wdiff draft-ietf-taps-arch-05.txt draft-ietf-taps-arch-06.txt

TAPS Working Group T. Pauly, Ed.
Internet-Draft Apple Inc.
Intended status: Standards Track B. Trammell, Ed.
Expires: May 7, 25 June 2020 Google
A. Brunstrom
Karlstad University
G. Fairhurst
University of Aberdeen
C. Perkins
University of Glasgow
P. Tiesel
TU Berlin
C. Wood
Apple Inc.
November 04,
23 December 2019

An Architecture for Transport Services
draft-ietf-taps-arch-05
draft-ietf-taps-arch-06

Abstract

This document provides describes an overview of the architecture of Transport
Services, a model for exposing transport
protocol features to applications for network communication. In contrast to what is
provided by most existing Application Programming Interfaces (APIs), communication, the
Transport Services architecture. The Transport Services Application
Programming Interface (API) is based on an asynchronous, event-driven
interaction pattern; it pattern. It uses messages for representing data transfer
to applications; applications, and it assumes an implementation that can use
multiple IP addresses, multiple protocols, and multiple paths, and
provide multiple application streams. This document further defines
the
common set of terminology and concepts to be used in definitions of
Transport Services APIs and implementations.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 7, 25 June 2020.

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Please review these documents carefully, as they describe your rights
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3 4
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Specification of Requirements . . . . . . . . . . . . . . 5
2. API Model . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Event-Driven API . . . . . . . . . . . . . . . . . . . . 6 7
2.2. Data Transfer Using Messages . . . . . . . . . . . . . . 7
2.3. Flexibile Implementation . . . . . . . . . . . . . . . . 8
3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 8 9
3.1. Common APIs for Common Features . . . . . . . . . . . . . 9
3.2. Access to Specialized Features . . . . . . . . . . . . . 9
3.3. Scope for API and Implementation Definitions . . . . . . 10
4. Transport Services Architecture and Concepts . . . . . . . . 11
4.1. Transport Services API Concepts . . . . . . . . . . . . . 12
4.1.1. Connection Connections and Related Objects . . . . . . . . . . . . . . . . . 14
4.1.2. Pre-Establishment . . . . . . . . . . . . . . . . . . 15
4.1.3. Establishment Actions . . . . . . . . . . . . . . . . 16
4.1.4. Data Transfer Objects and Actions . . . . . . . . . . 17
4.1.5. Event Handling . . . . . . . . . . . . . . . . . . . 18
4.1.6. Termination Actions . . . . . . . . . . . . . . . . . 18
4.2. Transport System Implementation Concepts . . . . . . . . 18 19
4.2.1. Candidate Gathering . . . . . . . . . . . . . . . . . 20
4.2.2. Candidate Racing . . . . . . . . . . . . . . . . . . 20
4.2.3. Protocol Stack Equivalence . . . . . . . . . . . . . 20 21
4.2.4. Separating Connection Groups . . . . . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23 24
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Normative References . . . . . . . . . . . . . . . . . . 24
8.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 26

1. Introduction

Many application programming interfaces (APIs) to perform transport
networking have been deployed, perhaps the most widely known and
imitated being the BSD socket() Socket [POSIX] interface. 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() "send()" and recv()
"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 architecture is to provide a
common, flexible, and reusable interface for transport protocols. 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. It can also support
applications by offering racing and fallback mechanisms, which
otherwise need to be implemented in each application separately.

This document is 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 that all APIs and
implementations are identical, a common minimal set of features
represented in a consistent fashion will enable applications to be
easily ported from one system to another.

1.1. Background

The Transport Services architecture is based on the survey of
Services Provided
services provided by IETF Transport Protocols transport protocols and Congestion Control
Mechanisms congestion control
mechanisms [RFC8095], and the distilled minimal set of the features
offered by transport protocols [I-D.ietf-taps-minset]. 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 architecture also considers
the impact of transport security protocols on the feature-set exposed
by transport services [I-D.ietf-taps-transport-security].

