draft-ietf-taps-arch-05.txt   draft-ietf-taps-arch-06.txt 
TAPS Working Group T. Pauly, Ed. TAPS Working Group T. Pauly, Ed.
Internet-Draft Apple Inc. Internet-Draft Apple Inc.
Intended status: Standards Track B. Trammell, Ed. Intended status: Standards Track B. Trammell, Ed.
Expires: May 7, 2020 Google Expires: 25 June 2020 Google
A. Brunstrom A. Brunstrom
Karlstad University Karlstad University
G. Fairhurst G. Fairhurst
University of Aberdeen University of Aberdeen
C. Perkins C. Perkins
University of Glasgow University of Glasgow
P. Tiesel P. Tiesel
TU Berlin TU Berlin
C. Wood C. Wood
Apple Inc. Apple Inc.
November 04, 2019 23 December 2019
An Architecture for Transport Services An Architecture for Transport Services
draft-ietf-taps-arch-05 draft-ietf-taps-arch-06
Abstract Abstract
This document provides an overview of the architecture of Transport This document describes an architecture for exposing transport
Services, a model for exposing transport protocol features to protocol features to applications for network communication, the
applications for network communication. In contrast to what is Transport Services architecture. The Transport Services Application
provided by most existing Application Programming Interfaces (APIs), Programming Interface (API) is based on an asynchronous, event-driven
Transport Services is based on an asynchronous, event-driven interaction pattern. It uses messages for representing data transfer
interaction pattern; it uses messages for representing data transfer to applications, and it assumes an implementation that can use
to applications; and it assumes an implementation that can use
multiple IP addresses, multiple protocols, and multiple paths, and multiple IP addresses, multiple protocols, and multiple paths, and
provide multiple application streams. This document further defines provide multiple application streams. This document further defines
the common set of terminology and concepts to be used in definitions common terminology and concepts to be used in definitions of
of Transport Services APIs and implementations. Transport Services APIs and implementations.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on May 7, 2020. This Internet-Draft will expire on 25 June 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Specification of Requirements . . . . . . . . . . . . . . 5 1.3. Specification of Requirements . . . . . . . . . . . . . . 5
2. API Model . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. API Model . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Event-Driven API . . . . . . . . . . . . . . . . . . . . 6 2.1. Event-Driven API . . . . . . . . . . . . . . . . . . . . 7
2.2. Data Transfer Using Messages . . . . . . . . . . . . . . 7 2.2. Data Transfer Using Messages . . . . . . . . . . . . . . 7
2.3. Flexibile Implementation . . . . . . . . . . . . . . . . 8 2.3. Flexibile Implementation . . . . . . . . . . . . . . . . 8
3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 8 3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Common APIs for Common Features . . . . . . . . . . . . . 9 3.1. Common APIs for Common Features . . . . . . . . . . . . . 9
3.2. Access to Specialized Features . . . . . . . . . . . . . 9 3.2. Access to Specialized Features . . . . . . . . . . . . . 9
3.3. Scope for API and Implementation Definitions . . . . . . 10 3.3. Scope for API and Implementation Definitions . . . . . . 10
4. Transport Services Architecture and Concepts . . . . . . . . 11 4. Transport Services Architecture and Concepts . . . . . . . . 11
4.1. Transport Services API Concepts . . . . . . . . . . . . . 12 4.1. Transport Services API Concepts . . . . . . . . . . . . . 12
4.1.1. Connection Objects . . . . . . . . . . . . . . . . . 14 4.1.1. Connections and Related Objects . . . . . . . . . . . 14
4.1.2. Pre-Establishment . . . . . . . . . . . . . . . . . . 15 4.1.2. Pre-Establishment . . . . . . . . . . . . . . . . . . 15
4.1.3. Establishment Actions . . . . . . . . . . . . . . . . 16 4.1.3. Establishment Actions . . . . . . . . . . . . . . . . 16
4.1.4. Data Transfer Objects and Actions . . . . . . . . . . 17 4.1.4. Data Transfer Objects and Actions . . . . . . . . . . 17
4.1.5. Event Handling . . . . . . . . . . . . . . . . . . . 18 4.1.5. Event Handling . . . . . . . . . . . . . . . . . . . 18
4.1.6. Termination Actions . . . . . . . . . . . . . . . . . 18 4.1.6. Termination Actions . . . . . . . . . . . . . . . . . 18
4.2. Transport System Implementation Concepts . . . . . . . . 18 4.2. Transport System Implementation Concepts . . . . . . . . 19
4.2.1. Candidate Gathering . . . . . . . . . . . . . . . . . 20 4.2.1. Candidate Gathering . . . . . . . . . . . . . . . . . 20
4.2.2. Candidate Racing . . . . . . . . . . . . . . . . . . 20 4.2.2. Candidate Racing . . . . . . . . . . . . . . . . . . 20
4.2.3. Protocol Stack Equivalence . . . . . . . . . . . . . 20 4.2.3. Protocol Stack Equivalence . . . . . . . . . . . . . 21
4.2.4. Separating Connection Groups . . . . . . . . . . . . 22 4.2.4. Separating Connection Groups . . . . . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Normative References . . . . . . . . . . . . . . . . . . 24 8.1. Normative References . . . . . . . . . . . . . . . . . . 24
8.2. Informative References . . . . . . . . . . . . . . . . . 24 8.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction 1. Introduction
Many application programming interfaces (APIs) to perform transport Many application programming interfaces (APIs) to perform transport
networking have been deployed, perhaps the most widely known and networking have been deployed, perhaps the most widely known and
imitated being the BSD socket() [POSIX] interface. The naming of imitated being the BSD Socket [POSIX] interface. The naming of
objects and functions across these APIs is not consistent, and varies objects and functions across these APIs is not consistent, and varies
depending on the protocol being used. For example, sending and depending on the protocol being used. For example, sending and
receiving streams of data is conceptually the same for both an receiving streams of data is conceptually the same for both an
unencrypted Transmission Control Protocol (TCP) stream and operating unencrypted Transmission Control Protocol (TCP) stream and operating
on an encrypted Transport Layer Security (TLS) [RFC8446] stream over on an encrypted Transport Layer Security (TLS) [RFC8446] stream over
TCP, but applications cannot use the same socket send() and recv() TCP, but applications cannot use the same socket "send()" and
calls on top of both kinds of connections. Similarly, terminology "recv()" calls on top of both kinds of connections. Similarly,
for the implementation of transport protocols varies based on the terminology for the implementation of transport protocols varies
context of the protocols themselves: terms such as "flow", "stream", based on the context of the protocols themselves: terms such as
"message", and "connection" can take on many different meanings. "flow", "stream", "message", and "connection" can take on many
This variety can lead to confusion when trying to understand the different meanings. This variety can lead to confusion when trying
similarities and differences between protocols, and how applications to understand the similarities and differences between protocols, and
can use them effectively. how applications can use them effectively.
The goal of the Transport Services architecture is to provide a The goal of the Transport Services architecture is to provide a
common, flexible, and reusable interface for transport protocols. As common, flexible, and reusable interface for transport protocols. As
applications adopt this interface, they will benefit from a wide set applications adopt this interface, they will benefit from a wide set
of transport features that can evolve over time, and ensure that the of transport features that can evolve over time, and ensure that the
system providing the interface can optimize its behavior based on the system providing the interface can optimize its behavior based on the
application requirements and network conditions, without requiring application requirements and network conditions, without requiring
changes to the applications. This flexibility enables faster changes to the applications. This flexibility enables faster
deployment of new features and protocols. It can also support deployment of new features and protocols. It can also support
applications by offering racing and fallback mechanisms, which applications by offering racing and fallback mechanisms, which
otherwise need to be implemented in each application separately. otherwise need to be implemented in each application separately.
