatically assign messages and connections to
underlying transport connections to utilize multi-streaming multistreaming and
pooled connections.

Allowing applications to interact with messages is backwards- backward-
compatible with existing protocols and APIs because it does not
change the wire format of any protocol. Instead, it provides the
Protocol Stack with additional information to allow it to make better
use of modern transport services, Transport Services, while simplifying the application's
role in parsing data. For protocols that inherently use a streaming
abstraction, framers (Section 4.1.5) bridge the gap between the two
abstractions.

2.3. Flexible Implementation

The Socket API for protocols like TCP is generally limited to
connecting to a single address over a single interface (IP source
address). It also presents a single stream to the application.
Software layers built upon this API often propagate this limitation
of a single-address single-stream model. The Transport Services
architecture
Architecture is designed: designed to:

* to handle multiple candidate endpoints, protocols, and paths;

* to support candidate protocol racing to select the most optimal stack
in each situation;

* to support multipath and multistreaming protocols;

* to provide state caching and application control over it.

A Transport Services Implementation is intended to be flexible at
connection establishment time, considering many different options and
trying to select the most optimal combinations by racing them and
measuring the results (see Section Sections 4.2.1 and Section 4.2.2). This requires
applications to specify identifiers for the Local and Remote Endpoint
that are higher-level at a higher level than IP addresses, such as a hostname or URL, which
URL. These identifiers are used by a Transport Services
Implementation for resolution, path selection, and racing. An
implementation can further implement fallback mechanisms if
connection establishment of for one protocol fails or performance is detected
determined to be unsatisfactory.

Information used in connection establishment (e.g. (e.g., cryptographic
resumption tokens, information about usability of certain protocols
on the path, results of racing in previous connections) are is cached in
the Transport Services Implementation. Applications have control
over whether this information is used for a specific establishment,
in order to allow tradeoffs trade-offs between efficiency and linkability.

Flexibility after connection establishment is also important.
Transport protocols that can migrate between multiple network-layer
interfaces need to be able to process and react to interface changes.
Protocols that support multiple application-layer streams need to
support initiating and receiving new streams using existing
connections.

2.4. Coexistence

While the architecture of the Transport Services System is designed
as an enhanced replacement for the Socket API, it need not replace it
entirely on a system or platform; indeed, coexistence has been
recommended for incremental deployability [RFC8170]. The
architecture is therefore designed such that it can run alongside
(or, indeed, on top of) an existing Socket API implementation; only
applications built to on the Transport Services API are managed by the
system's Transport Services Implementation.

3. API and Implementation Requirements

One goal of the architecture is to redefine the interface between
applications and transports in a way that allows the transport layer
to evolve and improve without fundamentally changing the contract
with the application. This requires a careful consideration of how to
expose the capabilities of protocols. The architecture also
encompasses system policies that can influence and inform how
transport protocols use a network path or interface.

There are several ways the Transport Services System can offer
flexibility to an application: it can application. It can:

* provide access to transport protocols and protocol features; it can

* use these protocols across multiple paths that could have
different performance and functional characteristics; and it can

* communicate with different remote systems to optimize performance,
robustness to failure, or some other metric.

Beyond these, if the Transport Services API remains the same over
time, new protocols and features can be added to the Transport
Services Implementation without requiring changes in applications for
adoption. Similarly, this can provide a common basis for utilizing
information about a network path or interface, enabling evolution
below the transport layer.

The normative requirements described in this section allow Transport
Services APIs and the Transport Services Implementation to provide
this functionality without causing incompatibility or introducing
security vulnerabilities.

3.1. Provide Common APIs for Common Features

Any functionality that is common across multiple transport protocols
SHOULD be made accessible through a unified set of calls using the
Transport Services API. As a baseline, any Transport Services API
SHOULD allow access to the minimal set of features offered by
transport protocols [RFC8923]. If that minimal set is updated or
expanded in the future, the Transport Services API ought to be
extended to match.

An application can specify constraints and preferences for the
protocols, features, and network interfaces it will use via
Properties. Properties are used by an application to declare its
preferences for how the transport service should operate at each
stage in the lifetime of a connection. Transport Properties are
subdivided into the following:

* Selection Properties, which specify which paths and Protocol
Stacks can be used and are preferred by the application;

* Connection Properties, which inform decisions made during
connection establishment and fine-tune the established connection;
and

* Message Properties, which can be set on individual Messages.

It is RECOMMENDED that the Transport Services API offers offer properties
that are common to multiple transport protocols. This enables a
Transport Services System to appropriately select between protocols
that offer equivalent features. Similarly, it is RECOMMENDED that
the Properties offered by the Transport Services API are be applicable to
a variety of network layer network-layer interfaces and paths, which permits to permit racing of
different network paths without affecting the applications using the
API. Each is expected to have a default value.