One of the key insights to come from identifying the minimal set of
features provided by transport protocols [I-D.ietf-taps-minset] 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 a system implementing
Transport Services (referred to as Automatable Features). Among the
Functional and Optimizing Features, some were common across all or
nearly all transport protocols, while others could be seen as
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. However, 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 architecture in three
sections:

* Section 2 describes how the API model of Transport Services
differs from traditional socket-based APIs. Specifically, it
offers asynchronous event-driven interaction, the use of messages
for data transfer, and the ability to easily adopt different
transport protocols.

* Section 3 explains the design principles that guide behind the Transport
Services API. These principles are intended to make sure that
transport protocols can continue to be enhanced and evolve without
requiring too many changes by application developers.

* Section 4 presents the Transport Services architecture diagram and
defines the concepts that are used by both the API and
implementation documents. The Preconnection allows applications
to configure connection properties, and the Connection represents
an object that can be used to send and receive Messages.

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.

2. API Model

The traditional model of using sockets for networking can be
represented as follows:

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

* The socket 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
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 stub resolver.

+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
| |
+---------------------+ +-----------------------+ | Socket
+------------+ +------------+ +--------------+
| stub | | Stream API | | Socket Datagram API |
+---------------------+ +-----------------------+
| resolver |
+-----------------------------------------------------+ +------------+ +--------------+
+------------+ | |
+---------------------------------+
| TCP UDP |
| Kernel Protocol Implementation Networking Stack |
+-----------------------------------------------------+
+---------------------------------+
|
+-----------------------------------------------------+
| Network Layer Interface |
+-----------------------------------------------------+

Figure 1: socket() Socket API Model

The Transport Services architecture maintains evolves this general model of
interaction, but aims aiming to both modernize the API surface exposed for presented to
applications by the transport protocols layer and enrich the capabilities of
the transport system implementation. It combines interfaces for
multiple interaction patterns into a unified whole. 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.

+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services API |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport System Implementation |
| (UDP, (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
mechanism for an application to create network connections and
transfer data. The implementation [I-D.ietf-taps-impl] is
responsible for mapping the API to the various available transport
protocols 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 sockets API: it presents
an asynchronous, event-driven API; the
Transport Services API is asynchronous and event-driven; it uses
messages for representing data transfer to applications; applications, and it
assumes an implementation that can use multiple IP addresses,
multiple protocols, multiple paths, and provide multiple application
streams.

2.1. Event-Driven API

Originally, sockets presented a blocking interface for establishing
connections and transferring data. However, most modern applications
interact with the network asynchronously. When sockets are presented
as Emulation of an
asynchronous interface, they interface using sockets generally use uses 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.

All

In contrast to sockets, all interaction with a Transport Services
system is expected to be asynchronous, and use an event-driven model unlike sockets
(see Section 4.1.5. 4.1.5). For example, if the application wants to read,
its call to read will not fail, complete immediately, but will deliver an
event containing the received data once it is available. Error
handling is also asynchronous; a failure to send results in an
asynchronous send error as an event.

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

Using asynchronous events allows for a much simpler 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 sockets represent network resources as file system
objects that may be temporarily unavailable.

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

2.2. Data Transfer Using Messages

Sockets provide a message interface for datagram protocols like UDP,
but provide an unstructured stream abstraction for TCP. While TCP
does indeed provide the ability to send and receive data as streams,
most applications need to interpret structure within these streams.
For example, HTTP/1.1 uses character delimiters to segment messages
over a stream [RFC7230]; TLS record headers carry a version, content
type, and length [RFC8446]; and HTTP/2 uses frames to segment its
headers and bodies [RFC7540].