This document is developed in parallel with the specification of the This document was developed in parallel with the specification of the
Transport Services API [I-D.ietf-taps-interface] and Implementation Transport Services API [I-D.ietf-taps-interface] and Implementation
Guidelines [I-D.ietf-taps-impl]. Although following the Transport Guidelines [I-D.ietf-taps-impl]. Although following the Transport
Services Architecture does not require that all APIs and Services Architecture does not require that all APIs and
implementations are identical, a common minimal set of features implementations are identical, a common minimal set of features
represented in a consistent fashion will enable applications to be represented in a consistent fashion will enable applications to be
easily ported from one system to another. easily ported from one system to another.
1.1. Background 1.1. Background
The Transport Services architecture is based on the survey of The Transport Services architecture is based on the survey of
Services Provided by IETF Transport Protocols and Congestion Control services provided by IETF transport protocols and congestion control
Mechanisms [RFC8095], and the distilled minimal set of the features mechanisms [RFC8095], and the distilled minimal set of the features
offered by transport protocols [I-D.ietf-taps-minset]. These offered by transport protocols [I-D.ietf-taps-minset]. These
documents identified common features and patterns across all documents identified common features and patterns across all
transport protocols developed thus far in the IETF. transport protocols developed thus far in the IETF.
Since transport security is an increasingly relevant aspect of using Since transport security is an increasingly relevant aspect of using
transport protocols on the Internet, this architecture also considers transport protocols on the Internet, this architecture also considers
the impact of transport security protocols on the feature-set exposed the impact of transport security protocols on the feature-set exposed
by transport services [I-D.ietf-taps-transport-security]. by transport services [I-D.ietf-taps-transport-security].
One of the key insights to come from identifying the minimal set of One of the key insights to come from identifying the minimal set of
features provided by transport protocols [I-D.ietf-taps-minset] was features provided by transport protocols [I-D.ietf-taps-minset] was
that features either require application interaction and guidance that features either require application interaction and guidance
(referred to as Functional or Optimizing Features), or else can be (referred to in that document as Functional or Optimizing Features),
handled automatically by a system implementing Transport Services or else can be handled automatically by a system implementing
(referred to as Automatable Features). Among the Functional and Transport Services (referred to as Automatable Features). Among the
Optimizing Features, some were common across all or nearly all Functional and Optimizing Features, some were common across all or
transport protocols, while others could be seen as features that, if nearly all transport protocols, while others could be seen as
specified, would only be useful with a subset of protocols, but would features that, if specified, would only be useful with a subset of
not harm the functionality of other protocols. For example, some protocols, but would not harm the functionality of other protocols.
protocols can deliver messages faster for applications that do not For example, some protocols can deliver messages faster for
require messages to arrive in the order in which they were sent. applications that do not require messages to arrive in the order in
However, this functionality needs to be explicitly allowed by the which they were sent. However, this functionality needs to be
application, since reordering messages would be undesirable in many explicitly allowed by the application, since reordering messages
cases. would be undesirable in many cases.
1.2. Overview 1.2. Overview
This document describes the Transport Services architecture in three This document describes the Transport Services architecture in three
sections: sections:
o Section 2 describes how the API model of Transport Services * Section 2 describes how the API model of Transport Services
differs from traditional socket-based APIs. Specifically, it differs from traditional socket-based APIs. Specifically, it
offers asynchronous event-driven interaction, the use of messages offers asynchronous event-driven interaction, the use of messages
for data transfer, and the ability to easily adopt different for data transfer, and the ability to easily adopt different
transport protocols. transport protocols.
o Section 3 explains the design principles that guide the Transport * Section 3 explains the design principles behind the Transport
Services API. These principles are intended to make sure that Services API. These principles are intended to make sure that
transport protocols can continue to be enhanced and evolve without transport protocols can continue to be enhanced and evolve without
requiring too many changes by application developers. requiring too many changes by application developers.
o Section 4 presents the Transport Services architecture diagram and * Section 4 presents the Transport Services architecture diagram and
defines the concepts that are used by both the API and defines the concepts that are used by both the API and
implementation documents. The Preconnection allows applications implementation documents. The Preconnection allows applications
to configure connection properties, and the Connection represents to configure connection properties, and the Connection represents
an object that can be used to send and receive Messages. an object that can be used to send and receive Messages.
1.3. Specification of Requirements 1.3. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. API Model 2. API Model
The traditional model of using sockets for networking can be The traditional model of using sockets for networking can be
represented as follows: represented as follows:
o Applications create connections and transfer data using the socket * Applications create connections and transfer data using the Socket
API. API.
o The socket API provides the interface to the implementations of * The Socket API provides the interface to the implementations of
TCP and UDP (typically implemented in the system's kernel). TCP and UDP (typically implemented in the system's kernel).
o TCP and UDP in the kernel send and receive data over the available * TCP and UDP in the kernel send and receive data over the available
network layer interfaces. 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 | | Application |
+-----------------------------------------------------+ +-----------------------------------------------------+
| | | | |
+---------------------+ +-----------------------+ +------------+ +------------+ +--------------+
| Socket Stream API | | Socket Datagram API | | stub | | Stream API | | Datagram API |
+---------------------+ +-----------------------+ | resolver | +------------+ +--------------+
| | +------------+ | |
+-----------------------------------------------------+ +---------------------------------+
| TCP UDP | | TCP UDP |
| Kernel Protocol Implementation | | Kernel Networking Stack |
+-----------------------------------------------------+ +---------------------------------+
| |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Network Layer Interface | | Network Layer Interface |
+-----------------------------------------------------+ +-----------------------------------------------------+
Figure 1: socket() API Model Figure 1: Socket API Model
The Transport Services architecture maintains this general model of The Transport Services architecture evolves this general model of
interaction, but aims to both modernize the API surface exposed for interaction, aiming to both modernize the API surface presented to
transport protocols and enrich the capabilities of the transport applications by the transport layer and enrich the capabilities of
system implementation. 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 | | Application |
+-----------------------------------------------------+ +-----------------------------------------------------+
| |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Transport Services API | | Transport Services API |
+-----------------------------------------------------+ +-----------------------------------------------------+
| |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Transport System Implementation | | Transport System Implementation |
| (UDP, TCP, SCTP, DCCP, TLS, QUIC, etc) | | (Using: DNS, UDP, TCP, SCTP, DCCP, TLS, QUIC, etc) |
+-----------------------------------------------------+ +-----------------------------------------------------+
| |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Network Layer Interface | | Network Layer Interface |
+-----------------------------------------------------+ +-----------------------------------------------------+
Figure 2: Transport Services API Model Figure 2: Transport Services API Model
The Transport Services API [I-D.ietf-taps-interface] defines the The Transport Services API [I-D.ietf-taps-interface] defines the
mechanism for an application to create network connections and mechanism for an application to create network connections and
transfer data. The implementation [I-D.ietf-taps-impl] is transfer data. The implementation [I-D.ietf-taps-impl] is
responsible for mapping the API to the various available transport responsible for mapping the API to the various available transport
protocols and managing the available network interfaces and paths. protocols and managing the available network interfaces and paths.
There are key differences between the architecture of the Transport There are key differences between the architecture of the Transport
Services system and the architecture of the sockets API: it presents Services system and the architecture of the sockets API: the
an asynchronous, event-driven API; it uses messages for representing Transport Services API is asynchronous and event-driven; it uses
data transfer to applications; and it assumes an implementation that messages for representing data transfer to applications, and it
can use multiple IP addresses, multiple protocols, multiple paths, assumes an implementation that can use multiple IP addresses,
and provide multiple application streams. multiple protocols, multiple paths, and provide multiple application
streams.
2.1. Event-Driven API 2.1. Event-Driven API
Originally, sockets presented a blocking interface for establishing Originally, sockets presented a blocking interface for establishing
connections and transferring data. However, most modern applications connections and transferring data. However, most modern applications
interact with the network asynchronously. When sockets are presented interact with the network asynchronously. Emulation of an
as an asynchronous interface, they generally use a try-and-fail asynchronous interface using sockets generally uses a try-and-fail
model. If the application wants to read, but data has not yet been 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 received from the peer, the call to read will fail. The application
then waits and can try again later. then waits and can try again later.