It is RECOMMENDED that the default values for Properties are be selected
to ensure correctness for the widest set of applications, while
providing the widest set of options for selection. For example,
since both applications that require reliability and those that do
not require reliability can function correctly when a protocol
provides reliability, reliability ought to be enabled by default. As
another example, the default value for a Property regarding the
selection of network interfaces ought to permit as many interfaces as
possible.

Applications using the Transport Services API need to be designed to
be robust to the automated selection provided by the Transport
Services System. This automated selection is constrained by the
properties and preferences expressed by the application and requires
applications to explicitly set properties that define any necessary
constraints on protocol, path, and interface selection.

3.2. Allow Access to Specialized Features

There are applications that will need to control fine-grained details
of transport protocols to optimize their behavior and ensure
compatibility with remote systems. It is therefore RECOMMENDED that
the Transport Services API and the Transport Services Implementation
permit more specialized protocol features to be used.

Some specialized feature features could be needed by an application only when
using a specific protocol, protocol and not when using others. For example, if
an application is using TCP, it could require control over the User
Timeout Option for TCP [RFC5482]; these options [RFC5482]. Such features would not take
effect for other transport protocols. In such cases, the API ought
to expose the features in such a way that they take effect when a
particular protocol is selected, selected but do not imply that only that
protocol could be used. For example, if the API allows an
application to specify a preference to use for using the User Timeout
Option, communication would not fail when a protocol such as UDP is
selected.

Other specialized features, however, can also be strictly required by
an application and thus further constrain the set of protocols that
can be used. For example, if an application requires support for
automatic handover or failover for a connection, only Protocol Stacks
that provide this feature are eligible to be used, e.g., Protocol
Stacks that include a multipath protocol or a protocol that supports
connection migration. A Transport Services API needs to allow
applications to define such requirements and constrain the options
available to a Transport Services Implementation. Since such options
are not part of the core/common features, it will generally be simple
for an application to modify its set of constraints and change the
set of allowable protocol features without changing the core
implementation.

To control these specialized features, the application can declare
its preference – preference: whether the presence of a specific feature is
prohibited, should be avoided, can be ignored, is preferred, or is
required in the pre-establishment preestablishment phase. An implementation of a
Transport Services API would honor this preference and allow the
application to query the availability of each specialized feature
after a successful establishment.

3.3. Select Between Equivalent Protocol Stacks

A Transport Services Implementation can attempt to use, and select between
between, multiple Protocol Stacks based on the Selection and
Connection Properties communicated by the application, along with any
Security Parameters. The implementation can only attempt to use
multiple Protocol Stacks when they are "equivalent", which means that
the stacks can provide the same Transport Properties and interface
expectations as requested by the application. Equivalent Protocol
Stacks can be safely swapped or raced in parallel (see Section 4.2.2)
during connection establishment.

The following two examples show non-equivalent Protocol Stacks:

* If the application requires preservation of message boundaries, a
Protocol Stack that runs UDP as the top-level interface to the
application is not equivalent to a Protocol Stack that runs TCP as
the top-level interface. A UDP stack would allow an application
to read out message boundaries based on datagrams sent from the
remote system, whereas TCP does not preserve message boundaries on
its own, own but needs a framing protocol on top to determine message
boundaries.

* If the application specifies that it requires reliable
transmission of data, then a Protocol Stack using UDP without any
reliability layer on top would not be allowed to replace a
Protocol Stack using TCP.

The following example shows Equivalent equivalent Protocol Stacks:

* If the application does not require reliable transmission of data,
then a Protocol Stack that adds reliability could be regarded as
an Equivalent equivalent Protocol Stack as long as providing this would not
conflict with any other application-requested properties.

A Transport Services Implementation can race different security
protocols, e.g., if the System Policy is explicitly configured to
consider them equivalent. A Transport Services implementation Implementation SHOULD
only race Protocol Stacks where the transport security protocols
within the stacks are identical. To ensure that security protocols
are not incorrectly swapped, a Transport Services Implementation MUST
only select Protocol Stacks that meet application requirements
([RFC8922]).
[RFC8922]. A Transport Services Implementation MUST NOT
automatically fall back from secure protocols to insecure protocols, protocols
or fall back to weaker versions of secure protocols. A Transport
Services Implementation MAY allow applications to explicitly specify
which versions of a protocol ought to be permitted, e.g., to allow a
minimum version of TLS 1.2 in case if TLS 1.3 is not available.

A Transport Services Implementation MAY specify security properties
relating to how the system operates (e.g., requirements,
prohibitions, and preferences for the use of DNS Security Extensions
(DNSSEC) or DNS over HTTPS (DoH)).