The Transport Services API represents data as messages, so that it
more closely matches the way applications use the network. Messages
seamlessly work with transport protocols that support datagrams or
records, but can also be used over a stream by defining an
application-layer framer Section 4.1.4. When framing protocols are
placed on top of unstructured streams, the messages used in the API
represent the framed messages within the stream. In the absence of a
framer, protocols that deal only in byte streams, such as TCP,
represent their data in each direction as a single, long message. Providing
a message-based abstraction provides many benefits, such as:

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

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

* the ability to manage dependencies between messages, when the
transport system could decide to not deliver a message, either
following packet loss or because it has missed a deadline. In
particular, this can avoid (re-)sending data that relies on a
previous transmission that was never received.

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

Allowing applications to interact with messages is backwards-
compatible with existings protocols and APIs, as it does not change
the wire format of any protocol. Instead, it gives the protocol
stack additional information to allow it to make better use of modern
transport services, while simplifying the application's role in
parsing data. For protocols which natively use a streaming
abstraction, framers (Section 4.1.4) bridge the gap between the two
abstractions.

2.3. Flexibile Implementation

Sockets, for protocols like TCP, are generally limited to connecting
to a single address over a single interface. They also present a
single stream to the application. Software layers built upon sockets
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; and support
multipath and multistreaming protocols.

Transport Services implementations are meant to be flexible at
connection establishment time, considering many different options and
trying to select the most optimal combinations (Section 4.2.1 and
Section 4.2.2). This requires applications to provide higher-level
endpoints than IP addresses, such as hostnames and URLs, which are
used by a Transport Services implementation for resolution, path
selection, and racing.

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.

3. Design Principles

The goal of the Transport Services 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.

There are several degrees in which a Transport Services system is
intended to offer flexibility to an application: it can provide
access to multiple sets of 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 API for the
system remains the same over time, new protocols and features could
be added to the system's implementation without requiring changes in
applications for adoption.

3.1. Common APIs for Common Features

Functionality that is common across multiple transport protocols
SHOULD be accessible through a unified set of API calls. An
application ought to be able to implement logic for its basic use of
transport networking (establishing the transport, and sending and
receiving data) once, and expect that implementation to continue to
function as the transports change.

Any Transport Services API is REQUIRED to allow access to the
distilled minimal set of features offered by transport protocols
[I-D.ietf-taps-minset].

3.2. 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. A Transport Services system
therefore SHOULD also permit more specialized protocol features to be
used. The interface for these specialized options ought to be
exposed differently from the common options to ensure flexibility.

A specialized feature could be required by an application only when
using a specific protocol, and not when using others. For example,
if an application is using UDP, it could require control over the
checksum or fragmentation behavior for UDP; if it used a protocol to
frame its data over a byte stream like TCP, it would not need these
options. 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 for
constrained checksum usage, to use a
partial checksum, communication would not fail when a protocol such
as TCP is selected, which uses a checksum covering the entire
payload.

Other specialized features, however, could be strictly required by an
application and thus constrain the set of protocols that can be used.
For example, if an application requires encryption of its transport
data, support for automatic
handover or failover for a Connection, only protocol stacks that include a transport security function
provide this feature are eligible to be used. used, e.g., protocol stacks
that include a multipath protocol or a protocol that supports
connection migration. A Transport Services API MUST allow
applications to define such requirements and constrain the system's
options. 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.

3.3. Scope for API and Implementation Definitions

The Transport Services API is envisioned as the abstract model for a
family of APIs that share a common way to expose transport features
and encourage flexibility. The abstract API definition
[I-D.ietf-taps-interface] describes this interface and how it can be
exposed to application developers.

Implementations that provide the Transport Services API
[I-D.ietf-taps-impl] will vary due to system-specific support and the
needs of the deployment scenario. It is expected that all
implementations of Transport Services will offer the entire mandatory
API. All implementations are REQUIRED to offer an API that is
sufficient to use the distilled minimal set of features offered by
transport protocols [I-D.ietf-taps-minset], including API support for
TCP and UDP transport. However, some features provided by this API
will not be functional in certain implementations. For example, it
is possible that some very constrained devices might not have a full
TCP implementation beneath the API.