All interaction with a Transport Services system is expected to be In contrast to sockets, all interaction with a Transport Services
asynchronous, and use an event-driven model unlike sockets system is expected to be asynchronous, and use an event-driven model
Section 4.1.5. For example, if the application wants to read, its (see Section 4.1.5). For example, if the application wants to read,
call to read will not fail, but will deliver an event containing the its call to read will not complete immediately, but will deliver an
received data once it is available. 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 The Transport Services API also delivers events regarding the
lifetime of a connection and changes in the available network links, lifetime of a connection and changes in the available network links,
which were not previously made explicit in sockets. which were not previously made explicit in sockets.
Using asynchronous events allows for a much simpler interaction model Using asynchronous events allows for a more natural interaction model
when establishing connections and transferring data. Events in time when establishing connections and transferring data. Events in time
more closely reflect the nature of interactions over networks, as more closely reflect the nature of interactions over networks, as
opposed to how sockets represent network resources as file system opposed to how sockets represent network resources as file system
objects that may be temporarily unavailable. 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 2.2. Data Transfer Using Messages
Sockets provide a message interface for datagram protocols like UDP, Sockets provide a message interface for datagram protocols like UDP,
but provide an unstructured stream abstraction for TCP. While TCP but provide an unstructured stream abstraction for TCP. While TCP
does indeed provide the ability to send and receive data as streams, does indeed provide the ability to send and receive data as streams,
most applications need to interpret structure within these streams. most applications need to interpret structure within these streams.
For example, HTTP/1.1 uses character delimiters to segment messages For example, HTTP/1.1 uses character delimiters to segment messages
over a stream [RFC7230]; TLS record headers carry a version, content over a stream [RFC7230]; TLS record headers carry a version, content
type, and length [RFC8446]; and HTTP/2 uses frames to segment its type, and length [RFC8446]; and HTTP/2 uses frames to segment its
headers and bodies [RFC7540]. headers and bodies [RFC7540].
The Transport Services API represents data as messages, so that it The Transport Services API represents data as messages, so that it
more closely matches the way applications use the network. Messages more closely matches the way applications use the network. Providing
seamlessly work with transport protocols that support datagrams or a message-based abstraction provides many benefits, such as:
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:
o the ability to associate deadlines with messages, for applications * the ability to associate deadlines with messages, for applications
that care about timing; that care about timing;
o the ability to provide control of reliability, choosing which * the ability to provide control of reliability, choosing which
messages to retransmit in the event of packet loss, and how best messages to retransmit when there is packet loss, and how best to
to make use of the data that arrived; make use of the data that arrived;
o the ability to manage dependencies between messages, when the * the ability to manage dependencies between messages, when the
transport system could decide to not deliver a message, either transport system could decide to not deliver a message, either
following packet loss or because it has missed a deadline. In following packet loss or because it has missed a deadline. In
particular, this can avoid (re-)sending data that relies on a particular, this can avoid (re-)sending data that relies on a
previous transmission that was never received. previous transmission that was never received.
o the ability to automatically assign messages and connections to * the ability to automatically assign messages and connections to
underlaying transport connections to utilize multi-streaming and underlying transport connections to utilize multi-streaming and
pooled connections. pooled connections.
Allowing applications to interact with messages is backwards- Allowing applications to interact with messages is backwards-
compatible with existings protocols and APIs, as it does not change compatible with existings protocols and APIs, as it does not change
the wire format of any protocol. Instead, it gives the protocol the wire format of any protocol. Instead, it gives the protocol
stack additional information to allow it to make better use of modern stack additional information to allow it to make better use of modern
transport services, while simplifying the application's role in transport services, while simplifying the application's role in
parsing data. 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 2.3. Flexibile Implementation
Sockets, for protocols like TCP, are generally limited to connecting Sockets, for protocols like TCP, are generally limited to connecting
to a single address over a single interface. They also present a to a single address over a single interface. They also present a
single stream to the application. Software layers built upon sockets single stream to the application. Software layers built upon sockets
often propagate this limitation of a single-address single-stream often propagate this limitation of a single-address single-stream
model. The Transport Services architecture is designed to handle model. The Transport Services architecture is designed to handle
multiple candidate endpoints, protocols, and paths; and support multiple candidate endpoints, protocols, and paths; and support
multipath and multistreaming protocols. multipath and multistreaming protocols.
skipping to change at page 9, line 41 skipping to change at page 10, line 8
exposed differently from the common options to ensure flexibility. exposed differently from the common options to ensure flexibility.
A specialized feature could be required by an application only when A specialized feature could be required by an application only when
using a specific protocol, and not when using others. For example, using a specific protocol, and not when using others. For example,
if an application is using UDP, it could require control over the if an application is using UDP, it could require control over the
checksum or fragmentation behavior for UDP; if it used a protocol to 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 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 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, 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, but do not imply that only that protocol could be used. For example,
if the API allows an application to specify a preference for if the API allows an application to specify a preference to use a
constrained checksum usage, communication would not fail when a partial checksum, communication would not fail when a protocol such
protocol such as TCP is selected, which uses a checksum covering the as TCP is selected, which uses a checksum covering the entire
entire payload. payload.
Other specialized features, however, could be strictly required by an Other specialized features, however, could be strictly required by an
application and thus constrain the set of protocols that can be used. application and thus constrain the set of protocols that can be used.
For example, if an application requires encryption of its transport For example, if an application requires support for automatic
data, only protocol stacks that include a transport security function handover or failover for a Connection, only protocol stacks that
are eligible to be used. A Transport Services API MUST allow 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 MUST allow
applications to define such requirements and constrain the system's applications to define such requirements and constrain the system's
options. Since such options are not part of the core/common options. Since such options are not part of the core/common
features, it will generally be simple for an application to modify features, it will generally be simple for an application to modify
its set of constraints and change the set of allowable protocol its set of constraints and change the set of allowable protocol
features without changing the core implementation. features without changing the core implementation.
3.3. Scope for API and Implementation Definitions 3.3. Scope for API and Implementation Definitions
The Transport Services API is envisioned as the abstract model for a The Transport Services API is envisioned as the abstract model for a
family of APIs that share a common way to expose transport features family of APIs that share a common way to expose transport features
skipping to change at page 10, line 41 skipping to change at page 11, line 10
particular transport protocols. The mappings of these API features particular transport protocols. The mappings of these API features
to specific implementations of each feature is explained in the to specific implementations of each feature is explained in the
[I-D.ietf-taps-impl] along with the implications of the feature on [I-D.ietf-taps-impl] along with the implications of the feature on
existing protocols. It is expected that [I-D.ietf-taps-interface] existing protocols. It is expected that [I-D.ietf-taps-interface]
will be updated and supplemented as new protocols and protocol will be updated and supplemented as new protocols and protocol
features are developed. features are developed.
It is important to note that neither the Transport Services API It is important to note that neither the Transport Services API
[I-D.ietf-taps-interface] nor the Implementation document [I-D.ietf-taps-interface] nor the Implementation document
[I-D.ietf-taps-impl] define new protocols or protocol capabilities [I-D.ietf-taps-impl] define new protocols or protocol capabilities
that affect what is communicated across the network. The Transport that affect what is communicated across the network: this implies
Services system MUST be deployable on one side only. A Transport that a Transport Services system MUST be deployable on only one side
Services system acting as a connection initiator can communicate with of a connection. A Transport Services system acting as a connection
any existing system that implements the transport protocol(s) initiator can communicate with any existing system that implements
selected by the Transport Services system. Similarly, a Transport the transport protocol(s) selected by the Transport Services system.
Services system acting as a listener can receive connections for any Similarly, a Transport Services system acting as a listener can
protocol that is supported by the system, from existing initiators. receive connections for any protocol that is supported by the system,
from existing initiators.
4. Transport Services Architecture and Concepts 4. Transport Services Architecture and Concepts
The concepts defined in this document are intended primarily for use The concepts defined in this document are intended primarily for use
in the documents and specifications that describe the Transport in the documents and specifications that describe the Transport
Services architecture and API. While the specific terminology can be Services architecture and API. While the specific terminology can be
used in some implementations, it is expected that there will remain a used in some implementations, it is expected that there will remain a
variety of terms used by running code. variety of terms used by running code.
