Internet DRAFT - draft-ietf-taps-arch
draft-ietf-taps-arch
TAPS Working Group T. Pauly, Ed.
Internet-Draft Apple Inc.
Intended status: Standards Track B. Trammell, Ed.
Expires: 12 May 2024 Google Switzerland GmbH
A. Brunstrom
Karlstad University
G. Fairhurst
University of Aberdeen
C. Perkins
University of Glasgow
9 November 2023
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 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, and describes how a Transport Services
Implementation can use multiple IP addresses, multiple protocols, and
multiple paths, and 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 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 of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 May 2024.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 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
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
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7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
8.1. Normative References . . . . . . . . . . . . . . . . . . 32
8.2. Informative References . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
Many 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). The naming of objects and functions across these APIs is not
consistent and varies depending on the protocol being used. For
example, 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 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, 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., 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, and 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 each with equivalent protocol stacks that are used to
identify an optimal combination of transport protocol instance such
as TCP, UDP, or another transport, together with configuration of
parameters and interfaces. A Connection represents an object that,
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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] and implementation
guidelines [I-D.ietf-taps-impl]. Although following the Transport
Services 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], 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 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 require application interaction and guidance (referred to in
that document as Functional or Optimizing Features), or else 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 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 Transport Services System in three
sections:
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* 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] and described in the implementation
guidelines [I-D.ietf-taps-impl]. 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. It provides a short description of
key terms that are later defined in this document.
* Application: An entity that uses the transport layer for 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.
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* Clone: A Connection that was created from another Connection, and
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 other Transport
Services 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.
* 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, or obtain details about the
transmission after receiving a Message.
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* 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 object that represents a Connection that has not
yet been established.
* Preference: A preference to prohibit, avoid, ignore, prefer, or
require a specific 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 [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.
* Server: The peer responsible for responding to a Connection
initiation.
* Socket: The combination of a destination IP address and a
destination port number [RFC8303].
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* 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 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 and preferences [RFC8095].
* Transport Service: A set of transport features, without an
association to any given framing protocol, that provides a
complete service to an application.
* Transport Services Implementation: This 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.
* 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 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.
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+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
| | |
+------------+ +------------+ +--------------+
| DNS stub | | Stream API | | Datagram API |
| resolver | +------------+ +--------------+
+------------+ | |
+---------------------------------+
| TCP UDP |
| Kernel Networking Stack |
+---------------------------------+
|
+-----------------------------------------------------+
| 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] 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 2). This offers generic functions and also
the protocol-specific mappings for TCP, UDP, UDP-Lite, and other
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protocol layers. These mapping are extensible. Future documents
could define similar mappings for new layers and for other transport
protocols, such as QUIC [RFC9000]. 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] is the
component of the Transport Services System that implements the
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 of
the Transport Services 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, multiple paths, and 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 the application wants to 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.
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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, 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 and
pooled connections.
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Allowing applications to interact with messages is backwards-
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, 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 is designed:
* 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 4.2.1 and Section 4.2.2). This
requires applications to specify identifiers for the Local and Remote
Endpoint that are higher-level than IP addresses, such as a hostname
or URL, which are used by a Transport Services Implementation for
resolution, path selection, and racing. An implementation can
further implement fallback mechanisms if connection establishment of
one protocol fails or performance is detected to be unsatisfactory.
Information used in connection establishment (e.g. cryptographic
resumption tokens, information about usability of certain protocols
on the path, results of racing in previous connections) are cached in
the Transport Services Implementation. Applications have control
over whether this information is used for a specific establishment,
in order to allow tradeoffs 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.
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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 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 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 Transport Services Implementation to provide this
functionality without causing incompatibility or introducing security
vulnerabilities.
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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 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, set on individual Messages.
It is RECOMMENDED that the Transport Services API offers 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 applicable
to a variety of network layer interfaces and paths, which permits
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 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.
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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.
A specialized feature could be needed by an application only when
using a specific 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 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, 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 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 – whether the presence of a specific feature is
prohibited, should be avoided, can be ignored, is preferred, or is
required in the pre-establishment 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.
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3.3. Select Between Equivalent Protocol Stacks
A Transport Services Implementation can attempt and select 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, 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 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 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 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]). A Transport Services Implementation MUST NOT
automatically fall back from secure protocols to insecure protocols,
or 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 TLS 1.3 is not available.