To preserve flexibility and compatibility with future protocols, top-
level features in the Transport Services API SHOULD avoid referencing
particular transport protocols. The mappings of these API features
to specific implementations of each feature is explained in the
[I-D.ietf-taps-impl] along with the implications of the feature on
existing protocols. It is expected that [I-D.ietf-taps-interface]
will be updated and supplemented as new protocols and protocol
features are developed.

It is important to note that neither the Transport Services API
[I-D.ietf-taps-interface] nor the Implementation document
[I-D.ietf-taps-impl] define new protocols or protocol capabilities
that affect what is communicated across the network. The network: this implies
that a Transport Services system MUST be deployable on only one side only.
of a connection. A Transport Services system acting as a connection
initiator can communicate with any existing system that implements
the transport protocol(s) selected by the Transport Services system.
Similarly, a Transport Services system acting as a listener can
receive connections for any protocol that is supported by the system,
from existing initiators.

4. Transport Services Architecture and Concepts

The concepts defined in this document are intended primarily for use
in the documents and specifications that describe the Transport
Services architecture and API. 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 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 when building systems that implement Transport
Services.

The following diagram summarizes the top-level concepts in the
architecture and how they relate to one another.

+-----------------------------------------------------+
| Application |
+-+----------------+------^-------+--------^----------+
| | | | |
pre- | data | events
establishment | transfer | |
| establishment | termination |
| | | | |
| +--v------v-------v+ |
+-v-------------+ Connection(s) +-------+----------+
| Transport +--------+---------+ |
| Services | |
| API | |
+------------------------|----------------------------+
|
+------------------------|----------------------------+
| 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 Transport Services
Architecture

4.1. Transport Services API Concepts

Fundamentally, a Transport Services API needs to provide connection
objects (Section 4.1.1) that allow applications to establish
communication, and then send and receive data. These could be
exposed as handles or referenced objects, depending on the language.

Beyond the connection objects, there are several high-level groups of
actions that any Transport Services API implementing this
specification MUST provide:

* Pre-Establishment (Section 4.1.2) encompasses the properties that
an application can pass to describe its intent, requirements,
prohibitions, and preferences for its networking operations. For
any system that provides generic Transport Services, these
properties SHOULD be defined to apply to multiple transport protocols.
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 4.1.3) focuses on the actions that an
application takes on the connection objects to prepare for data
transfer.

* Data Transfer (Section 4.1.4) 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 (Section 4.1.5) defines the set categories of properties about notifications
which an application can receive notifications during the lifetime of transport
objects. Events MAY also provide opportunities for the
application to interact with the underlying transport by querying
state or updating maintenance options.

* Termination (Section 4.1.6) focuses on the methods by which data
transmission is stopped, and state is torn down in the transport.

The diagram below provides a high-level view of the actions and
events during the lifetime of a connection. 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), Pre-Establishment) or have been
omitted for brevity.

Pre-Establishment : Established : Termination
----------------- : ----------- : -----------
: :
+-- Local Endpoint : Message :
+-- Remote Endpoint : Receive() | :
+-- Transport Properties : Send() | :
| : | Close() :
| +---------------+ Initiate() +------------+ +-----+------+ Abort() :
+-->|
+---+ Preconnection |------------->| Connection |-----------> Closed
|
+---------------+ Rendezvous() +------------+ Conn. :
| : ^ | Finished :
+-- Local Endpoint : | | :
Listen() | : | | :
+-- Remote Endpoint
| : | v :
|
v : | Connection :
+-- Selection Properties
+----------+ : | Ready :
+-- Connection Properties : | :
+-- Message Properties :
| :
| : | :
| +----------+ : | :
+-->| Listener |----------------------+ :
+----------+ Connection Received :
^ : :
| : :
Listen()
: :