The architecture divides the concepts for Transport Services into two The architecture divides the concepts for Transport Services into two
skipping to change at page 12, line 37 skipping to change at page 12, line 37
| (Candidate Racing) | +-----------------+ | | (Candidate Racing) | +-----------------+ |
| | | System | | | | | System | |
| | | Policy | | | | | Policy | |
| +----------v-----+ +-----------------+ | | +----------v-----+ +-----------------+ |
| | Protocol | | | | Protocol | |
+-------------+ Stack(s) +----------------------+ +-------------+ Stack(s) +----------------------+
+-------+--------+ +-------+--------+
V V
Network Layer Interface Network Layer Interface
Figure 3: Concepts and Relationships in the Transport Services Figure 3: Concepts and Relationships in the Transport Services
Architecture Architecture
4.1. Transport Services API Concepts 4.1. Transport Services API Concepts
Fundamentally, a Transport Services API needs to provide connection Fundamentally, a Transport Services API needs to provide connection
objects (Section 4.1.1) that allow applications to establish objects (Section 4.1.1) that allow applications to establish
communication, and then send and receive data. These could be communication, and then send and receive data. These could be
exposed as handles or referenced objects, depending on the language. exposed as handles or referenced objects, depending on the language.
Beyond the connection objects, there are several high-level groups of Beyond the connection objects, there are several high-level groups of
actions that any Transport Services API implementing this actions that any Transport Services API implementing this
specification MUST provide: specification MUST provide:
o Pre-Establishment (Section 4.1.2) encompasses the properties that * Pre-Establishment (Section 4.1.2) encompasses the properties that
an application can pass to describe its intent, requirements, an application can pass to describe its intent, requirements,
prohibitions, and preferences for its networking operations. For prohibitions, and preferences for its networking operations. For
any system that provides generic Transport Services, these any system that provides generic Transport Services, these
properties SHOULD be defined to apply to multiple transport properties SHOULD apply to multiple transport protocols.
protocols. Properties specified during Pre-Establishment can have Properties specified during Pre-Establishment can have a large
a large impact on the rest of the interface: they modify how impact on the rest of the interface: they modify how establishment
establishment occurs, they influence the expectations around data occurs, they influence the expectations around data transfer, and
transfer, and they determine the set of events that will be they determine the set of events that will be supported.
supported.
o Establishment (Section 4.1.3) focuses on the actions that an * Establishment (Section 4.1.3) focuses on the actions that an
application takes on the connection objects to prepare for data application takes on the connection objects to prepare for data
transfer. transfer.
o Data Transfer (Section 4.1.4) consists of how an application * Data Transfer (Section 4.1.4) consists of how an application
represents the data to be sent and received, the functions represents the data to be sent and received, the functions
required to send and receive that data, and how the application is required to send and receive that data, and how the application is
notified of the status of its data transfer. notified of the status of its data transfer.
o Event Handling (Section 4.1.5) defines the set of properties about * Event Handling (Section 4.1.5) defines categories of notifications
which an application can receive notifications during the lifetime which an application can receive during the lifetime of transport
of transport objects. Events MAY also provide opportunities for objects. Events MAY also provide opportunities for the
the application to interact with the underlying transport by application to interact with the underlying transport by querying
querying state or updating maintenance options. state or updating maintenance options.
o Termination (Section 4.1.6) focuses on the methods by which data * Termination (Section 4.1.6) focuses on the methods by which data
transmission is stopped, and state is torn down in the transport. transmission is stopped, and state is torn down in the transport.
The diagram below provides a high-level view of the actions and The diagram below provides a high-level view of the actions and
events during the lifetime of a connection. Note that some actions events during the lifetime of a connection. Note that some actions
are alternatives (e.g., whether to initiate a connection or to listen are alternatives (e.g., whether to initiate a connection or to listen
for incoming connections), others are optional (e.g., setting for incoming connections), while others are optional (e.g., setting
Connection and Message Properties in Pre-Establishment), or have been Connection and Message Properties in Pre-Establishment) or have been
omitted for brevity. omitted for brevity.
Pre-Establishment : Established : Termination Pre-Establishment : Established : Termination
----------------- : ----------- : ----------- ----------------- : ----------- : -----------
: Close() : : :
+---------------+ Initiate() +------------+ Abort() : +-- Local Endpoint : Message :
+-->| Preconnection |------------->| Connection |-----------> Closed +-- Remote Endpoint : Receive() | :
| +---------------+ Rendezvous() +------------+ Conn. : +-- Transport Properties : Send() | :
| : ^ | Finished : | : | Close() :
+-- Local Endpoint : | | : | +---------------+ Initiate() +-----+------+ Abort() :
| : | | : +---+ Preconnection |------------->| Connection |-----------> Closed
+-- Remote Endpoint : | v : +---------------+ Rendezvous() +------------+ Conn. :
| : | Connection : | : ^ | Finished :
+-- Selection Properties : | Ready : Listen() | : | | :
+-- Connection Properties : | : | : | v :
+-- Message Properties : | : v : | Connection :
| : | : +----------+ : | Ready :
| +----------+ : | : | Listener |----------------------+ :
+-->| Listener |----------------------+ : +----------+ Connection Received :
+----------+ Connection Received : : :
^ : :
| : :
Listen() : :
Figure 4: The lifetime of a connection Figure 4: The lifetime of a connection
4.1.1. Connection Objects 4.1.1. Connections and Related Objects
o Preconnection: A Preconnection object is a representation of a * Preconnection: A Preconnection object is a representation of a
potential connection. It has state that describes parameters of a potential connection. It has state that describes parameters of a
Connection that might exist in the future: the Local Endpoint from Connection that might exist in the future: the Local Endpoint from
which that Connection will be established, the Remote Endpoint which that Connection will be established, the Remote Endpoint
(Section 4.1.2) to which it will connect, and Selection Properties (Section 4.1.2) to which it will connect, and Transport Properties
(Section 4.1.2) that influence the paths and protocols a that influence the paths and protocols a Connection will use. A
Connection will use. A Preconnection can be fully specified such Preconnection can be fully specified such that it represents a
that it represents a single possible Connection, or it can be single possible Connection, or it can be partially specified such
partially specified such that it represents a family of possible that it represents a family of possible Connections. The Local
Connections. The Local Endpoint (Section 4.1.2) MUST be specified Endpoint (Section 4.1.2) MUST be specified if the Preconnection is
if the Preconnection is used to Listen for incoming connections. used to Listen for incoming connections. The Local Endpoint is
The Local Endpoint is OPTIONAL if it is used to Initiate OPTIONAL if it is used to Initiate connections. The Remote
connections. The Remote Endpoint MUST be specified in the Endpoint MUST be specified in the Preconnection that is used to
Preconnection that is used to Initiate connections. The Remote Initiate connections. The Remote Endpoint is OPTIONAL if it is
Endpoint is OPTIONAL if it is used to Listen for incoming used to Listen for incoming connections. The Local Endpoint and
connections. The Local Endpoint and the Remote Endpoint MUST both the Remote Endpoint MUST both be specified if a peer-to-peer
be specified if a peer-to-peer Rendezvous is to occur based on the Rendezvous is to occur based on the Preconnection.
Preconnection.
o Transport Properties: Transport Properties can be specified as * Transport Properties: Transport Properties allow the application
part of a Preconnection to allow the application to configure the to express their requirements, prohibitions, and preferences and
Transport System and express their requirements, prohibitions, and configure the Transport System. There are three kinds of
preferences. There are three kinds of Transport Properties: Transport Properties:
* Selection Properties (Section 4.1.2) - Selection Properties (Section 4.1.2) that can only be specified
on a Preconnection.
* Connection Properties (Section 4.1.2) - Connection Properties (Section 4.1.2) that can be specified on
a Preconnection and changed on the Connection.