3.4. Maintain Interoperability

It is important to note that neither the Transport Services API
[I-D.ietf-taps-interface]
[RFC9622] nor the guidelines for implementation of the Transport Service
Services System [I-D.ietf-taps-impl] [RFC9623] define new protocols or protocol
capabilities that affect what is communicated across the network. A
Transport Services System MUST NOT require that a peer on the other
side of a connection uses use the same API or implementation. A Transport
Services Implementation acting as a connection initiator is able to
communicate with any existing Endpoint that implements the transport
protocol(s) and all the required properties selected. Similarly, a
Transport Services Implementation acting as a Listener can receive
connections for any protocol that is supported from an existing
initiator that implements the protocol, independent independently of whether or
not the initiator uses the Transport Services System or not. System.

A Transport Services Implemenation Implementation makes decisions that select
protocols and interfaces. In normal use, a given version of a
Transport Services System SHOULD result in consistent protocol and
interface selection decisions for the same network conditions conditions, given
the same set of Properties. This is intended to provide predictable
outcomes to the application using the API.

3.5. Support Monitoring

The Transport Services API increases the layer of abstraction for
applications, and it enables greater automation below the API. Such
increased abstraction comes at the cost of increased complexity when
application programmers, users users, or system administrators try to
understand why any issues and failures may be happening. Transport
Services systems should therefore offer monitoring functions that
provide relevant debug and diagnostics information. For example,
such monitoring functions could indicate the protocol(s) in use, the
number of open connections per protocol, and any statistics that
these protocols may offer.

4. Transport Services Architecture and Concepts

This section of the document describes the architecture non-
normatively non-normatively and explains
the operation of a Transport Services Implementation. The concepts
defined in this document are intended primarily for use in the
documents and specifications that describe the Transport Services
System. This includes the architecture, the Transport Services API API,
and the associated Transport Services Implementation. While the
specific terminology can be used in some implementations, it is
expected that there will remain a variety of terms used by running
code.

The architecture divides the concepts for the Transport Services
System into two categories:

1. API concepts, which are intended to be exposed to applications;
and

2. System-implementation concepts, which are intended to be
internally used by a Transport Services Implementation.

The following diagram summarizes the top-level concepts in a
Transport Services System and how they relate to one another.

+-----------------------------------------------------+
| Application |
+-+----------------+------^-------+--------^----------+
| | | | |
pre- | data | events
establishment | transfer | |
| establishment | termination |
| | | | |
| +--v------v-------v+ |
+-v-------------+ Connection(s) +-------+----------+
| Transport +--------+---------+ |
| Services | |
| API | +-------------+ |
+------------------------+--+ Framer(s) |-----------+
| +-------------+
+------------------------|----------------------------+
| Transport | |
| System | +-----------------+ |
| Implementation | | Cached | |
| | | State | |
| (Candidate Gathering) | +-----------------+ |
| | |
| (Candidate Racing) | +-----------------+ |
| | | System | |
| | | Policy | |
| +----------v-----+ +-----------------+ |
| | Protocol | |
+-------------+ Stack(s) +----------------------+
+-------+--------+
V
+-----------------------------------------------------+
| Network Layer Network-Layer Interface |
+-----------------------------------------------------+

Figure 3: Concepts and Relationships in the Architecture of the
Transport Services System

The Transport Services Implementation includes the Cached State and
System Policy.

The System Policy provides input from an operating system or other
global preferences that can constrain or influence how an
implementation will gather Candidate Paths and Protocol Stacks and
race the candidates when establishing a Connection. As the details
of System Policy configuration and enforcement are largely platform- dependent
on the platform and implementation- dependent, implementation and do not affect application-level application-
level interoperability, the Transport Services API
[I-D.ietf-taps-interface] [RFC9622] does not
specify an interface for reading or writing System Policy.

The Cached State is the state and history that the Transport Services
Implementation keeps for each set of associated Endpoints that have
previously been used. An application ought to explicitly request any
required or desired properties via the Transport Services API.

4.1. Transport Services API Concepts

Fundamentally, a Transport Services API needs to provide Connection
objects (Section 4.1.2) that allow applications to establish
communication,
communication and then send and receive data. These could be exposed
as handles or referenced objects, depending on the chosen programming
language.

Beyond the Connection objects, there are several high-level groups of
actions that any Transport Services API needs to provide:

* Pre-establishment Preestablishment (Section 4.1.3) encompasses the properties that
an application can pass to describe its intent, requirements,
prohibitions, and preferences for its networking operations.
These properties apply to multiple transport protocols, unless
otherwise specified. Properties specified during pre-
establishment preestablishment
can have a large impact on the rest of the interface: they modify
how establishment occurs, they influence the expectations around data
transfer, and they determine the set of events that will be supported.

* Establishment (Section 4.1.4) focuses on the actions that an
application takes on the Connection objects to prepare for data
transfer.

* Data Transfer transfer (Section 4.1.5) consists of how an application
represents the data to be sent and received, the functions
required to send and receive that data, and how the application is
notified of the status of its data transfer.

* Event Handling handling (Section 4.1.6) defines categories of notifications
that an application can receive during the lifetime of a
Connection. Events also provide opportunities for the application
to interact with the underlying transport by querying state or
updating maintenance options.