Figure 4: The lifetime of a connection

4.1.1. Connection Connections and Related Objects

* Preconnection: A Preconnection object is a representation of a
potential connection. It has state that describes parameters of a
Connection that might exist in the future: the Local Endpoint from
which that Connection will be established, the Remote Endpoint
(Section 4.1.2) to which it will connect, and Selection Transport Properties
(Section 4.1.2)
that influence the paths and protocols a Connection will use. A
Preconnection can be fully specified such that it represents a
single possible Connection, or it can be partially specified such
that it represents a family of possible Connections. The Local
Endpoint (Section 4.1.2) MUST be specified if the Preconnection is
used to Listen for incoming connections. The Local Endpoint is
OPTIONAL if it is used to Initiate connections. The Remote
Endpoint MUST be specified in the Preconnection that is used to
Initiate connections. The Remote Endpoint is OPTIONAL if it is
used to Listen for incoming connections. The Local Endpoint and
the Remote Endpoint MUST both be specified if a peer-to-peer
Rendezvous is to occur based on the Preconnection.

* Transport Properties: Transport Properties can be specified as
part of a Preconnection to allow the application
to configure the
Transport System and express their requirements, prohibitions, and
preferences. preferences and
configure the Transport System. There are three kinds of
Transport Properties:

- Selection Properties (Section 4.1.2)

* Connection Properties (Section 4.1.2)

* and Message Properties (Section 4.1.4); note that Message
Properties can also only be specified
on a Preconnection.

- Connection Properties (Section 4.1.2) that can be specified on
a Preconnection and changed on the Connection.

- Message Properties (Section 4.1.4) that can be specified as
defaults on a Preconnection or a Connection, and can also be
specified during data transfer to affect specific Messages.

* Connection: A Connection object represents one or more active
transport protocol instances that can send and/or receive Messages
between local and remote systems. It holds state pertaining to
the underlying transport protocol instances and any ongoing data
transfers. This represents, for example, an active connection in
a connection-oriented protocol such as TCP, or a fully-specified
5-tuple for a connectionless protocol such as UDP. It can also
represent a pool of transport protocol instance, instances, e.g., a set of
TCP and QUIC connections to equivalent endpoints, or a stream of a
multi-streaming transport protocol instance.

o Connections can be
created from a Preconnection or by a Listener.

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

4.1.2. Pre-Establishment

* Endpoint: An Endpoint represents an identifier for one side of a
transport connection. Endpoints can be Local Endpoints or Remote
Endpoints, and respectively represent an identity that the
application uses for the source or destination of a connection.
An Endpoint can be specified at various levels, levels of abstraction, and
an Endpoint
with wider scope at a higher level of abstraction (such as a hostname)
can be resolved to more concrete identities (such as IP
addresses).

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

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

* Selection Properties: The Selection Properties consist of the
options that an application can set to influence the selection of
paths between the local and remote systems, to influence the
selection of transport protocols, or to configure the behavior of
generic transport protocol features. These options can take the
form of requirements, prohibitions, or preferences. Examples of
options that influence path selection include the interface type
(such as a Wi-Fi Ethernet connection, or a Cellular LTE connection),
requirements around the Maximum Transmission Unit (MTU) or path
MTU (PMTU), or preferences for throughput and latency properties.
Examples of options that influence protocol selection and
configuration of transport protocol features include reliability,
service class, multipath support, and fast open support.

* Connection Properties: The Connection Properties are used to
configure protocol-specific options and control per-connection
behavior of the Transport System. For System; for example, a protocol-
specific protocol-specific
Connection Property can express that if UDP is used, the
implementation ought to use checksums. 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 MAY can be used in this way
by an implementation during connection establishment. Connection
Properties SHOULD be are specified on a Preconnection prior to Connection
establishment, but MAY and can be modified on the Connection later.
Changes made to Connection Properties after Connection
establishment take effect on a best-effort basis. Such changes do not affect protocol or path
selection, but only modify the manner in which a connection sends
and receives data.