* and Message Properties (Section 4.1.4); note that Message - Message Properties (Section 4.1.4) that can be specified as
Properties can also be specified during data transfer to affect defaults on a Preconnection or a Connection, and can also be
specific Messages. specified during data transfer to affect specific Messages.
o Connection: A Connection object represents one or more active * Connection: A Connection object represents one or more active
transport protocol instances that can send and/or receive Messages transport protocol instances that can send and/or receive Messages
between local and remote systems. It holds state pertaining to between local and remote systems. It holds state pertaining to
the underlying transport protocol instances and any ongoing data the underlying transport protocol instances and any ongoing data
transfers. This represents, for example, an active connection in transfers. This represents, for example, an active connection in
a connection-oriented protocol such as TCP, or a fully-specified a connection-oriented protocol such as TCP, or a fully-specified
5-tuple for a connectionless protocol such as UDP. It can also 5-tuple for a connectionless protocol such as UDP. It can also
represent a pool of transport protocol instance, e.g., a set of represent a pool of transport protocol instances, e.g., a set of
TCP and QUIC connections to equivalent endpoints, or a stream of a TCP and QUIC connections to equivalent endpoints, or a stream of a
multi-streaming transport protocol instance. multi-streaming transport protocol instance. Connections can be
created from a Preconnection or by a Listener.
o Listener: A Listener object accepts incoming transport protocol * Listener: A Listener object accepts incoming transport protocol
connections from remote systems and generates corresponding connections from remote systems and generates corresponding
Connection objects. It is created from a Preconnection object Connection objects. It is created from a Preconnection object
that specifies the type of incoming connections it will accept. that specifies the type of incoming connections it will accept.
4.1.2. Pre-Establishment 4.1.2. Pre-Establishment
o Endpoint: An Endpoint represents an identifier for one side of a * Endpoint: An Endpoint represents an identifier for one side of a
transport connection. Endpoints can be Local Endpoints or Remote transport connection. Endpoints can be Local Endpoints or Remote
Endpoints, and respectively represent an identity that the Endpoints, and respectively represent an identity that the
application uses for the source or destination of a connection. application uses for the source or destination of a connection.
An Endpoint can be specified at various levels, and an Endpoint An Endpoint can be specified at various levels of abstraction, and
with wider scope (such as a hostname) can be resolved to more an Endpoint at a higher level of abstraction (such as a hostname)
concrete identities (such as IP addresses). can be resolved to more concrete identities (such as IP
addresses).
o Remote Endpoint: The Remote Endpoint represents the application's * Remote Endpoint: The Remote Endpoint represents the application's
identifier for a peer that can participate in a transport identifier for a peer that can participate in a transport
connection. For example, the combination of a DNS name for the connection; for example, the combination of a DNS name for the
peer and a service name/port. peer and a service name/port.
o Local Endpoint: The Local Endpoint represents the application's * Local Endpoint: The Local Endpoint represents the application's
identifier for itself that it uses for transport connections. For identifier for itself that it uses for transport connections; for
example, a local IP address and port. example, a local IP address and port.
o Selection Properties: The Selection Properties consist of the * Selection Properties: The Selection Properties consist of the
options that an application can set to influence the selection of options that an application can set to influence the selection of
paths between the local and remote systems, to influence the paths between the local and remote systems, to influence the
selection of transport protocols, or to configure the behavior of selection of transport protocols, or to configure the behavior of
generic transport protocol features. These options can take the generic transport protocol features. These options can take the
form of requirements, prohibitions, or preferences. Examples of form of requirements, prohibitions, or preferences. Examples of
options that influence path selection include the interface type options that influence path selection include the interface type
(such as a Wi-Fi Ethernet connection, or a Cellular LTE (such as a Wi-Fi connection, or a Cellular LTE connection),
connection), requirements around the Maximum Transmission Unit requirements around the Maximum Transmission Unit (MTU) or path
(MTU) or path MTU (PMTU), or preferences for throughput and MTU (PMTU), or preferences for throughput and latency properties.
latency properties. Examples of options that influence protocol Examples of options that influence protocol selection and
selection and configuration of transport protocol features include configuration of transport protocol features include reliability,
reliability, service class, multipath support, and fast open service class, multipath support, and fast open support.
support.
o Connection Properties: The Connection Properties are used to * Connection Properties: The Connection Properties are used to
configure protocol-specific options and control per-connection configure protocol-specific options and control per-connection
behavior of the Transport System. For example, a protocol- behavior of the Transport System; for example, a protocol-specific
specific Connection Property can express that if UDP is used, the Connection Property can express that if UDP is used, the
implementation ought to use checksums. Note that the presence of implementation ought to use checksums. Note that the presence of
such a property does not require that a specific protocol will be such a property does not require that a specific protocol will be
used. In general, these properties do not explicitly determine used. In general, these properties do not explicitly determine
the selection of paths or protocols, but MAY be used in this way the selection of paths or protocols, but can be used in this way
by an implementation during connection establishment. Connection by an implementation during connection establishment. Connection
Properties SHOULD be specified on a Preconnection prior to Properties are specified on a Preconnection prior to Connection
Connection establishment, but MAY be modified later. Changes made establishment, and can be modified on the Connection later.
to Connection Properties after establishment take effect on a Changes made to Connection Properties after Connection
best-effort basis. Such changes do not affect protocol or path establishment take effect on a best-effort basis.
selection, but only modify the manner in which a connection sends
and receives data.
4.1.3. Establishment Actions 4.1.3. Establishment Actions
o Initiate: The primary action that an application can take to * Initiate: The primary action that an application can take to
create a Connection to a Remote Endpoint, and prepare any required create a Connection to a Remote Endpoint, and prepare any required
local or remote state to enable the transmission of Messages. For local or remote state to enable the transmission of Messages. For
some protocols, this will initiate a client-to-server style some protocols, this will initiate a client-to-server style
handshake; for other protocols, this will just establish local handshake; for other protocols, this will just establish local
state. The process of identifying options for connecting, such as state (e.g., with connectionless protocols such as UDP). The
resolution of the Remote Endpoint, occurs in response the Initiate process of identifying options for connecting, such as resolution
call. of the Remote Endpoint, occurs in response to the Initiate call.
o Listen: The action of marking a Listener as willing to accept * Listen: Enables a listener to accept incoming Connections. The
incoming Connections. The Listener will then create Connection Listener will then create Connection objects as incoming
objects as incoming connections are accepted (Section 4.1.5). connections are accepted (Section 4.1.5). 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.
o Rendezvous: The action of establishing a peer-to-peer connection * Rendezvous: The action of establishing a peer-to-peer connection
with a Remote Endpoint. It simultaneously attempts to initiate a with a Remote Endpoint. It simultaneously attempts to initiate a
connection to a Remote Endpoint whilst listening for an incoming connection to a Remote Endpoint while listening for an incoming
connection from that endpoint. This corresponds, for example, to connection from that endpoint. The process of identifying options
a TCP simultaneous open [RFC0793]. The process of identifying for the connection, such as resolution of the Remote Endpoint,
options for the connection, such as resolution of the Remote occurs during the Rendezvous call. As with Listeners, the set of
Endpoint, occurs during the Rendezvous call. If successful, the local paths and endpoints is constrained by Selection Properties.
rendezvous call returns a Connection object to represent the If successful, the Rendezvous call returns a Connection object to
established peer-to-peer connection. 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 4.1.4. Data Transfer Objects and Actions
o Message: A Message object is a unit of data that can be * Message: A Message object is a unit of data that can be
represented as bytes that can be transferred between two systems represented as bytes that can be transferred between two systems
over a transport connection. The bytes within a Message are over a transport connection. The bytes within a Message are
assumed to be ordered within the Message. If an application does assumed to be ordered within the Message. If an application does
not care about the order in which a peer receives two distinct not care about the order in which a peer receives two distinct
spans of bytes, those spans of bytes are considered independent spans of bytes, those spans of bytes are considered independent
Messages. Boundaries of a Message might or might not be Messages.
understood or transmitted by transport protocols. Specifically,
what one application considers to be two Messages sent on a
stream-based transport can be treated as a single Message by the
application on the other side.