* Termination (Section 4.1.7) focuses on the methods by which data
transmission is stopped, stopped and connection state is torn down.

The diagram below provides a high-level view of the actions and
events during the lifetime of a Connection object. Note that some
actions are alternatives (e.g., whether to initiate a connection or
to
listen for incoming connections), while others are optional (e.g.,
setting Connection and Message Properties in pre-establishment) preestablishment) or
have been omitted for brevity and simplicity.

Pre-establishment

Figure 4: The lifetime Lifetime of a Connection object Object

In this diagram, the lifetime of a Connection object is divided into
three phases: pre-establishment, preestablishment, the Established state, and
Termination.

Pre-establishment
termination of a connection.

Preestablishment is based around a Preconnection object, that
contains object containing
various sub-objects that describe the properties and parameters of
desired Connections (Local and Remote Endpoints, Transport
Properties, and Security Parameters). A Preconnection can be used to
start listening for inbound connections, connections -- in which case a Listener
object is created, created -- or can be used to establish a new connection
directly using Initiate (for outbound connections) or Rendezvous (for
peer-to-peer connections).

Once a Connection is in the Established state, an application can
send and receive Message objects, objects and can receive state updates.

Closing or aborting a connection, either locally or from the peer,
can terminate a connection.

4.1.1. Endpoint Objects

An Endpoint Identifier specifies one side of a transport connection.
Endpoints can be Local Endpoints or Remote Endpoints, and the
Endpoint Identifiers can respectively represent an identity that the
application uses for the source or destination of a connection. An
Endpoint Identifier can be specified at various levels of
abstraction. An Endpoint Identifier at a higher level of abstraction
(such as a hostname) can be resolved to more concrete identities
(such as IP addresses). A Remote Endpoint Identifier can also
represent a multicast group or anycast address. In the case of
multicast, this selects a multicast transport will be selected for communication.

Remote Endpoint Identifier: The Remote Endpoint Identifier
represents the application's identifier for a peer that can
participate in a transport connection; connection, for example, the
combination of a DNS name for the peer and a service name/port.

Local Endpoint Identifier: The Local Endpoint Identifier represents
the application's identifier for itself that it uses for transport connections;
connections, for example, a local IP address and port.

4.1.2. Connections and Related Objects

Connection: A Connection object represents one or more active
transport protocol instances that can send and/or receive Messages
between Local and Remote Endpoints. It is an abstraction that
represents the communication. The Connection object holds state
pertaining to the underlying transport protocol instances and any
ongoing data transfers. For example, an active Connection can
represent a connection-oriented protocol such as TCP, or it can
represent a fully-specified fully specified 5-tuple for a connectionless protocol
such as UDP, where the Connection remains an abstraction at the
endpoints. It can also represent a pool of transport protocol
instances, e.g., a set of TCP and QUIC connections to equivalent
endpoints, or a stream of a multi-streaming multistreaming transport protocol
instance. Connections can be created from a Preconnection or by a
Listener.

Preconnection: A Preconnection object is a representation of a
Connection that has not yet been established. It has state that
describes parameters of the Connection: the Local Endpoint
Identifier from which that Connection will be established, the
Remote Endpoint Identifier (Section 4.1.3) to which it will connect, and Transport
Properties that influence the paths and protocols a Connection
will use. A Preconnection can be either fully specified
(representing a single possible Connection), Connection) or it
can be partially specified
(representing a family of possible Connections). The Local
Endpoint (Section 4.1.3) is required for a Preconnection used to
Listen for incoming Connections, Connections but is optional if it is used to
Initiate a Connection. The Remote Endpoint Identifier is required
in a Preconnection that is used to Initiate a Connection, Connection but is
optional if it is used to Listen for incoming Connections. The
Local Endpoint Identifier and the Remote Endpoint Identifier are
both required if a peer-to-peer Rendezvous is to occur based on
the Preconnection.

Transport Properties: Transport Properties allow the application to
express their requirements, prohibitions, and preferences and configure
a Transport Services Implementation. There are three kinds of
Transport Properties:

Selection Properties (Section 4.1.3): Selection Properties can
only be specified on a Preconnection.

Connection Properties (Section 4.1.3): Connection Properties can
be specified on a Preconnection and changed on the Connection.

Message Properties (Section 4.1.5): Message Properties can be
specified as defaults on a Preconnection or a Connection, Connection and
can also be specified during data transfer to affect specific
Messages.

Listener: A Listener object accepts incoming transport protocol
connections from Remote Endpoints and generates corresponding
Connection objects. It is created from a Preconnection object
that specifies the type of incoming Connections it will accept.

4.1.3. Pre-establishment

* Preestablishment

Selection Properties: The Selection Properties consist of the properties
that an application can set to influence the selection of paths
between the Local and Remote Endpoints, to influence the selection of
transport protocols, or to configure the behavior of generic
transport protocol features. These properties can take the form
of requirements, prohibitions, or preferences. Examples of
properties that influence path selection include the interface
type (such as a Wi-Fi connection, connection or a Cellular LTE connection),
requirements around the largest Message that can be sent, or
preferences for throughput and latency. Examples of properties
that influence protocol selection and configuration of transport
protocol features include reliability, multipath support, and fast
open support.