4.1.3. 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.
state (e.g., with connectionless protocols such as UDP). The
process of identifying options for connecting, such as resolution
of the Remote Endpoint, occurs in response to the Initiate call.

* Listen: The action of marking Enables a Listener as willing listener to accept incoming Connections. The
Listener will then create Connection objects as incoming
connections are accepted (Section 4.1.5).

o Listeners by default
register with multiple paths, protocols, and local endpoints,
unless constrained by Selection Properties and/or the specified
Local Endpoint(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 whilst while listening for an incoming
connection from that endpoint. This corresponds, for example, to
a TCP simultaneous open [RFC0793]. The process of identifying options
for the connection, such as resolution of the Remote Endpoint,
occurs during the Rendezvous call. As with Listeners, the set of
local paths and endpoints is constrained by Selection Properties.
If successful, the
rendezvous Rendezvous call 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 [RFC0793] will
be performed. However, if the set of Local Endpoints includes
server reflexive candidates, such as those provided by STUN, a
Rendezvous action will race candidates in the style of the ICE
algorithm [RFC8445] to perform NAT binding discovery and initiate
a peer-to-peer connection.

4.1.4. 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 systems
over a transport connection. The bytes within a Message are
assumed to be ordered within the Message. 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. Boundaries of a

* Message might or might not be
understood or transmitted by transport protocols. Specifically,
what one application considers Properties: Message Properties are used to be two Messages sent on a
stream-based transport specify details
about Message transmission. They can be treated as a single Message by the
application specified directly on the other side.

o Message Properties: Message Properties
individual Messages, or can be used to annotate
specific Messages. 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), but can also include properties
that specific protocols encode and communicate to the Remote
Endpoint. Message Properties MAY be set on a Preconnection to
define defaults properties for sending. When receiving Messages, Message Properties can contain per-protocol properties for
properties that are sent between
information about the endpoints.

o received Message, such as metadata generated
at the receiver and information signalled by the remote endpoint.

* Send: The action to transmit a Message or partial Message over a Connection to the
remote system. The interface to Send MAY include Message
Properties specific to how the Message content is to be sent. The
status of the Send operation can MUST be delivered back to the sending
application in an event (Section 4.1.5).

* Receive: An action that indicates that the application is ready to
asynchronously accept a Message over a Connection from a remote
system, while the Message content itself will be delivered in an
event (Section 4.1.5). The interface to Receive MAY 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 to define how application-level application-layer Messages are
transmitted over a transport protocol. This is particularly
relevant for protocols that otherwise present unstructured
streams, such as TCP.

4.1.5. Event Handling

This section provides the top-level

The following categories of events events that can be delivered to an application. This list is not exhaustive.

o
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 Finished: Signals to an application that a given
Connection is no longer usable for sending or receiving Messages.
The event SHOULD deliver 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 passively received a Connection.

* Message Received: Delivers received Message content to the
application, based on a Receive action. This MAY include an error
if the Receive action cannot be satisfied 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 Message has been processed by the
protocol stack.

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

4.1.6. 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 system if the transport protocol allows this.

o 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. Indicating the end of a stream in
the Transport Services architecture is possible using Message
Properties when sending.)

* Abort: The action the application takes on a Connection to
indicate a Close and also indicate that the transport system
SHOULD NOT attempt to deliver any outstanding data. This is
intended for immediate termination of a connection, without
cleaning up state.

4.2. Transport System Implementation Concepts

This section defines the set of objects 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.

* Connection Group: A set of Connections that share properties and
caches. For multiplexing transport protocols, only Connections
within the same Connection Group are allowed to be multiplexed
together. An application can explicitly define Connection Groups
to control caching boundaries, as discussed in Section 4.2.4.

* Path: Represents an available set of properties that a local
system can use to communicate with a remote system, 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 over IP), or 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. 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, of which there can be several.
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: Represents 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.

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
Endpoints provided by the application, as well as the policies and
heuristics of a Transport Services system.