o Message Properties: Message Properties can be used to annotate * Message Properties: Message Properties are used to specify details
specific Messages. These properties might only apply to how about Message transmission. They can be specified directly on
Message is sent (such as how the transport will treat individual Messages, or can be set on a Preconnection or
Connection as defaults. These properties might only apply to how
a Message is sent (such as how the transport will treat
prioritization and reliability), but can also include properties prioritization and reliability), but can also include properties
that specific protocols encode and communicate to the Remote that specific protocols encode and communicate to the Remote
Endpoint. Message Properties MAY be set on a Preconnection to Endpoint. When receiving Messages, Message Properties can contain
define defaults properties for sending. When receiving Messages, information about the received Message, such as metadata generated
Message Properties can contain per-protocol properties for at the receiver and information signalled by the remote endpoint.
properties that are sent between the endpoints.
o Send: The action to transmit a Message or partial Message over a * Send: The action to transmit a Message over a Connection to the
Connection to the remote system. The interface to Send MAY remote system. The interface to Send MAY include Message
include Message Properties specific to how the Message content is Properties specific to how the Message content is to be sent. The
to be sent. The status of the Send operation can be delivered status of the Send operation MUST be delivered back to the sending
back to the sending application in an event (Section 4.1.5). application in an event (Section 4.1.5).
o Receive: An action that indicates that the application is ready to * Receive: An action that indicates that the application is ready to
asynchronously accept a Message over a Connection from a remote asynchronously accept a Message over a Connection from a remote
system, while the Message content itself will be delivered in an system, while the Message content itself will be delivered in an
event (Section 4.1.5). The interface to Receive MAY include event (Section 4.1.5). The interface to Receive MAY include
Message Properties specific to the Message that is to be delivered Message Properties specific to the Message that is to be delivered
to the application. to the application.
o Framer: A Framer is a data translation layer that can be added to * Framer: A Framer is a data translation layer that can be added to
a Connection to define how application-level Messages are a Connection to define how application-layer Messages are
transmitted over a transport protocol. This is particularly transmitted over a transport protocol. This is particularly
relevant for protocols that otherwise present unstructured relevant for protocols that otherwise present unstructured
streams, such as TCP. streams, such as TCP.
4.1.5. Event Handling 4.1.5. Event Handling
This section provides the top-level categories of events events that The following categories of events can be delivered to an
can be delivered to an application. This list is not exhaustive. application:
o Connection Ready: Signals to an application that a given * Connection Ready: Signals to an application that a given
Connection is ready to send and/or receive Messages. If the Connection is ready to send and/or receive Messages. If the
Connection relies on handshakes to establish state between peers, Connection relies on handshakes to establish state between peers,
then it is assumed that these steps have been taken. then it is assumed that these steps have been taken.
o Connection Finished: Signals to an application that a given * Connection Finished: Signals to an application that a given
Connection is no longer usable for sending or receiving Messages. Connection is no longer usable for sending or receiving Messages.
The event SHOULD deliver a reason or error to the application that The event SHOULD deliver a reason or error to the application that
describes the nature of the termination. describes the nature of the termination.
o Connection Received: Signals to an application that a given * Connection Received: Signals to an application that a given
Listener has passively received a Connection. Listener has received a Connection.
o Message Received: Delivers received Message content to the * Message Received: Delivers received Message content to the
application, based on a Receive action. This MAY include an error application, based on a Receive action. This MAY include an error
if the Receive action cannot be satisfied due to the Connection if the Receive action cannot be satisfied due to the Connection
being closed. being closed.
o Message Sent: Notifies the application of the status of its Send * Message Sent: Notifies the application of the status of its Send
action. This might indicate a failure if the Message cannot be action. This might indicate a failure if the Message cannot be
sent, or an indication that Message has been processed by the sent, or an indication that Message has been processed by the
protocol stack. protocol stack.
o Path Properties Changed: Notifies the application that some * Path Properties Changed: Notifies the application that some
property of the Connection has changed that might influence how property of the Connection has changed that might influence how
and where data is sent and/or received. and where data is sent and/or received.
4.1.6. Termination Actions 4.1.6. Termination Actions
o Close: The action an application takes on a Connection to indicate * Close: The action an application takes on a Connection to indicate
that it no longer intends to send data, is no longer willing to that it no longer intends to send data, is no longer willing to
receive data, and that the protocol SHOULD signal this state to receive data, and that the protocol SHOULD signal this state to
the remote system if the transport protocol allows this. the remote system 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. Indicating the end of a stream in
the Transport Services architecture is possible using Message
Properties when sending.)
o Abort: The action the application takes on a Connection to * Abort: The action the application takes on a Connection to
indicate a Close and also indicate that the transport system indicate a Close and also indicate that the transport system
SHOULD NOT attempt to deliver any outstanding data. 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 4.2. Transport System Implementation Concepts
This section defines the set of objects used internally to a system This section defines the set of objects used internally to a system
or library to implement the functionality needed to provide a or library to implement the functionality needed to provide a
transport service across a network, as required by the abstract transport service across a network, as required by the abstract
interface. interface.
o Connection Group: A set of Connections that share properties and * Connection Group: A set of Connections that share properties and
caches. For multiplexing transport protocols, only Connections caches. For multiplexing transport protocols, only Connections
within the same Connection Group are allowed to be multiplexed within the same Connection Group are allowed to be multiplexed
together. An application can explicitly define Connection Groups together. An application can explicitly define Connection Groups
to control caching boundaries, as discussed in Section 4.2.4. to control caching boundaries, as discussed in Section 4.2.4.
o Path: Represents an available set of properties that a local * Path: Represents an available set of properties that a local
system can use to communicate with a remote system, such as system can use to communicate with a remote system, such as
routes, addresses, and physical and virtual network interfaces. routes, addresses, and physical and virtual network interfaces.
o Protocol Instance: A single instance of one protocol, including * Protocol Instance: A single instance of one protocol, including
any state necessary to establish connectivity or send and receive any state necessary to establish connectivity or send and receive
Messages. Messages.
o Protocol Stack: A set of Protocol Instances (including relevant * Protocol Stack: A set of Protocol Instances (including relevant
application, security, transport, or Internet protocols) that are application, security, transport, or Internet protocols) that are
used together to establish connectivity or send and receive used together to establish connectivity or send and receive
Messages. A single stack can be simple (a single transport Messages. A single stack can be simple (a single transport
protocol instance over IP), or complex (multiple application protocol instance over IP), or complex (multiple application
protocol streams going through a single security and transport protocol streams going through a single security and transport
protocol, over IP; or, a multi-path transport protocol over protocol, over IP; or, a multi-path transport protocol over
multiple transport sub-flows). multiple transport sub-flows).
o Candidate Path: One path that is available to an application and * Candidate Path: One path that is available to an application and
conforms to the Selection Properties and System Policy. Candidate conforms to the Selection Properties and System Policy. Candidate
Paths are identified during the gathering phase (Section 4.2.1) Paths are identified during the gathering phase (Section 4.2.1)
and can be used during the racing phase (Section 4.2.2). and can be used during the racing phase (Section 4.2.2).
o Candidate Protocol Stack: One protocol stack that can be used by * Candidate Protocol Stack: One protocol stack that can be used by
an application for a connection, of which there can be several. an application for a connection, of which there can be several.
Candidate Protocol Stacks are identified during the gathering Candidate Protocol Stacks are identified during the gathering
phase (Section 4.2.1) and are started during the racing phase phase (Section 4.2.1) and are started during the racing phase
(Section 4.2.2). (Section 4.2.2).
o System Policy: Represents the input from an operating system or * System Policy: Represents the input from an operating system or
other global preferences that can constrain or influence how an other global preferences that can constrain or influence how an
implementation will gather candidate paths and protocol stacks implementation will gather candidate paths and protocol stacks
(Section 4.2.1) and race the candidates during establishment (Section 4.2.1) and race the candidates during establishment
(Section 4.2.2). Specific aspects of the System Policy either (Section 4.2.2). Specific aspects of the System Policy either
apply to all Connections or only certain ones, depending on the apply to all Connections or only certain ones, depending on the
runtime context and properties of the Connection. runtime context and properties of the Connection.
o Cached State: The state and history that the implementation keeps * Cached State: The state and history that the implementation keeps
for each set of associated Endpoints that have been used for each set of associated Endpoints that have been used
previously. This can include DNS results, TLS session state, previously. This can include DNS results, TLS session state,
previous success and quality of transport protocols over certain previous success and quality of transport protocols over certain
paths. paths.
