An Architecture for Transport Services
draft-ietf-taps-arch-08
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draft-ietf-taps-arch-08
TAPS Working Group T. Pauly, Ed.
Internet-Draft Apple Inc.
Intended status: Standards Track B. Trammell, Ed.
Expires: 14 January 2021 Google Switzerland GmbH
A. Brunstrom
Karlstad University
G. Fairhurst
University of Aberdeen
C. Perkins
University of Glasgow
P. Tiesel
TU Berlin
C.A. Wood
Cloudflare
13 July 2020
An Architecture for Transport Services
draft-ietf-taps-arch-08
Abstract
This document describes an architecture for exposing transport
protocol features to applications for network communication, the
Transport Services architecture. The Transport Services Application
Programming Interface (API) is based on an asynchronous, event-driven
interaction pattern. It uses messages for representing data transfer
to applications, and it describes how implementations can use
multiple IP addresses, multiple protocols, and multiple paths, and
provide multiple application streams. This document further defines
common terminology and concepts to be used in definitions of
Transport Services APIs and implementations.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 14 January 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Specification of Requirements . . . . . . . . . . . . . . 5
2. API Model . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Event-Driven API . . . . . . . . . . . . . . . . . . . . 7
2.2. Data Transfer Using Messages . . . . . . . . . . . . . . 7
2.3. Flexibile Implementation . . . . . . . . . . . . . . . . 8
3. API and Implementation Requirements . . . . . . . . . . . . . 9
3.1. Provide Common APIs for Common Features . . . . . . . . . 9
3.2. Allow Access to Specialized Features . . . . . . . . . . 10
3.3. Select Equivalent Protocol Stacks . . . . . . . . . . . . 11
3.4. Maintain Interoperability . . . . . . . . . . . . . . . . 12
4. Transport Services Architecture and Concepts . . . . . . . . 12
4.1. Transport Services API Concepts . . . . . . . . . . . . . 13
4.1.1. Connections and Related Objects . . . . . . . . . . . 15
4.1.2. Pre-Establishment . . . . . . . . . . . . . . . . . . 16
4.1.3. Establishment Actions . . . . . . . . . . . . . . . . 17
4.1.4. Data Transfer Objects and Actions . . . . . . . . . . 18
4.1.5. Event Handling . . . . . . . . . . . . . . . . . . . 19
4.1.6. Termination Actions . . . . . . . . . . . . . . . . . 20
4.1.7. Connection Groups . . . . . . . . . . . . . . . . . . 20
4.2. Transport Services Implementation Concepts . . . . . . . 20
4.2.1. Candidate Gathering . . . . . . . . . . . . . . . . . 21
4.2.2. Candidate Racing . . . . . . . . . . . . . . . . . . 22
4.2.3. Separating Connection Groups . . . . . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
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8.1. Normative References . . . . . . . . . . . . . . . . . . 24
8.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Many application programming interfaces (APIs) to perform transport
networking have been deployed, perhaps the most widely known and
imitated being the BSD Socket [POSIX] interface (Socket API). The
naming of objects and functions across these APIs is not consistent,
and varies depending on the protocol being used. For example,
sending and receiving streams of data is conceptually the same for
both an unencrypted Transmission Control Protocol (TCP) stream and
operating on an encrypted Transport Layer Security (TLS) [RFC8446]
stream over TCP, but applications cannot use the same socket "send()"
and "recv()" calls on top of both kinds of connections. Similarly,
terminology for the implementation of transport protocols varies
based on the context of the protocols themselves: terms such as
"flow", "stream", "message", and "connection" can take on many
different meanings. This variety can lead to confusion when trying
to understand the similarities and differences between protocols, and
how applications can use them effectively.
The goal of the Transport Services architecture is to provide a
common, flexible, and reusable interface for transport protocols. As
applications adopt this interface, they will benefit from a wide set
of transport features that can evolve over time, and ensure that the
system providing the interface can optimize its behavior based on the
application requirements and network conditions, without requiring
changes to the applications. This flexibility enables faster
deployment of new features and protocols. It can also support
applications by offering racing and fallback mechanisms, which
otherwise need to be implemented in each application separately.
This document was developed in parallel with the specification of the
Transport Services API [I-D.ietf-taps-interface] and Implementation
Guidelines [I-D.ietf-taps-impl]. Although following the Transport
Services Architecture does not require that all APIs and
implementations are identical, a common minimal set of features
represented in a consistent fashion will enable applications to be
easily ported from one system to another.
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1.1. Background
The Transport Services architecture is based on the survey of
services provided by IETF transport protocols and congestion control
mechanisms [RFC8095], and the distilled minimal set of the features
offered by transport protocols [I-D.ietf-taps-minset]. These
documents identified common features and patterns across all
transport protocols developed thus far in the IETF.
Since transport security is an increasingly relevant aspect of using
transport protocols on the Internet, this architecture also considers
the impact of transport security protocols on the feature-set exposed
by Transport Services [I-D.ietf-taps-transport-security].