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

4.2.2. Candidate Racing

Connection establishment attempts for a set of candidates may be
performed simultaneously, synchronously, serially, or 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 for local
addresses and DNS servers, attempts across different Paths will
perform separate DNS resolution stepss, steps, which can lead to further
racing of the resolved Remote Endpoints.

* 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, such as IP addresses
resolved from a DNS hostname.

4.2.3. Protocol Stack Equivalence

The Transport Services architecture defines a mechanism that allows
applications to easily use different network paths and Protocol
Stacks. In some cases, changing which Protocol Stacks or network
paths are used will require updating the preferences expressed by the
application that uses the Transport Services system. For example, an
application can enable the use of a multipath or multistreaming
transport protocol by modifying the properties in its Pre-Connection
configuration. In some cases, however, the Transport Services system
will be able to automatically change Protocol Stacks without an
update to the application, either by selecting a new stack entirely,
or by racing multiple candidate Protocol Stacks during connection
establishment. This functionality in the API can be a powerful
driver of new protocol adoption, but needs to be constrained
carefully to avoid unexpected behavior that can lead to functional or
security problems.

If two different Protocol Stacks can be safely swapped, or raced in
parallel (see Section 4.2.2), then they are considered to be
"equivalent". Equivalent Protocol Stacks need to meet the following
criteria:

1. Both stacks MUST offer the same interface to requested by the application
for connection establishment and data transmission. For example,
if
one an application requires preservation of message boundaries, a
Protocol Stack has that runs UDP as the top-level interface to the
application, then it
application is not equivalent to a Protocol Stack that runs TCP
as the top-level interface. Among other differences,
the 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.

2. Both stacks MUST offer the transport services that are required requested
by the application. For example, if an 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. However, if the
application does not require reliability, 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.

3. Both stacks MUST offer the same security properties. The
inclusion of transport security protocols
[I-D.ietf-taps-transport-security] in a Protocol Stack adds
additional restrictions to Protocol Stack equivalence. and parameters as
requested by the application [I-D.ietf-taps-transport-security].
Security features and properties, such as cryptographic
algorithms, peer authentication, and identity privacy vary across
security protocols, and across versions of security protocols.
Protocol equivalence ought not to be assumed for different
protocols or protocol versions, even if they offer similar
application configuration options. To ensure that security
protocols are not incorrectly swapped, Transport Services systems
SHOULD only automatically generate equivalent Protocol Stacks
when the transport security protocols within the stacks are
identical. Specifically, a transport system would consider
protocols identical only if they are of the same type and
version. For example, the same version of TLS running over two
different transport protocol stacks are considered equivalent,
whereas TLS 1.2 and TLS 1.3 [RFC8446] are not considered
equivalent. However, Transport Services systems MAY allow
applications to indicate that they consider two different
transport protocols equivalent, e.g., to allow fallback to TLS
1.2 if TLS 1.3 is not available.

4.2.4. Separating Connection Groups

By default, all stored properties of the implementation are shared
within a process, implementation, such as cached
protocol state, cached path state, and heuristics. heuristics, may be shared
(e.g. across multiple connections in an application). This provides
efficiency and convenience for the application, since the Transport
System implementation can automatically optimize behavior.

There are several reasons, however, that an application might want to
explicitly isolate some Connections within a single process. Connections. These reasons include:

* Privacy concerns about re-using cached protocol state that can
lead to linkability. Sensitive state may include TLS session
state [RFC8446] and HTTP cookies [RFC6265].

* 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 (for example, when Connections
are multiplexed HTTP/2 or QUIC streams).

* Performance concerns about Connections introducing head-of-line
blocking due to multiplexing or needing to share state on a single
thread.

The Transport Services API SHOULD allow applications to explicitly
define Connection Groups that force separation of Cached State and
Protocol Stacks. For example, a web browser application might use
Connection Groups with separate caches for different tabs in the
browser to decrease linkability.