4.2.1. Candidate Gathering 4.2.1. Candidate Gathering
o Path Selection: Path Selection represents the act of choosing one * Candidate Path Selection: Candidate Path Selection represents the
or more paths that are available to use based on the Selection act of choosing one or more paths that are available to use based
Properties provided by the application, the policies and 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. heuristics of a Transport Services system.
o Protocol Selection: Protocol Selection represents the act of * Candidate Protocol Selection: Candidate Protocol Selection
choosing one or more sets of protocol options that are available represents the act of choosing one or more sets of protocol stacks
to use based on the Transport Properties provided by the that are available to use based on the Transport Properties
application, and the heuristics or policies within the Transport provided by the application, and the heuristics or policies within
Services system. the Transport Services system.
4.2.2. Candidate Racing 4.2.2. Candidate Racing
o Protocol Option Racing: Protocol Racing is the act of attempting Connection establishment attempts for a set of candidates may be
to establish, or scheduling attempts to establish, multiple performed simultaneously, synchronously, serially, or some
Protocol Stacks that differ based on the composition of protocols combination of all of these. We refer to this process as racing,
or the options used for protocols. borrowing terminology from Happy Eyeballs [RFC8305].
o Path Racing: Path Racing is the act of attempting to establish, or * 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 scheduling attempts to establish, multiple Protocol Stacks that
differ based on a selection from the available Paths. Since differ based on a selection from the available Paths. Since
different Paths will have distinct configurations for local different Paths will have distinct configurations for local
addresses and DNS servers, attempts across different Paths will addresses and DNS servers, attempts across different Paths will
perform separate DNS resolution stepss, which can lead to further perform separate DNS resolution steps, which can lead to further
racing of the resolved Remote Endpoints. racing of the resolved Remote Endpoints.
o Remote Endpoint Racing: Remote Endpoint Racing is the act of * Remote Endpoint Racing: Remote Endpoint Racing is the act of
attempting to establish, or scheduling attempts to establish, attempting to establish, or scheduling attempts to establish,
multiple Protocol Stacks that differ based on the specific multiple Protocol Stacks that differ based on the specific
representation of the Remote Endpoint, such as IP addresses representation of the Remote Endpoint, such as IP addresses
resolved from a DNS hostname. resolved from a DNS hostname.
4.2.3. Protocol Stack Equivalence 4.2.3. Protocol Stack Equivalence
The Transport Services architecture defines a mechanism that allows The Transport Services architecture defines a mechanism that allows
applications to easily use different network paths and Protocol applications to easily use different network paths and Protocol
Stacks. In some cases, changing which Protocol Stacks or network Stacks. In some cases, changing which Protocol Stacks or network
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establishment. This functionality in the API can be a powerful establishment. This functionality in the API can be a powerful
driver of new protocol adoption, but needs to be constrained driver of new protocol adoption, but needs to be constrained
carefully to avoid unexpected behavior that can lead to functional or carefully to avoid unexpected behavior that can lead to functional or
security problems. security problems.
If two different Protocol Stacks can be safely swapped, or raced in If two different Protocol Stacks can be safely swapped, or raced in
parallel (see Section 4.2.2), then they are considered to be parallel (see Section 4.2.2), then they are considered to be
"equivalent". Equivalent Protocol Stacks need to meet the following "equivalent". Equivalent Protocol Stacks need to meet the following
criteria: criteria:
1. Both stacks MUST offer the same interface to the application for 1. Both stacks MUST offer the interface requested by the application
connection establishment and data transmission. For example, if for connection establishment and data transmission. For example,
one Protocol Stack has UDP as the top-level interface to the if an application requires preservation of message boundaries, a
application, then it is not equivalent to a Protocol Stack that Protocol Stack that runs UDP as the top-level interface to the
runs TCP as the top-level interface. Among other differences, application is not equivalent to a Protocol Stack that runs TCP
the UDP stack would allow an application to read out message as the top-level interface. A UDP stack would allow an
boundaries based on datagrams sent from the remote system, application to read out message boundaries based on datagrams
whereas TCP does not preserve message boundaries on its own. 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.
2. Both stacks MUST offer the transport services that are required 2. Both stacks MUST offer the transport services that are requested
by the application. For example, if an application specifies by the application. For example, if an application specifies
that it requires reliable transmission of data, then a Protocol that it requires reliable transmission of data, then a Protocol
Stack using UDP without any reliability layer on top would not be Stack using UDP without any reliability layer on top would not be
allowed to replace a Protocol Stack using TCP. However, if the allowed to replace a Protocol Stack using TCP. However, if the
application does not require reliability, then a Protocol Stack application does not require reliability, then a Protocol Stack
that adds reliability could be regarded as an equivalent Protocol that adds reliability could be regarded as an equivalent Protocol
Stack as long providing this would not conflict with any other Stack as long as providing this would not conflict with any other
application-requested properties. application-requested properties.
3. Both stacks MUST offer the same security properties. The 3. Both stacks MUST offer security protocols and parameters as
inclusion of transport security protocols requested by the application [I-D.ietf-taps-transport-security].
[I-D.ietf-taps-transport-security] in a Protocol Stack adds Security features and properties, such as cryptographic
additional restrictions to Protocol Stack equivalence. Security algorithms, peer authentication, and identity privacy vary across
features and properties, such as cryptographic algorithms, peer security protocols, and across versions of security protocols.
authentication, and identity privacy vary across security Protocol equivalence ought not to be assumed for different
protocols, and across versions of security protocols. Protocol protocols or protocol versions, even if they offer similar
equivalence ought not to be assumed for different protocols or application configuration options. To ensure that security
protocol versions, even if they offer similar application protocols are not incorrectly swapped, Transport Services systems
configuration options. To ensure that security protocols are not SHOULD only automatically generate equivalent Protocol Stacks
incorrectly swapped, Transport Services systems SHOULD only when the transport security protocols within the stacks are
automatically generate equivalent Protocol Stacks when the identical. Specifically, a transport system would consider
transport security protocols within the stacks are identical. protocols identical only if they are of the same type and
Specifically, a transport system would consider protocols version. For example, the same version of TLS running over two
identical only if they are of the same type and version. For different transport protocol stacks are considered equivalent,
example, the same version of TLS running over two different whereas TLS 1.2 and TLS 1.3 [RFC8446] are not considered
transport protocol stacks are considered equivalent, whereas TLS equivalent. However, Transport Services systems MAY allow
1.2 and TLS 1.3 [RFC8446] are not considered equivalent. 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 4.2.4. Separating Connection Groups
By default, all stored properties of the implementation are shared By default, stored properties of the implementation, such as cached
within a process, such as cached protocol state, cached path state, protocol state, cached path state, and heuristics, may be shared
and heuristics. This provides efficiency and convenience for the (e.g. across multiple connections in an application). This provides
application, since the Transport System implementation can efficiency and convenience for the application, since the Transport
automatically optimize behavior. System implementation can automatically optimize behavior.
There are several reasons, however, that an application might want to There are several reasons, however, that an application might want to
isolate some Connections within a single process. These reasons explicitly isolate some Connections. These reasons include:
include:
o Privacy concerns about re-using cached protocol state that can * Privacy concerns about re-using cached protocol state that can
lead to linkability. Sensitive state may include TLS session lead to linkability. Sensitive state may include TLS session
state [RFC8446] and HTTP cookies [RFC6265]. state [RFC8446] and HTTP cookies [RFC6265].
o Privacy concerns about allowing Connections to multiplex together, * Privacy concerns about allowing Connections to multiplex together,
which can tell a Remote Endpoint that all of the Connections are which can tell a Remote Endpoint that all of the Connections are
coming from the same application (for example, when Connections coming from the same application (for example, when Connections
are multiplexed HTTP/2 or QUIC streams). are multiplexed HTTP/2 or QUIC streams).
o Performance concerns about Connections introducing head-of-line * Performance concerns about Connections introducing head-of-line
blocking due to multiplexing or needing to share state on a single blocking due to multiplexing or needing to share state on a single
thread. thread.