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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] nor the guidelines for implementation of
the Transport Service System [I-D.ietf-taps-impl] 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 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 of whether the initiator uses the Transport
Services System or not.
A Transport Services Implemenation 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 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 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.
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4. Transport Services Architecture and Concepts
This section of the document describes the architecture 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 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 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.
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+-----------------------------------------------------+
| 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 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.
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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-
and implementation- dependent, and do not affect application-level
interoperability, the Transport Services API
[I-D.ietf-taps-interface] 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, 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 (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 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 (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.
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* Event 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, 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) or
have been omitted for brevity and simplicity.
Pre-establishment : Established : Termination
----------------- : ----------- : -----------
: :
+-- Local Endpoint : Message :
+-- Remote Endpoint : Receive() | :
+-- Transport Properties : Send() | :
+-- Security Parameters : | :
| : | :
| InitiateWithSend() | Close() :
| +---------------+ Initiate() +-----+------+ Abort() :
+---+ Preconnection |------------->| Connection |-----------> Closed
+---------------+ Rendezvous() +------------+ :
Listen() | : | | :
| : | v :
v : | Connection :
+----------+ : | Ready :
| Listener |----------------------+ :
+----------+ Connection Received :
: :
Figure 4: The lifetime of a Connection object
In this diagram, the lifetime of a Connection object is divided into
three phases: pre-establishment, the Established state, and
Termination.
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Pre-establishment is based around a Preconnection object, that
contains 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, in which case a
Listener object is 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, and 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 for communication.
* Remote Endpoint Identifier: The Remote Endpoint Identifier
represents the application's identifier for a peer that can
participate in a transport 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; 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 can
represent a fully-specified 5-tuple for a connectionless protocol
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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 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), 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, but
optional if it is used to Initiate a Connection. The Remote
Endpoint Identifier is required in a Preconnection that used to
Initiate a 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, 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.
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4.1.3. Pre-establishment
* 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, 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.
* 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 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, but
can be used by an implementation during connection establishment.
Connection Properties are specified on a Preconnection prior to
Connection 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 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, 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
handshake; for other protocols, this will just establish local
state (e.g., with connectionless protocols such as UDP). The
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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 candidates, such as those provided by STUN
(Session Traversal Utilities for NAT) [RFC5389], a Rendezvous
action will race candidates in the style of the ICE (Interactive
Connection 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, or can be set on a Preconnection or
Connection as defaults. These properties might only apply to how
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a Message is sent (such as how the transport will treat
prioritization and 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 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 action to transmit 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 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,
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 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.
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* 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, or an indication 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, is no longer willing to
receive data, and that the protocol should signal this state to
the Remote Endpoint if the transport protocol allows this. (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, and
immediately drop the connection. This is intended for immediate,
usually abnormal, termination of a connection.
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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, 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 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.
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* 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 over IP), or it can be complex (multiple
application protocol streams going through a single security and
transport protocol, over IP; or, a multi-path transport protocol
over multiple transport sub-flows).
* 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 and 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 all
Connections or only certain ones, 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, 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 it ought to explicitly request
any required or desired properties via the Transport Services API.
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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.
* 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.
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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 concerns about re-using cached protocol state that 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 HTTP/2 or QUIC
stream.
5. IANA Considerations
This document has no actions for IANA.
6. Security and Privacy Considerations
The Transport Services System does not recommend 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, or other information about past communications that was
cached by the Transport Services System is used during establishment.
This allows applications to make tradeoffs 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. "incognito mode") that align with this functionality.
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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, or in other
another security protocol handshake that is not equivalent to TLS).
A Transport Services System MUST NOT automatically fall back from
secure protocols to insecure protocols, or to weaker versions of
secure protocols (see Section 3.3). For example, if an application
requests a specific version of 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. Normative References
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[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>.
[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. 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 Standard for Information Technology
-- Portable Operating System Interface (POSIX). Open
group Technical Standard: Base Specifications, Issue 7",
2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/rfc/rfc5389>.
[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>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/rfc/rfc6265>.
[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/rfc/rfc7556>.
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[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
Authors' Addresses
Tommy Pauly (editor)
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: tpauly@apple.com
Brian Trammell (editor)
Google Switzerland GmbH
Gustav-Gull-Platz 1
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
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
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