The interface to specify these groups MAY expose fine-grained tuning
for which properties and cached state is allowed to be shared with
other Connections. For example, an application might want to allow
sharing TCP Fast Open cookies across groups, but not TLS session
state.

5. IANA Considerations

RFC-EDITOR: Please remove this section before publication.

This document has no actions for IANA.

6. Security Considerations

The Transport Services architecture 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. Transport Services APIs will expose similar functionality
[I-D.ietf-taps-transport-security].

As described above in Section 4.2.3, if a Transport Services system
races between two different Protocol Stacks, both MUST use the same
security protocols and options.

Clients

Applications need to ensure that they use security APIs are used
appropriately. In cases where clients 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. For
example, clients applications ought not to use PSK material created for the
Encapsulating Security Protocol (ESP, part of IPsec) [RFC4303] with
QUIC, and clients applications ought not to use private keys intended for
server authentication as a keys for client authentication.

Moreover, Transport Services systems MUST NOT automatically fall back
from secure protocols to insecure protocols, or to weaker versions of
secure protocols. For example, if a client an application requests TLS, but
the desired version of TLS is not available, its connection will
fail.
Clients Applications are thus responsible for implementing security
protocol fallback or version fallback by creating multiple Transport
Services Connections, if so desired.

7. Acknowledgements

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

This work has been supported by Leibniz Prize project funds 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 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

[I-D.ietf-taps-interface]
Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
Kuehlewind, M., Perkins, C., Tiesel, P., Wood, C., and T.
Pauly, "An Abstract Application Layer Interface to
Transport Services", draft-ietf-taps-interface-04 (work Work in
progress), July 2019. Progress, Internet-Draft,
draft-ietf-taps-interface-05, 4 November 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-taps-
interface-05.txt>.

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

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

8.2. Informative References

[I-D.ietf-taps-impl]
Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
"Implementing Interfaces to Transport Services", draft-
ietf-taps-impl-04 (work Work in progress), July 2019.
Progress, Internet-Draft, draft-ietf-taps-impl-05, 4
November 2019, <http://www.ietf.org/internet-drafts/draft-
ietf-taps-impl-05.txt>.

[I-D.ietf-taps-minset]
Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for End Systems", draft-ietf-taps-minset-11 (work Work in progress), Progress, Internet-
Draft, draft-ietf-taps-minset-11, 27 September 2018. 2018,
<http://www.ietf.org/internet-drafts/draft-ietf-taps-
minset-11.txt>.

[I-D.ietf-taps-transport-security]
Wood, C.,
Enghardt, T., Pauly, T., Perkins, C., and K. Rose, K., and C.
Wood, "A Survey of Transport the Interaction Between Security Protocols", draft-
ietf-taps-transport-security-09 (work
Protocols and Transport Services", Work in progress),
September 2019. Progress,
Internet-Draft, draft-ietf-taps-transport-security-10, 17
November 2019, <http://www.ietf.org/internet-drafts/draft-
ietf-taps-transport-security-10.txt>.

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

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.

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

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.

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

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

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

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

Authors' Addresses

Tommy Pauly (editor)
Apple Inc.
One Apple Park Way
Cupertino, California 95014 95014,
United States of America

Email: tpauly@apple.com

Brian Trammell (editor)
Google
Gustav-Gull-Platz 1
CH- 8004 Zurich
Switzerland

Email: ietf@trammell.ch

Anna Brunstrom
Karlstad University
Universitetsgatan 2
SE- 651 88 Karlstad
Sweden

Email: anna.brunstrom@kau.se
Godred Fairhurst
University of Aberdeen
Fraser Noble Building
Aberdeen, AB24 3UE
Scotland

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

Philipp S. Tiesel
TU Berlin
Einsteinufer 25
10587 Berlin
Germany

Email: philipp@tiesel.net

Chris Wood
Apple Inc.
One Apple Park Way
Cupertino, California 95014 95014,
United States of America

Email: cawood@apple.com