The Transport Services API SHOULD allow applications to explicitly The Transport Services API SHOULD allow applications to explicitly
define Connection Groups that force separation of Cached State and define Connection Groups that force separation of Cached State and
Protocol Stacks. For example, a web browser application might use Protocol Stacks. For example, a web browser application might use
Connection Groups with separate caches for different tabs in the Connection Groups with separate caches for different tabs in the
browser to decrease linkability. browser to decrease linkability.
The interface to specify these groups MAY expose fine-grained tuning The interface to specify these groups MAY expose fine-grained tuning
skipping to change at page 23, line 20 skipping to change at page 23, line 40
interface. Each provided interface translates to an existing interface. Each provided interface translates to an existing
protocol-specific interface provided by supported security protocols. protocol-specific interface provided by supported security protocols.
For example, trust verification callbacks are common parts of TLS For example, trust verification callbacks are common parts of TLS
APIs. Transport Services APIs will expose similar functionality APIs. Transport Services APIs will expose similar functionality
[I-D.ietf-taps-transport-security]. [I-D.ietf-taps-transport-security].
As described above in Section 4.2.3, if a Transport Services system As described above in Section 4.2.3, if a Transport Services system
races between two different Protocol Stacks, both MUST use the same races between two different Protocol Stacks, both MUST use the same
security protocols and options. security protocols and options.
Clients need to ensure that security APIs are used appropriately. In Applications need to ensure that they use security APIs
cases where clients use an interface to provide sensitive keying appropriately. In cases where applications use an interface to
material, e.g., access to private keys or copies of pre-shared keys provide sensitive keying material, e.g., access to private keys or
(PSKs), key use needs to be validated. For example, clients ought copies of pre-shared keys (PSKs), key use needs to be validated. For
not to use PSK material created for the Encapsulating Security example, applications ought not to use PSK material created for the
Protocol (ESP, part of IPsec) [RFC4303] with QUIC, and clients ought Encapsulating Security Protocol (ESP, part of IPsec) [RFC4303] with
not to use private keys intended for server authentication as a keys QUIC, and applications ought not to use private keys intended for
for client authentication. server authentication as keys for client authentication.
Moreover, Transport Services systems MUST NOT automatically fall back Moreover, Transport Services systems MUST NOT automatically fall back
from secure protocols to insecure protocols, or to weaker versions of from secure protocols to insecure protocols, or to weaker versions of
secure protocols. For example, if a client requests TLS, but the secure protocols. For example, if an application requests TLS, but
desired version of TLS is not available, its connection will fail. the desired version of TLS is not available, its connection will
Clients are thus responsible for implementing security protocol fail. Applications are thus responsible for implementing security
fallback or version fallback by creating multiple Transport Services protocol fallback or version fallback by creating multiple Transport
Connections, if so desired. Services Connections, if so desired.
7. Acknowledgements 7. Acknowledgements
This work has received funding from the European Union's Horizon 2020 This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreements No. 644334 research and innovation programme under grant agreements No. 644334
(NEAT) and No. 688421 (MAMI). (NEAT) and No. 688421 (MAMI).
This work has been supported by Leibniz Prize project funds of DFG - This work has been supported by Leibniz Prize project funds of DFG -
German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
FE 570/4-1). FE 570/4-1).
skipping to change at page 24, line 13 skipping to change at page 24, line 34
Eyeballs, that heavily influenced this work. Eyeballs, that heavily influenced this work.
8. References 8. References
8.1. Normative References 8.1. Normative References
[I-D.ietf-taps-interface] [I-D.ietf-taps-interface]
Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G., Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
Kuehlewind, M., Perkins, C., Tiesel, P., Wood, C., and T. Kuehlewind, M., Perkins, C., Tiesel, P., Wood, C., and T.
Pauly, "An Abstract Application Layer Interface to Pauly, "An Abstract Application Layer Interface to
Transport Services", draft-ietf-taps-interface-04 (work in Transport Services", Work in Progress, Internet-Draft,
progress), July 2019. 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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References 8.2. Informative References
[I-D.ietf-taps-impl] [I-D.ietf-taps-impl]
Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K., Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
Jones, T., Tiesel, P., Perkins, C., and M. Welzl, Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
"Implementing Interfaces to Transport Services", draft- "Implementing Interfaces to Transport Services", Work in
ietf-taps-impl-04 (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] [I-D.ietf-taps-minset]
Welzl, M. and S. Gjessing, "A Minimal Set of Transport Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for End Systems", draft-ietf-taps-minset-11 (work Services for End Systems", Work in Progress, Internet-
in progress), September 2018. Draft, draft-ietf-taps-minset-11, 27 September 2018,
<http://www.ietf.org/internet-drafts/draft-ietf-taps-
minset-11.txt>.
[I-D.ietf-taps-transport-security] [I-D.ietf-taps-transport-security]
Wood, C., Enghardt, T., Pauly, T., Perkins, C., and K. Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
Rose, "A Survey of Transport Security Protocols", draft- Wood, "A Survey of the Interaction Between Security
ietf-taps-transport-security-09 (work in progress), Protocols and Transport Services", Work in Progress,
September 2019. 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 [POSIX] "IEEE Std. 1003.1-2008 Standard for Information Technology
-- Portable Operating System Interface (POSIX). Open -- Portable Operating System Interface (POSIX). Open
group Technical Standard: Base Specifications, Issue 7", group Technical Standard: Base Specifications, Issue 7",
n.d.. 2008.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981, RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>. <https://www.rfc-editor.org/info/rfc793>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005, RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>. <https://www.rfc-editor.org/info/rfc4303>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
skipping to change at page 25, line 29 skipping to change at page 26, line 11
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>. <https://www.rfc-editor.org/info/rfc7540>.
[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind, [RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095, Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017, DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>. <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 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
Authors' Addresses Authors' Addresses
Tommy Pauly (editor) Tommy Pauly (editor)
Apple Inc. Apple Inc.
One Apple Park Way One Apple Park Way
Cupertino, California 95014 Cupertino, California 95014,
United States of America United States of America
Email: tpauly@apple.com Email: tpauly@apple.com
Brian Trammell (editor) Brian Trammell (editor)
Google Google
Gustav-Gull-Platz 1 Gustav-Gull-Platz 1
8004 Zurich CH- 8004 Zurich
Switzerland Switzerland
Email: ietf@trammell.ch Email: ietf@trammell.ch
Anna Brunstrom Anna Brunstrom
Karlstad University Karlstad University
Universitetsgatan 2 Universitetsgatan 2
651 88 Karlstad SE- 651 88 Karlstad
Sweden Sweden
Email: anna.brunstrom@kau.se Email: anna.brunstrom@kau.se
Godred Fairhurst Godred Fairhurst
University of Aberdeen University of Aberdeen
Fraser Noble Building Fraser Noble Building
Aberdeen, AB24 3UE Aberdeen, AB24 3UE
Scotland
Email: gorry@erg.abdn.ac.uk Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/ URI: http://www.erg.abdn.ac.uk/
Colin Perkins Colin Perkins
University of Glasgow University of Glasgow
School of Computing Science School of Computing Science
Glasgow G12 8QQ Glasgow G12 8QQ
United Kingdom United Kingdom
skipping to change at page 26, line 40 skipping to change at page 27, line 31
TU Berlin TU Berlin
Einsteinufer 25 Einsteinufer 25
10587 Berlin 10587 Berlin
Germany Germany
Email: philipp@tiesel.net Email: philipp@tiesel.net
Chris Wood Chris Wood
Apple Inc. Apple Inc.
One Apple Park Way One Apple Park Way
Cupertino, California 95014 Cupertino, California 95014,
United States of America United States of America
Email: cawood@apple.com Email: cawood@apple.com
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