*
support for TCP Fast Open.

Connection Properties: The Connection Properties are used to configure
protocol-specific options and control per-connection behavior of a
Transport Services Implementation; for example, a
protocol-specific protocol-
specific Connection Property can express that if TCP is used, the
implementation ought to use the User Timeout Option. Note that
the presence of such a property does not require that a specific
protocol will be used. In general, these properties do not explicitly
determine the selection of paths or protocols, protocols but can be used by
an implementation during connection establishment. Connection
Properties are specified on a Preconnection prior to Connection establishment,
establishment and can be modified on the Connection later.
Changes made to Connection Properties after Connection
establishment take effect on a best-effort basis.

Security Parameters: Security Parameters define an application's
requirements for authentication and encryption on a Connection.
They are used by Transport Security transport security protocols (such as those
described in [RFC8922]) to establish secure Connections. Examples
of parameters that can be set include local identities, private
keys, supported cryptographic algorithms, and requirements for
validating trust of remote identities. Security Parameters are
primarily associated with a Preconnection object, but properties
related to identities can be associated directly with Endpoints.

4.1.4. Establishment Actions

Initiate: The primary action that an application can take to create
a Connection to a Remote Endpoint, Endpoint and prepare any required local
or remote state to enable the transmission of Messages. For some
protocols, this will initiate a client-to-server style client-to-server-style handshake;
for other protocols, this will just establish local state (e.g.,
with connectionless protocols such as UDP). The process of
identifying options for connecting, such as resolution of the
Remote Endpoint Identifier, occurs in response to the Initiate
call.

Listen: Enables a Listener to accept incoming connections. The
Listener will then create Connection objects as incoming
connections are accepted (Section 4.1.6). Listeners by default
register with multiple paths, protocols, and Local Endpoints,
unless constrained by Selection Properties and/or the specified
Local Endpoint Identifier(s). Connections can be accepted on any
of the available paths or endpoints.

Rendezvous: The action of establishing a peer-to-peer connection
with a Remote Endpoint. It simultaneously attempts to initiate a
connection to a Remote Endpoint while listening for an incoming
connection from that Endpoint. The process of identifying options
for the connection, such as resolution of the Remote Endpoint
Identifier(s), occurs in response to the Rendezvous call. As with
Listeners, the set of local paths and endpoints is constrained by
Selection Properties. If successful, the Rendezvous call
generates and asynchronously returns a Connection object to
represent the established peer-to-peer connection. The processes
by which connections are initiated during a Rendezvous action will
depend on the set of Local and Remote Endpoints configured on the
Preconnection. For example, if the Local and Remote Endpoints are
TCP host candidates, then a TCP simultaneous open [RFC9293] might
be performed. However, if the set of Local Endpoints includes
server reflexive
server-reflexive candidates, such as those provided by STUN
(Session Traversal Utilities for NAT) [RFC5389], [RFC8489], a Rendezvous
action will race candidates in the style of the ICE (Interactive
Connection
Connectivity Establishment) algorithm [RFC8445] to perform NAT
binding discovery and initiate a peer-to-peer connection.

4.1.5. Data Transfer Objects and Actions

Message: A Message object is a unit of data that can be represented
as bytes that can be transferred between two endpoints over a
transport connection. The bytes within a Message are assumed to
be ordered. If an application does not care about the order in
which a peer receives two distinct spans of bytes, those spans of
bytes are considered independent Messages. Messages are sent in
the payload of IP packets. One packet can carry one or more
Messages or parts of a Message.

Message Properties: Message Properties are used to specify details
about Message transmission. They can be specified directly on
individual Messages, Messages or can be set on a Preconnection or Connection
as defaults. These properties might only apply to how a Message
is sent (such as how the transport will treat prioritization and reliability),
reliability) but can also include properties that specific
protocols encode and communicate to the Remote Endpoint. When
receiving Messages, Message Properties can contain information
about the received Message, such as metadata generated at the
receiver and information signalled signaled by the Remote Endpoint. For
example, a Message can be marked with a Message Property
indicating that it is the final Message on a Connection.

Send: The Send action to transmit transmits a Message over a Connection to the
Remote Endpoint. The interface to Send can accept Message
Properties specific to how the Message content is to be sent. The
status of the Send operation is delivered back to the sending
application in an event (Section 4.1.6).

Receive: An The Receive action that indicates that the application is ready
to asynchronously accept a Message over a Connection from a Remote
Endpoint, while the Message content itself will be delivered in an
event (Section 4.1.6). The interface to Receive can include
Message Properties specific to the Message that is to be delivered
to the application.