One of the key insights to come from identifying the minimal set of
features provided by transport protocols [I-D.ietf-taps-minset] was
that features either require application interaction and guidance
(referred to in that document as Functional or Optimizing Features),
or else can be handled automatically by a system implementing
Transport Services (referred to as Automatable Features). Among the
Functional and Optimizing Features, some were common across all or
nearly all transport protocols, while others could be seen as
features that, if specified, would only be useful with a subset of
protocols, but would not harm the functionality of other protocols.
For example, some protocols can deliver messages faster for
applications that do not require messages to arrive in the order in
which they were sent. However, this functionality needs to be
explicitly allowed by the application, since reordering messages
would be undesirable in many cases.
1.2. Overview
This document describes the Transport Services architecture in three
sections:
* Section 2 describes how the API model of Transport Services
differs from traditional socket-based APIs. Specifically, it
offers asynchronous event-driven interaction, the use of messages
for data transfer, and the flexibility to use different transport
protocols and paths without requiring major changes to the
application.
* Section 3 explains the fundamental requirements for a Transport
Services API. These principles are intended to make sure that
transport protocols can continue to be enhanced and evolve without
requiring significant changes by application developers.
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* Section 4 presents the Transport Services architecture diagram and
defines the concepts that are used by both the API and
implementation documents. The Preconnection allows applications
to configure Connection Properties, and the Connection represents
an object that can be used to send and receive Messages.
1.3. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. API Model
The traditional model of using sockets for networking can be
represented as follows:
* Applications create connections and transfer data using the Socket
API.
* The Socket API provides the interface to the implementations of
TCP and UDP (typically implemented in the system's kernel).
* TCP and UDP in the kernel send and receive data over the available
network-layer interfaces.
* Sockets are bound directly to transport-layer and network-layer
addresses, obtained via a separate resolution step, usually
performed by a system-provided stub resolver.
+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
| | |
+------------+ +------------+ +--------------+
| stub | | Stream API | | Datagram API |
| resolver | +------------+ +--------------+
+------------+ | |
+---------------------------------+
| TCP UDP |
| Kernel Networking Stack |
+---------------------------------+
|
+-----------------------------------------------------+
| Network Layer Interface |
+-----------------------------------------------------+
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Figure 1: Socket API Model
The Transport Services architecture evolves this general model of
interaction, aiming to both modernize the API surface presented to
applications by the transport layer and enrich the capabilities of
the Transport Services implementation. It combines interfaces for
multiple interaction patterns into a unified whole. By combining
name resolution with connection establishment and data transfer in a
single API, it allows for more flexible implementations to provide
path and transport protocol agility on the application's behalf.
+-----------------------------------------------------+
| Application |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services API |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Transport Services Implementation |
| (Using: DNS, UDP, TCP, SCTP, DCCP, TLS, QUIC, etc) |
+-----------------------------------------------------+
|
+-----------------------------------------------------+
| Network Layer Interface |
+-----------------------------------------------------+
Figure 2: Transport Services API Model
The Transport Services API [I-D.ietf-taps-interface] defines the
mechanism for an application to create network connections and
transfer data. The implementation [I-D.ietf-taps-impl] is
responsible for mapping the API to the various available transport
protocols and managing the available network interfaces and paths.
There are key differences between the architecture of the Transport
Services system and the architecture of the Socket API: the Transport
Services API is asynchronous and event-driven; it uses messages for
representing data transfer to applications; and it describes how
implementations can use multiple IP addresses, multiple protocols,
multiple paths, and provide multiple application streams.
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2.1. Event-Driven API
Originally, sockets presented a blocking interface for establishing
connections and transferring data. However, most modern applications
interact with the network asynchronously. Emulation of an
asynchronous interface using sockets generally uses a try-and-fail
model. If the application wants to read, but data has not yet been
received from the peer, the call to read will fail. The application
then waits and can try again later.
In contrast to sockets, all interaction with a Transport Services
system is expected to be asynchronous, and use an event-driven model
(see Section 4.1.5). For example, if the application wants to read,
its call to read will not complete immediately, but will deliver an
event containing the received data once it is available. Error
handling is also asynchronous; a failure to send results in an
asynchronous send error as an event.
The Transport Services API also delivers events regarding the
lifetime of a connection and changes in the available network links,
which were not previously made explicit in sockets.
Using asynchronous events allows for a more natural interaction model
when establishing connections and transferring data. Events in time
more closely reflect the nature of interactions over networks, as
opposed to how sockets represent network resources as file system
objects that may be temporarily unavailable.
Separate from events, callbacks are also provided for asynchronous
interactions with the API not directly related to events on the
network or network interfaces.
2.2. Data Transfer Using Messages
Sockets provide a message interface for datagram protocols like UDP,
but provide an unstructured stream abstraction for TCP. While TCP
does indeed provide the ability to send and receive data as streams,
most applications need to interpret structure within these streams.
For example, HTTP/1.1 uses character delimiters to segment messages
over a stream [RFC7230]; TLS record headers carry a version, content
type, and length [RFC8446]; and HTTP/2 uses frames to segment its
headers and bodies [RFC7540].