Framer: A Framer is a data translation layer that can be added to a
Connection. Framers allow extending a Connection's Protocol Stack
to define how to encapsulate or encode outbound Messages, Messages and how
to decapsulate or decode inbound data into Messages. In this way,
message boundaries can be preserved when using a Connection
object, even with a protocol that otherwise presents unstructured
streams, such as TCP. This is designed based on the fact that
many of the current application protocols evolved over TCP, which
does not provide message boundary preservation, and since many of
these protocols require message boundaries to function, each application layer
application-layer protocol has defined its own framing. For
example, when an HTTP application sends and receives HTTP messages
over a byte-stream transport, it must parse the boundaries of HTTP
messages from the stream of bytes.

4.1.6. Event Handling

The following categories of events can be delivered to an
application:

Connection Ready: Signals to an application that a given Connection
is ready to send and/or receive Messages. If the Connection
relies on handshakes to establish state between peers, then it is
assumed that these steps have been taken.

Connection Closed: Signals to an application that a given Connection
is no longer usable for sending or receiving Messages. The event
delivers a reason or error to the application that describes the
nature of the termination.

Connection Received: Signals to an application that a given Listener
has received a Connection.

Message Received: Delivers received Message content to the
application, based on a Receive action. To allow an application
to limit the occurrence of such events, each call to Receive will
be paired with a single Receive event. This can include an error
if the Receive action cannot be satisfied, e.g., due to the
Connection being closed.

Message Sent: Notifies the application of the status of its Send
action. This might indicate a failure if the Message cannot be
sent,
sent or an indication might indicate that the Message has been processed by the
Transport Services System.

Path Properties Changed: Notifies the application that a property of
the Connection has changed that might influence how and where data
is sent and/or received.

4.1.7. Termination Actions

Close: The action an application takes on a Connection to indicate
that it no longer intends to send data, data or is no longer willing to
receive data, and that the data. The protocol should signal this state to the Remote
Endpoint if the transport protocol allows this. permits it. (Note that this is
distinct from the concept of "half-closing" a bidirectional
connection, such as when a FIN is sent in one direction of a TCP
connection [RFC9293]. The end of a stream can also be indicated
using Message Properties when sending.)

Abort: The action the application takes on a Connection to indicate a Close and also indicate
that the Transport Services System should not attempt to deliver
any outstanding data, data and that it should immediately close and drop
the connection. This is intended for immediate, usually abnormal,
termination of a connection.

4.1.8. Connection Groups

A Connection Group is a set of Connections that shares Connection
Properties and cached state generated by protocols. A Connection
Group represents state for managing Connections within a single
application,
application and does not require end-to-end protocol signaling. For
transport protocols that support multiplexing, only Connections
within the same Connection Group are allowed to be multiplexed
together.

The API allows a Connection to be created from another Connection.
This adds the new Connection to the Connection Group. A change to
one of the Connection Properties on any Connection in the Connection
Group automatically changes the Connection Property for all others.
All Connections in a Connection Group share the same set of
Connection Properties except for the Connection Priority. These
Connection Properties are said to be entangled.

Passive Connections can also be added to a Connection Group, e.g.,
when a Listener receives a new Connection that is just a new stream
of an already active multi-streaming already-active multistreaming protocol instance.

While Connection Groups are managed by the Transport Services
Implementation, an application can define different Connection
Contexts for different Connection Groups to explicitly control
caching boundaries, as discussed in Section 4.2.3.

4.2. Transport Services Implementation

This section defines the key architectural concepts for the Transport
Services Implementation within the Transport Services System.

The Transport Services System consists of the Transport Services
Implementation and the Transport Services API. The Transport
Services Implementation consists of all objects and protocol
instances used internally to a system or library to implement the
functionality needed to provide a transport service across a network,
as required by the abstract interface.

Path: Represents an available set of properties that a Local
Endpoint can use to communicate with a Remote Endpoint, such as
routes, addresses, and physical and virtual network interfaces.

Protocol Instance: A single instance of one protocol, including any
state necessary to establish connectivity or send and receive
Messages.

Protocol Stack: A set of Protocol Instances (including relevant
application, security, transport, or Internet protocols) that are
used together to establish connectivity or send and receive
Messages. A single stack can be simple (a single transport
protocol instance (e.g., one application
stream carried TCP running over IP), IP) or it can be complex (multiple (e.g,. multiple
application protocol streams going through a single security and
transport protocol, carried over IP; or, a multi-path multipath transport protocol
over
using multiple transport sub-flows).

* subflows over IP).

Candidate Path: One path that is available to an application and
conforms to the Selection Properties and System Policy, of which
there can be several. Candidate Paths are identified during the
gathering phase (Section 4.2.1) and can be used during the racing
phase (Section 4.2.2).

Candidate Protocol Stack: One Protocol Stack that can be used by an
application for a Connection, for which there can be several
candidates. Candidate Protocol Stacks are identified during the
gathering phase (Section 4.2.1) and are started during the racing
phase (Section 4.2.2).

System Policy: The input from an operating system or other global
preferences that can constrain or influence how an implementation
will gather candidate paths Candidate Paths and Candidate Protocol Stacks
(Section 4.2.1) and race the candidates during establishment
(Section 4.2.2). Specific aspects of the System Policy either apply to
either all Connections or only certain ones, Connections, depending on
the runtime context and properties of the Connection.

Cached State: The state and history that the implementation keeps
for each set of associated Endpoints that have been used
previously. This can include DNS results, TLS session state,
previous success and quality of transport protocols over certain
paths, as well as other information. This caching does not imply
that the same decisions are necessarily made for subsequent
connections,
connections; rather, it means that cached state is used by a
Transport Services Implementation to inform functions such as
choosing the candidates to be raced, selecting appropriate
transport parameters, etc. An application SHOULD NOT rely on
specific caching behaviour, instead behavior; instead, it ought to explicitly request
any required or desired properties via the Transport Services API.

4.2.1. Candidate Gathering

Candidate Path Selection: Candidate Path Selection represents the
act of choosing one or more paths that are available to use based
on the Selection Properties and any available Local and Remote
Endpoint Identifiers provided by the application, as well as the
policies and heuristics of a Transport Services implementation.

* Implementation.

Candidate Protocol Selection: Candidate Protocol Selection
represents the act of choosing one or more sets of Protocol Stacks
that are available to use based on the Transport Properties
provided by the application, and the heuristics or policies within
the Transport Services Implementation.

4.2.2. Candidate Racing

Connection establishment attempts for a set of candidates may be
performed simultaneously, synchronously, serially, or using some
combination of all of these. We refer to this process as racing,
borrowing terminology from Happy Eyeballs [RFC8305].

Protocol Option Racing: Protocol Option Racing is the act of
attempting to establish, or scheduling attempts to establish,
multiple Protocol Stacks that differ based on the composition of
protocols or the options used for protocols.

Path Racing: Path Racing is the act of attempting to establish, or
scheduling attempts to establish, multiple Protocol Stacks that
differ based on a selection from the available Paths. Since
different Paths will have distinct configurations (see [RFC7556])
for local addresses and DNS servers, attempts across different
Paths will perform separate DNS resolution steps, which can lead
to further racing of the resolved Remote Endpoint Identifiers.

Remote Endpoint Racing: Remote Endpoint Racing is the act of
attempting to establish, or scheduling attempts to establish,
multiple Protocol Stacks that differ based on the specific
representation of the Remote Endpoint Identifier, such as a
particular IP address that was resolved from a DNS hostname.

4.2.3. Separating Connection Contexts

A Transport Services Implementation can by default share stored
properties across Connections within an application, such as cached
protocol state, cached path state, and heuristics. This provides
efficiency and convenience for the application, since the Transport
Services System can automatically optimize behavior.

The Transport Services API can allow applications to explicitly
define Connection Contexts that force separation of Cached State and
Protocol Stacks. For example, a web browser application could use
Connection Contexts with separate caches when implementing different
tabs. Possible reasons to isolate Connections using separate
Connection Contexts include:

* Privacy include privacy concerns about re-using regarding:

* reusing cached protocol state that state, as this can lead to linkability.
Sensitive state could include TLS session state [RFC8446] and HTTP
cookies [RFC6265]. These concerns could be addressed using
Connection Contexts with separate caches, such as for different
browser tabs.

* Privacy concerns about allowing Connections to multiplex together, which can tell a
Remote Endpoint that all of the Connections are coming from the
same application. Using Connection Contexts avoids the
Connections being multiplexed in a an HTTP/2 or QUIC stream.

5. IANA Considerations

This document has no actions for IANA. IANA actions.

6. Security and Privacy Considerations

The Transport Services System does not recommend the use of specific
security protocols or algorithms. Its goal is to offer ease of use
for existing protocols by providing a generic security-related
interface. Each provided interface translates to an existing
protocol-specific interface provided by supported security protocols.
For example, trust verification callbacks are common parts of TLS
APIs; a Transport Services API exposes similar functionality
[RFC8922].

As described above in Section 3.3, if a Transport Services
Implementation races between two different Protocol Stacks, both need
to use the same security protocols and options. However, a Transport
Services Implementation can race different security protocols, e.g.,
if the application explicitly specifies that it considers them
equivalent.

The application controls whether information from previous racing
attempts,
attempts or other information about past communications that was
cached by the Transport Services System is used during establishment.
This allows applications to make tradeoffs trade-offs between efficiency
(through racing) and privacy (via information that might leak from
the cache toward an on-path observer). Some applications have
features (e.g. (e.g., "incognito mode") that align with this functionality.