The Transport Services API represents data as messages, so that it
more closely matches the way applications use the network. Providing
a message-based abstraction provides many benefits, such as:
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* the ability to associate deadlines with messages, for applications
that care about timing;
* the ability to provide control of reliability, choosing which
messages to retransmit when there is packet loss, and how best to
make use of the data that arrived;
* the ability to manage dependencies between messages, when the
Transport Services system could decide to not deliver a message,
either following packet loss or because it has missed a deadline.
In particular, this can avoid (re-)sending data that relies on a
previous transmission that was never received.
* the ability to automatically assign messages and connections to
underlying transport connections to utilize multi-streaming and
pooled connections.
Allowing applications to interact with messages is backwards-
compatible with existings protocols and APIs, as it does not change
the wire format of any protocol. Instead, it gives the protocol
stack additional information to allow it to make better use of modern
transport services, while simplifying the application's role in
parsing data. For protocols which natively use a streaming
abstraction, framers (Section 4.1.4) bridge the gap between the two
abstractions.
2.3. Flexibile Implementation
Sockets, for protocols like TCP, are generally limited to connecting
to a single address over a single interface. They also present a
single stream to the application. Software layers built upon sockets
often propagate this limitation of a single-address single-stream
model. The Transport Services architecture is designed to handle
multiple candidate endpoints, protocols, and paths; and support
multipath and multistreaming protocols.
Transport Services implementations are meant to be flexible at
connection establishment time, considering many different options and
trying to select the most optimal combinations (Section 4.2.1 and
Section 4.2.2). This requires applications to provide higher-level
endpoints than IP addresses, such as hostnames and URLs, which are
used by a Transport Services implementation for resolution, path
selection, and racing. Transport services implementations can
further implement fallback mechanisms if connection establishment of
one protocol fails or performance is detected to be unsatisfactory.
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Flexibility after connection establishment is also important.
Transport protocols that can migrate between multiple network-layer
interfaces need to be able to process and react to interface changes.
Protocols that support multiple application-layer streams need to
support initiating and receiving new streams using existing
connections.
3. API and Implementation Requirements
The goal of the Transport Services architecture is to redefine the
interface between applications and transports in a way that allows
the transport layer to evolve and improve without fundamentally
changing the contract with the application. This requires a careful
consideration of how to expose the capabilities of protocols.
There are several degrees in which a Transport Services system is
intended to offer flexibility to an application: it can provide
access to multiple sets of protocols and protocol features; it can
use these protocols across multiple paths that could have different
performance and functional characteristics; and it can communicate
with different remote systems to optimize performance, robustness to
failure, or some other metric. Beyond these, if the API for the
system remains the same over time, new protocols and features can be
added to the system's implementation without requiring changes in
applications for adoption.
The normative requirements described here allow Transport Services
APIs and Implementations to provide this functionlity without causing
incompatibility or introducing security vulnerabilities. The rest of
this document describes the architecture non-normatively.
3.1. Provide Common APIs for Common Features
Any functionality that is common across multiple transport protocols
SHOULD be made accessible through a unified set of Transport Services
API calls. As a baseline, any Transport Services API MUST allow
access to the minimal set of features offered by transport protocols
[I-D.ietf-taps-minset].
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An application can specify constraints and preferences for the
protocols, features, and network interfaces it will use via
Properties. A Transport Services API SHOULD offer Properties that
are common to multiple transport protocols, in order to enable the
system to appropriately select between protocols that offer
equivalent features. Similarly, a Transport Services API SHOULD
offer Properties that are applicable to a variety of network layer
interfaces and paths, in order to permit racing of different network
paths without affecting the applications using the system. Each
Property is expected to have a default value.
The default values for Properties SHOULD be selected to ensure
correctness for the widest set of applications, while providing the
widest set of options for selection. For example, since both
applications that require reliability and those that do not require
reliability can function correctly when a protocol provides
reliability, reliability ought to be enabled by default. As another
example, the default value for a Property regarding the selection of
network interfaces ought to permit as many interfaces as possible.
Applications using a Transport Services system interface are REQUIRED
to be robust to the automated selection provided by the system, where
the automated selection is constrained by the requirements and
preferences expressed by the application.
3.2. Allow Access to Specialized Features
There are applications that will need to control fine-grained details
of transport protocols to optimize their behavior and ensure
compatibility with remote systems. A Transport Services system
therefore SHOULD permit more specialized protocol features to be
used.
A specialized feature could be required by an application only when
using a specific protocol, and not when using others. For example,
if an application is using TCP, it could require control over the
User Timeout Option for TCP; these options would not take effect for
other transport protocols. In such cases, the API ought to expose
the features in such a way that they take effect when a particular
protocol is selected, but do not imply that only that protocol could
be used. For example, if the API allows an application to specify a
preference to use the User Timeout Option, communication would not
fail when a protocol such as QUIC is selected.
Other specialized features, however, can be strictly required by an
application and thus constrain the set of protocols that can be used.
For example, if an application requires support for automatic
handover or failover for a connection, only protocol stacks that
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provide this feature are eligible to be used, e.g., protocol stacks
that include a multipath protocol or a protocol that supports
connection migration. A Transport Services API needs to allow
applications to define such requirements and constrain the system's
options. Since such options are not part of the core/common
features, it will generally be simple for an application to modify
its set of constraints and change the set of allowable protocol
features without changing the core implementation.
3.3. Select Equivalent Protocol Stacks
A Transport Services implementation can select Protocol Stacks based
on the Properties communicated by the application. If two different
Protocol Stacks can be safely swapped, or raced in parallel (see
Section 4.2.2), then they are considered to be "equivalent".
Equivalent Protocol Stacks are defined as stacks that can provide the
same transport properties and interface expectations as requested by
the application.
The following two examples show non-equivalent Protocol Stacks:
* If the application requires preservation of message boundaries, a
Protocol Stack that runs UDP as the top-level interface to the
application is not equivalent to a Protocol Stack that runs TCP as
the top-level interface. A UDP stack would allow an application
to read out message boundaries based on datagrams sent from the
remote system, whereas TCP does not preserve message boundaries on
its own, but needs a framing protocol on top to determine message
boundaries.
* If the application specifies that it requires reliable
transmission of data, then a Protocol Stack using UDP without any
reliability layer on top would not be allowed to replace a
Protocol Stack using TCP.
The following example shows equivalent Protocol Stacks:
* If the application does not require reliable transmission of data,
then a Protocol Stack that adds reliability could be regarded as
an equivalent Protocol Stack as long as providing this would not
conflict with any other application-requested properties.
To ensure that security protocols are not incorrectly swapped,
Transport Services systems MUST only select Protocol Stacks that meet
application requirements ([I-D.ietf-taps-transport-security]).
Systems SHOULD only race Protocol Stacks where the transport security
protocols within the stacks are identical. Transport Services
systems MUST NOT automatically fall back from secure protocols to
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insecure protocols, or to weaker versions of secure protocols. A
Transport Services system MAY allow applications to explicitly
specify that fallback to a specific other version of a protocol is
allowed, e.g., to allow fallback to TLS 1.2 if TLS 1.3 is not
available.
3.4. Maintain Interoperability
It is important to note that neither the Transport Services API
[I-D.ietf-taps-interface] nor the Implementation document
[I-D.ietf-taps-impl] define new protocols or protocol capabilities
that affect what is communicated across the network. Use of a
Transport Services system MUST NOT require that a peer on the other
side of a connection uses the same API or implementation. A
Transport Services system acting as a connection initiator is able to
communicate with any existing system that implements the transport
protocol(s) and all the required properties selected by the Transport
Services system. Similarly, a Transport Services system acting as a
listener can receive connections for any protocol that is supported
by the system from existing initiators that implement the protocol,
independent of whether the initiator uses a Transport Services system
or not.
In normal use, a Transport Services system SHOULD result in
consistent protocol and interface selection decisions for the same
network conditions given the same set of Properties. This is
intended to provide predictable outcomes to the application using the
API.
4. Transport Services Architecture and Concepts
The concepts defined in this document are intended primarily for use
in the documents and specifications that describe the Transport
Services architecture and API. While the specific terminology can be
used in some implementations, it is expected that there will remain a
variety of terms used by running code.
The architecture divides the concepts for Transport Services into two
categories:
1. API concepts, which are intended to be exposed to applications;
and
2. System-implementation concepts, which are intended to be
internally used when building systems that implement Transport
Services.
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The following diagram summarizes the top-level concepts in the
architecture and how they relate to one another.
+-----------------------------------------------------+
| Application |
+-+----------------+------^-------+--------^----------+
| | | | |
pre- | data | events
establishment | transfer | |
| establishment | termination |
| | | | |
| +--v------v-------v+ |
+-v-------------+ Connection(s) +-------+----------+
| Transport +--------+---------+ |
| Services | |
| API | +-------------+ |
+------------------------+--+ Framer(s) |-----------+
| +-------------+
+------------------------|----------------------------+
| Transport | |
| System | +-----------------+ |
| Implementation | | Cached | |
| | | State | |
| (Candidate Gathering) | +-----------------+ |
| | |
| (Candidate Racing) | +-----------------+ |
| | | System | |
| | | Policy | |
| +----------v-----+ +-----------------+ |
| | Protocol | |
+-------------+ Stack(s) +----------------------+
+-------+--------+
V
Network Layer Interface
Figure 3: Concepts and Relationships in the Transport Services
Architecture
4.1. Transport Services API Concepts
Fundamentally, a Transport Services API needs to provide connection
objects (Section 4.1.1) that allow applications to establish
communication, and then send and receive data. These could be
exposed as handles or referenced objects, depending on the language.
Beyond the connection objects, there are several high-level groups of
actions that any Transport Services API needs to provide:
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* Pre-Establishment (Section 4.1.2) encompasses the properties that
an application can pass to describe its intent, requirements,
prohibitions, and preferences for its networking operations.