Applications need to ensure that they use security APIs
appropriately. In cases where applications use an interface to
provide sensitive keying material, e.g., access to private keys or
copies of pre-shared keys (PSKs), key use needs to be validated and
scoped to the intended protocols and roles. For example, if an
application provides a certificate to only be used as client
authentication for outbound TLS and QUIC connections, the Transport
Services System MUST NOT use this automatically in other contexts
(such as server authentication for inbound connections, connections or in other
another
security protocol handshake handshakes that is are not equivalent to TLS).

A Transport Services System MUST NOT automatically fall back from
secure protocols to insecure protocols, protocols or fall back to weaker
versions of secure protocols (see Section 3.3). For example, if an
application requests a specific version of TLS, TLS but the desired
version of TLS is not available, its connection will fail. As
described in Section 3.3, the Transport Services API can allow
applications to specify minimum versions that are allowed to be used
by the Transport Services System.

7. Acknowledgements

This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreements No. 644334
(NEAT), No. 688421 (MAMI) and No 815178 (5GENESIS).

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

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

Thanks to Reese Enghardt, Max Franke, Mirja Kuehlewind, Jonathan
Lennox, and Michael Welzl for the discussions and feedback that
helped shape the architecture of the system described here.
Particular thanks is also due to Philipp S. Tiesel and Christopher
A. Wood, who were both co-authors of this specification as it
progressed through the TAPS working group. Thanks as well to Stuart
Cheshire, Josh Graessley, David Schinazi, and Eric Kinnear for their
implementation and design efforts, including Happy Eyeballs, that
heavily influenced this work.

8. References

8.1.

7.1. Normative References

[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/rfc/rfc2119>.
<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/rfc/rfc8174>.

8.2. <https://www.rfc-editor.org/info/rfc8174>.

7.2. Informative References

[I-D.ietf-taps-impl]
Brunstrom, A., Pauly, T., Enghardt, R., Tiesel, P. S., and
M. Welzl, "Implementing Interfaces to Transport Services",
Work in Progress, Internet-Draft, draft-ietf-taps-impl-16,
5 June 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-taps-impl-16>.

[I-D.ietf-taps-interface]
Trammell, B., Welzl, M., Enghardt, R., Fairhurst, G.,
Kühlewind, M., Perkins, C., Tiesel, P. S., and T. Pauly,
"An Abstract Application Layer Interface to Transport
Services", Work in Progress, Internet-Draft, draft-ietf-
taps-interface-22, 6 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-taps-
interface-22>.

[POSIX] "IEEE Std. 1003.1-2008 "IEEE/Open Group Standard for Information Technology
-- -
Portable Operating System Interface (POSIX). Open
group Technical Standard: (POSIX(TM)) Base
Specifications, Issue 7",
2008.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389, 8", IEEE Std 1003.1-2024,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/rfc/rfc5389>. 10.1109/IEEESTD.2024.10555529, 2024,
<https://ieeexplore.ieee.org/document/10555529>.

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

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/rfc/rfc6265>.
<https://www.rfc-editor.org/info/rfc6265>.

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

[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/rfc/rfc8095>.
<https://www.rfc-editor.org/info/rfc8095>.

[RFC8170] Thaler, D., Ed., "Planning for Protocol Adoption and
Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
May 2017, <https://www.rfc-editor.org/rfc/rfc8170>. <https://www.rfc-editor.org/info/rfc8170>.

[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/rfc/rfc8303>.
<https://www.rfc-editor.org/info/rfc8303>.

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

[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/rfc/rfc8445>.
<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/rfc/rfc8446>.
<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>.

[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/rfc/rfc8922>.
<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/rfc/rfc8923>. <https://www.rfc-editor.org/info/rfc8923>.

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

[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/rfc/rfc9112>. <https://www.rfc-editor.org/info/rfc9112>.

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

[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/rfc/rfc9293>.
<https://www.rfc-editor.org/info/rfc9293>.

[RFC9622] Trammell, B., Ed., Welzl, M., Ed., Enghardt, R.,
Fairhurst, G., Kühlewind, M., Perkins, C. S., Tiesel, P.,
and T. Pauly, "An Abstract Application Programming
Interface (API) for Transport Services", RFC 9622,
DOI 10.17487/RFC9622, December 2024,
<https://www.rfc-editor.org/info/rfc9622>.

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

Acknowledgements

This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreements No. 644334
(NEAT), No. 688421 (MAMI), and No. 815178 (5GENESIS).

This work has been supported by:

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

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

Thanks to Reese Enghardt, Max Franke, Mirja Kühlewind, Jonathan
Lennox, and Michael Welzl for the discussions and feedback that
helped shape the architecture of the system described here.
Particular thanks are also due to Philipp S. Tiesel and Christopher
A. Wood, who were both coauthors of this specification as it
progressed through the Transport Services (TAPS) Working Group.
Thanks as well to Stuart Cheshire, Josh Graessley, David Schinazi,
and Eric Kinnear for their implementation and design efforts,
including Happy Eyeballs, that heavily influenced this work.

Authors' Addresses

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

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

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

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

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