These properties apply to multiple transport protocols, unless
otherwise specified. Properties specified during Pre-
Establishment can have a large impact on the rest of the
interface: they modify how establishment occurs, they influence
the expectations around data transfer, and they determine the set
of events that will be supported.
* Establishment (Section 4.1.3) focuses on the actions that an
application takes on the connection objects to prepare for data
transfer.
* Data Transfer (Section 4.1.4) consists of how an application
represents the data to be sent and received, the functions
required to send and receive that data, and how the application is
notified of the status of its data transfer.
* Event Handling (Section 4.1.5) defines categories of notifications
which an application can receive during the lifetime of transport
objects. Events also provide opportunities for the application to
interact with the underlying transport by querying state or
updating maintenance options.
* Termination (Section 4.1.6) focuses on the methods by which data
transmission is stopped, and state is torn down in the transport.
The diagram below provides a high-level view of the actions and
events during the lifetime of a Connection object. Note that some
actions are alternatives (e.g., whether to initiate a connection or
to listen for incoming connections), while others are optional (e.g.,
setting Connection and Message Properties in Pre-Establishment) or
have been omitted for brevity and simplicity.
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Pre-Establishment : Established : Termination
----------------- : ----------- : -----------
: :
+-- Local Endpoint : Message :
+-- Remote Endpoint : Receive() | :
+-- Transport Properties : Send() | :
+-- Security Parameters : | :
| : | :
| InitiateWithSend() | Close() :
| +---------------+ Initiate() +-----+------+ Abort() :
+---+ Preconnection |------------->| Connection |-----------> Closed
+---------------+ Rendezvous() +------------+ :
Listen() | : | | :
| : | v :
v : | Connection :
+----------+ : | Ready :
| Listener |----------------------+ :
+----------+ Connection Received :
: :
Figure 4: The lifetime of a Connection object
4.1.1. Connections and Related Objects
* Preconnection: A Preconnection object is a representation of a
potential Connection. It has state that describes parameters of a
Connection that might exist in the future: the Local Endpoint from
which that Connection will be established, the Remote Endpoint
(Section 4.1.2) to which it will connect, and Transport Properties
that influence the paths and protocols a Connection will use. A
Preconnection can be fully specified such that it represents a
single possible Connection, or it can be partially specified such
that it represents a family of possible Connections. The Local
Endpoint (Section 4.1.2) is required if the Preconnection is used
to Listen for incoming Connections. The Local Endpoint is
optional if it is used to Initiate Connections. The Remote
Endpoint is required in the Preconnection that is used to Initiate
Connections. The Remote Endpoint is optional if it is used to
Listen for incoming Connections. The Local Endpoint and the
Remote Endpoint are both required if a peer-to-peer Rendezvous is
to occur based on the Preconnection.
* Transport Properties: Transport Properties allow the application
to express their requirements, prohibitions, and preferences and
configure the Transport Services system. There are three kinds of
Transport Properties:
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- Selection Properties (Section 4.1.2) that can only be specified
on a Preconnection.
- Connection Properties (Section 4.1.2) that can be specified on
a Preconnection and changed on the Connection.
- Message Properties (Section 4.1.4) that can be specified as
defaults on a Preconnection or a Connection, and can also be
specified during data transfer to affect specific Messages.
* Connection: A Connection object represents one or more active
transport protocol instances that can send and/or receive Messages
between local and remote systems. It holds state pertaining to
the underlying transport protocol instances and any ongoing data
transfers. This represents, for example, an active Connection in
a connection-oriented protocol such as TCP, or a fully-specified
5-tuple for a connectionless protocol such as UDP. It can also
represent a pool of transport protocol instances, e.g., a set of
TCP and QUIC connections to equivalent endpoints, or a stream of a
multi-streaming transport protocol instance. Connections can be
created from a Preconnection or by a Listener.
* Listener: A Listener object accepts incoming transport protocol
connections from remote systems and generates corresponding
Connection objects. It is created from a Preconnection object
that specifies the type of incoming Connections it will accept.
4.1.2. Pre-Establishment
* Endpoint: An Endpoint represents an identifier for one side of a
transport connection. Endpoints can be Local Endpoints or Remote
Endpoints, and respectively represent an identity that the
application uses for the source or destination of a connection.
An Endpoint can be specified at various levels of abstraction, and
an Endpoint at a higher level of abstraction (such as a hostname)
can be resolved to more concrete identities (such as IP
addresses).
* Remote Endpoint: The Remote Endpoint represents the application's
identifier for a peer that can participate in a transport
connection; for example, the combination of a DNS name for the
peer and a service name/port.
* Local Endpoint: The Local Endpoint represents the application's
identifier for itself that it uses for transport connections; for
example, a local IP address and port.
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* Selection Properties: The Selection Properties consist of the
options that an application can set to influence the selection of
paths between the local and remote systems, to influence the
selection of transport protocols, or to configure the behavior of
generic transport protocol features. These options can take the
form of requirements, prohibitions, or preferences. Examples of
options that influence path selection include the interface type
(such as a Wi-Fi connection, or a Cellular LTE connection),
requirements around the largest Message that can be sent, or
preferences for throughput and latency properties. Examples of
options that influence protocol selection and configuration of
transport protocol features include reliability, multipath
support, and fast open support.
* Connection Properties: The Connection Properties are used to
configure protocol-specific options and control per-connection
behavior of the Transport Services system; for example, a
protocol-specific Connection Property can express that if TCP is
used, the implementation ought to use the User Timeout Option.
Note that the presence of such a property does not require that a
specific protocol will be used. In general, these properties do
not explicitly determine the selection of paths or protocols, but
can be used in this way by an implementation during connection
establishment. Connection Properties are specified on a
Preconnection prior to Connection establishment, and can be
modified on the Connection later. Changes made to Connection
Properties after Connection establishment take effect on a best-
effort basis.
* Security Parameters: Security Parameters define an application's
requirements for authentication and encryption on a Connection.
They are used by Transport Security protocols (such as those
described in [I-D.ietf-taps-transport-security]) to establish
secure Connections. Examples of parameters that can be set
include local identities, private keys, supported cryptographic
algorithms, and requirements for validating trust of remote
identities. Security Parameters are primarily associated with a
Preconnection object, but properties related to identities can be
associated directly with Endpoints.
4.1.3. Establishment Actions
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* Initiate: The primary action that an application can take to
create a Connection to a Remote Endpoint, and prepare any required
local or remote state to enable the transmission of Messages. For
some protocols, this will initiate a client-to-server style
handshake; for other protocols, this will just establish local
state (e.g., with connectionless protocols such as UDP). The
process of identifying options for connecting, such as resolution
of the Remote Endpoint, occurs in response to the Initiate call.
* Listen: Enables a listener to accept incoming Connections. The
Listener will then create Connection objects as incoming
connections are accepted (Section 4.1.5). 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.
* Rendezvous: The action of establishing a peer-to-peer connection
with a Remote Endpoint. It simultaneously attempts to initiate a
connection to a Remote Endpoint while listening for an incoming
connection from that endpoint. The process of identifying options
for the connection, such as resolution of the Remote Endpoint,
occurs in response to the Rendezvous call. As with Listeners, the
set of local paths and endpoints is constrained by Selection
Properties. If successful, the Rendezvous call returns a
Connection object to represent the established peer-to-peer
connection. The processes by which connections are initiated
during a Rendezvous action will depend on the set of Local and
Remote Endpoints configured on the Preconnection. For example, if
the Local and Remote Endpoints are TCP host candidates, then a TCP
simultaneous open [RFC0793] will be performed. However, if the
set of Local Endpoints includes server reflexive candidates, such
as those provided by STUN, a Rendezvous action will race
candidates in the style of the ICE algorithm [RFC8445] to perform
NAT binding discovery and initiate a peer-to-peer connection.
4.1.4. Data Transfer Objects and Actions
* Message: A Message object is a unit of data that can be
represented as bytes that can be transferred between two systems
over a transport connection. The bytes within a Message are
assumed to be ordered. If an application does not care about the
order in which a peer receives two distinct spans of bytes, those
spans of bytes are considered independent Messages.
* Message Properties: Message Properties are used to specify details
about Message transmission. They can be specified directly on
individual Messages, or can be set on a Preconnection or
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Connection as defaults. These properties might only apply to how
a Message is sent (such as how the transport will treat
prioritization and reliability), but can also include properties
that specific protocols encode and communicate to the Remote
Endpoint. When receiving Messages, Message Properties can contain
information about the received Message, such as metadata generated
at the receiver and information signalled by the remote endpoint.
For example, a Message can be marked with a Message Property
indicating that it is the final message on a connection if the
peer sent a TCP FIN.
* Send: The action to transmit a Message over a Connection to the
remote system. The interface to Send can accept Message
Properties specific to how the Message content is to be sent. The
status of the Send operation is delivered back to the sending
application in an event (Section 4.1.5).
* Receive: An action that indicates that the application is ready to
asynchronously accept a Message over a Connection from a remote
system, while the Message content itself will be delivered in an
event (Section 4.1.5). The interface to Receive can include
Message Properties specific to the Message that is to be delivered
to the application.
* Framer: A Framer is a data translation layer that can be added to
a Connection to define how application-layer Messages are
transmitted over a transport protocol. This is particularly
relevant for protocols that otherwise present unstructured
streams, such as TCP.
4.1.5. Event Handling
The following categories of events can be delivered to an
application:
* Connection Ready: Signals to an application that a given
Connection is ready to send and/or receive Messages. If the
Connection relies on handshakes to establish state between peers,
then it is assumed that these steps have been taken.
* Connection Closed: Signals to an application that a given
Connection is no longer usable for sending or receiving Messages.
The event delivers a reason or error to the application that
describes the nature of the termination.
* Connection Received: Signals to an application that a given
Listener has received a Connection.
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* Message Received: Delivers received Message content to the
application, based on a Receive action. This can include an error
if the Receive action cannot be satisfied due to the Connection
being closed.
* Message Sent: Notifies the application of the status of its Send
action. This might indicate a failure if the Message cannot be
sent, or an indication that the Message has been processed by the
protocol stack.
* Path Properties Changed: Notifies the application that some
property of the Connection has changed that might influence how
and where data is sent and/or received.
4.1.6. Termination Actions
* Close: The action an application takes on a Connection to indicate
that it no longer intends to send data, is no longer willing to
receive data, and that the protocol should signal this state to
the remote system if the transport protocol allows this. (Note
that this is distinct from the concept of "half-closing" a
bidirectional connection, such as when a FIN is sent in one
direction of a TCP connection. Indicating the end of a stream in
the Transport Services architecture is possible using Message
Properties when sending.)
* Abort: The action the application takes on a Connection to
indicate a Close and also indicate that the Transport Services
system should not attempt to deliver any outstanding data. This
is intended for immediate termination of a connection, without
cleaning up state.
4.1.7. Connection Groups
A Connection Group is a set of Connections that share properties and
caches. For multiplexing transport protocols, only Connections
within the same Connection Group are allowed to be multiplexed
together. An application can explicitly define Connection Groups to
control caching boundaries, as discussed in Section 4.2.3.
4.2. Transport Services Implementation Concepts
This section defines the set of objects used internally to a system
or library to implement the functionality needed to provide a
transport service across a network, as required by the abstract
interface.
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* Path: Represents an available set of properties that a local
system can use to communicate with a remote system, such as
routes, addresses, and physical and virtual network interfaces.
* Protocol Instance: A single instance of one protocol, including
any state necessary to establish connectivity or send and receive
Messages.
* Protocol Stack: A set of Protocol Instances (including relevant
application, security, transport, or Internet protocols) that are
used together to establish connectivity or send and receive
Messages. A single stack can be simple (a single transport
protocol instance over IP), or it can be complex (multiple
application protocol streams going through a single security and
transport protocol, over IP; or, a multi-path transport protocol
over multiple transport sub-flows).
* Candidate Path: One path that is available to an application and
conforms to the Selection Properties and System Policy, of which
there can be several. Candidate Paths are identified during the
gathering phase (Section 4.2.1) and can be used during the racing
phase (Section 4.2.2).
* Candidate Protocol Stack: One Protocol Stack that can be used by
an application for a connection, of which there can be several.
Candidate Protocol Stacks are identified during the gathering
phase (Section 4.2.1) and are started during the racing phase
(Section 4.2.2).
* System Policy: Represents the input from an operating system or
other global preferences that can constrain or influence how an
implementation will gather candidate paths and Protocol Stacks
(Section 4.2.1) and race the candidates during establishment
(Section 4.2.2). Specific aspects of the System Policy either
apply to all Connections or only certain ones, depending on the
runtime context and properties of the Connection.
* Cached State: The state and history that the implementation keeps
for each set of associated Endpoints that have been used
previously. This can include DNS results, TLS session state,
previous success and quality of transport protocols over certain
paths, as well as other information.
4.2.1. Candidate Gathering
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* Candidate Path Selection: Candidate Path Selection represents the
act of choosing one or more paths that are available to use based
on the Selection Properties and any available Local and Remote
Endpoints provided by the application, as well as the policies and
heuristics of a Transport Services system.
* Candidate Protocol Selection: Candidate Protocol Selection
represents the act of choosing one or more sets of Protocol Stacks
that are available to use based on the Transport Properties
provided by the application, and the heuristics or policies within
the Transport Services system.
4.2.2. Candidate Racing
Connection establishment attempts for a set of candidates may be
performed simultaneously, synchronously, serially, or some
combination of all of these. We refer to this process as racing,
borrowing terminology from Happy Eyeballs [RFC8305].
* Protocol Option Racing: Protocol Option Racing is the act of
attempting to establish, or scheduling attempts to establish,
multiple Protocol Stacks that differ based on the composition of
protocols or the options used for protocols.
* Path Racing: Path Racing is the act of attempting to establish, or
scheduling attempts to establish, multiple Protocol Stacks that
differ based on a selection from the available Paths. Since
different Paths will have distinct configurations for local
addresses and DNS servers, attempts across different Paths will
perform separate DNS resolution steps, which can lead to further
racing of the resolved Remote Endpoints.
* Remote Endpoint Racing: Remote Endpoint Racing is the act of
attempting to establish, or scheduling attempts to establish,
multiple Protocol Stacks that differ based on the specific
representation of the Remote Endpoint, such as a particular IP
address that was resolved from a DNS hostname.
4.2.3. Separating Connection Groups
By default, stored properties of the implementation, such as cached
protocol state, cached path state, and heuristics, may be shared
(e.g. across multiple connections in an application). This provides
efficiency and convenience for the application, since the Transport
Services implementation can automatically optimize behavior.
There are several reasons, however, that an application might want to
explicitly isolate some Connections. These reasons include:
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* Privacy concerns about re-using cached protocol state that can
lead to linkability. Sensitive state may include TLS session
state [RFC8446] and HTTP cookies [RFC6265].
* Privacy concerns about allowing Connections to multiplex together,
which can tell a Remote Endpoint that all of the Connections are
coming from the same application (for example, when Connections
are multiplexed HTTP/2 or QUIC streams).
* Performance concerns about Connections introducing head-of-line
blocking due to multiplexing or needing to share state on a single
thread.
The Transport Services API can allow applications to explicitly
define Connection Groups that force separation of Cached State and
Protocol Stacks. For example, a web browser application might use
Connection Groups with separate caches for different tabs in the
browser to decrease linkability.
The interface to specify a Connection Group can expose fine-grained
tuning for which properties and cached state is allowed to be shared
with other Connections. For example, an application might want to
allow sharing TCP Fast Open cookies across groups, but not TLS
session state.
5. IANA Considerations
RFC-EDITOR: Please remove this section before publication.
This document has no actions for IANA.
6. Security Considerations
The Transport Services architecture does not recommend use of
specific security protocols or algorithms. Its goal is to offer ease
of use for existing protocols by providing a generic security-related
interface. Each provided interface translates to an existing
protocol-specific interface provided by supported security protocols.
For example, trust verification callbacks are common parts of TLS
APIs. Transport Services APIs will expose similar functionality
[I-D.ietf-taps-transport-security].
As described above in Section 3.3, if a Transport Services system
races between two different Protocol Stacks, both need to use the
same security protocols and options. However, a Transport Services
system can race different security protocols, e.g., if the
application explicitly specifies that it considers them equivalent.
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Applications need to ensure that they use security APIs
appropriately. In cases where applications use an interface to
provide sensitive keying material, e.g., access to private keys or
copies of pre-shared keys (PSKs), key use needs to be validated. For
example, applications ought not to use PSK material created for the
Encapsulating Security Protocol (ESP, part of IPsec) [RFC4303] with
QUIC, and applications ought not to use private keys intended for
server authentication as keys for client authentication.
Moreover, Transport Services systems must not automatically fall back
from secure protocols to insecure protocols, or to weaker versions of
secure protocols (see Section 3.3). For example, if an application
requests a specific version of TLS, but the desired version of TLS is
not available, its connection will fail. Applications are thus
responsible for implementing security protocol fallback or version
fallback by creating multiple Transport Services Connections, if so
desired. Alternatively, a Transport Services system MAY allow
applications to specify that fallback to a specific other version of
a protocol is allowed.
7. Acknowledgements
This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreements No. 644334
(NEAT) and No. 688421 (MAMI).
This work has been supported by Leibniz Prize project funds of DFG -
German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
FE 570/4-1).
This work has been supported by the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.
Thanks to Theresa Enghardt, Max Franke, Mirja Kuehlewind, Jonathan
Lennox, and Michael Welzl for the discussions and feedback that
helped shape the architecture described here. Thanks as well to
Stuart Cheshire, Josh Graessley, David Schinazi, and Eric Kinnear for
their implementation and design efforts, including Happy Eyeballs,
that heavily influenced this work.
8. References
8.1. Normative References
[I-D.ietf-taps-interface]
Trammell, B., Welzl, M., Enghardt, T., Fairhurst, G.,
Kuehlewind, M., Perkins, C., Tiesel, P., Wood, C., and T.
Pauly, "An Abstract Application Layer Interface to
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Transport Services", Work in Progress, Internet-Draft,
draft-ietf-taps-interface-06, 9 March 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-taps-
interface-06.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.ietf-taps-impl]
Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
"Implementing Interfaces to Transport Services", Work in
Progress, Internet-Draft, draft-ietf-taps-impl-06, 9 March
2020, <http://www.ietf.org/internet-drafts/draft-ietf-
taps-impl-06.txt>.
[I-D.ietf-taps-minset]
Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for End Systems", Work in Progress, Internet-
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]
Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
Wood, "A Survey of the Interaction Between Security
Protocols and Transport Services", Work in Progress,
Internet-Draft, draft-ietf-taps-transport-security-12, 23
April 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-taps-transport-security-12.txt>.
[POSIX] "IEEE Std. 1003.1-2008 Standard for Information Technology
-- Portable Operating System Interface (POSIX). Open
group Technical Standard: Base Specifications, Issue 7",
2008.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
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[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Authors' Addresses
Tommy Pauly (editor)
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: tpauly@apple.com
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Brian Trammell (editor)
Google Switzerland GmbH
Gustav-Gull-Platz 1
CH- 8004 Zurich
Switzerland
Email: ietf@trammell.ch
Anna Brunstrom
Karlstad University
Universitetsgatan 2
651 88 Karlstad
Sweden
Email: anna.brunstrom@kau.se
Godred Fairhurst
University of Aberdeen
Fraser Noble Building
Aberdeen, AB24 3UE
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
Philipp S. Tiesel
TU Berlin
Einsteinufer 25
10587 Berlin
Germany
Email: philipp@tiesel.net
Christopher A. Wood
Cloudflare
101 Townsend St
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San Francisco,
United States of America
Email: caw@heapingbits.net
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