An Abstract Application Layer Interface to Transport Services
draft-ietf-taps-interface-13
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| Authors | Brian Trammell , Michael Welzl , Reese Enghardt , Gorry Fairhurst , Mirja Kühlewind , Colin Perkins , Philipp S. Tiesel , Christopher A. Wood , Tommy Pauly , Kyle Rose | ||
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draft-ietf-taps-interface-13
TAPS Working Group B. Trammell, Ed.
Internet-Draft Google Switzerland GmbH
Intended status: Standards Track M. Welzl, Ed.
Expires: 13 January 2022 University of Oslo
T. Enghardt
Netflix
G. Fairhurst
University of Aberdeen
M. Kuehlewind
Ericsson
C. Perkins
University of Glasgow
P. Tiesel
SAP SE
C.A. Wood
Cloudflare
T. Pauly
Apple Inc.
K. Rose
Akamai Technologies, Inc.
12 July 2021
An Abstract Application Layer Interface to Transport Services
draft-ietf-taps-interface-13
Abstract
This document describes an abstract application programming
interface, API, to the transport layer that enables the selection of
transport protocols and network paths dynamically at runtime. This
API enables faster deployment of new protocols and protocol features
without requiring changes to the applications. The specified API
follows the Transport Services Architecture by providing
asynchronous, atomic transmission of messages. It is intended to
replace the traditional BSD sockets API as the common interface to
the transport layer, in an environment where endpoints could select
from multiple interfaces and potential transport protocols.
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/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 13 January 2022.
Copyright Notice
Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology and Notation . . . . . . . . . . . . . . . . 5
1.2. Specification of Requirements . . . . . . . . . . . . . . 7
2. Overview of Interface Design . . . . . . . . . . . . . . . . 7
3. API Summary . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Usage Examples . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Server Example . . . . . . . . . . . . . . . . . . . 10
3.1.2. Client Example . . . . . . . . . . . . . . . . . . . 10
3.1.3. Peer Example . . . . . . . . . . . . . . . . . . . . 12
4. Transport Properties . . . . . . . . . . . . . . . . . . . . 13
4.1. Transport Property Names . . . . . . . . . . . . . . . . 13
4.2. Transport Property Types . . . . . . . . . . . . . . . . 14
5. Scope of the Interface Definition . . . . . . . . . . . . . . 15
6. Pre-Establishment Phase . . . . . . . . . . . . . . . . . . . 16
6.1. Specifying Endpoints . . . . . . . . . . . . . . . . . . 17
6.1.1. Using Multicast Endpoints . . . . . . . . . . . . . . 18
6.1.2. Endpoint Aliases . . . . . . . . . . . . . . . . . . 18
6.1.3. Endpoint Examples . . . . . . . . . . . . . . . . . . 19
6.2. Specifying Transport Properties . . . . . . . . . . . . . 20
6.2.1. Reliable Data Transfer (Connection) . . . . . . . . . 23
6.2.2. Preservation of Message Boundaries . . . . . . . . . 23
6.2.3. Configure Per-Message Reliability . . . . . . . . . . 24
6.2.4. Preservation of Data Ordering . . . . . . . . . . . . 24
6.2.5. Use 0-RTT Session Establishment with a Safely
Replayable Message . . . . . . . . . . . . . . . . . 24
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6.2.6. Multistream Connections in Group . . . . . . . . . . 24
6.2.7. Full Checksum Coverage on Sending . . . . . . . . . . 25
6.2.8. Full Checksum Coverage on Receiving . . . . . . . . . 25
6.2.9. Congestion control . . . . . . . . . . . . . . . . . 25
6.2.10. Keep alive . . . . . . . . . . . . . . . . . . . . . 26
6.2.11. Interface Instance or Type . . . . . . . . . . . . . 26
6.2.12. Provisioning Domain Instance or Type . . . . . . . . 27
6.2.13. Use Temporary Local Address . . . . . . . . . . . . . 28
6.2.14. Multipath Transport . . . . . . . . . . . . . . . . . 28
6.2.15. Advertisement of Alternative Addresses . . . . . . . 29
6.2.16. Direction of communication . . . . . . . . . . . . . 30
6.2.17. Notification of ICMP soft error message arrival . . . 30
6.2.18. Initiating side is not the first to write . . . . . . 31
6.3. Specifying Security Parameters and Callbacks . . . . . . 31
6.3.1. Specifying Security Parameters on a Pre-Connection . 31
6.3.2. Connection Establishment Callbacks . . . . . . . . . 33
7. Establishing Connections . . . . . . . . . . . . . . . . . . 33
7.1. Active Open: Initiate . . . . . . . . . . . . . . . . . . 34
7.2. Passive Open: Listen . . . . . . . . . . . . . . . . . . 35
7.3. Peer-to-Peer Establishment: Rendezvous . . . . . . . . . 36
7.4. Connection Groups . . . . . . . . . . . . . . . . . . . . 38
8. Managing Connections . . . . . . . . . . . . . . . . . . . . 39
8.1. Generic Connection Properties . . . . . . . . . . . . . . 41
8.1.1. Required Minimum Corruption Protection Coverage for
Receiving . . . . . . . . . . . . . . . . . . . . . . 41
8.1.2. Connection Priority . . . . . . . . . . . . . . . . . 42
8.1.3. Timeout for Aborting Connection . . . . . . . . . . . 42
8.1.4. Timeout for keep alive packets . . . . . . . . . . . 42
8.1.5. Connection Group Transmission Scheduler . . . . . . . 43
8.1.6. Capacity Profile . . . . . . . . . . . . . . . . . . 43
8.1.7. Policy for using Multipath Transports . . . . . . . . 45
8.1.8. Bounds on Send or Receive Rate . . . . . . . . . . . 46
8.1.9. Group Connection Limit . . . . . . . . . . . . . . . 46
8.1.10. Isolate Session . . . . . . . . . . . . . . . . . . . 46
8.1.11. Read-only Connection Properties . . . . . . . . . . . 47
8.2. TCP-specific Properties: User Timeout Option (UTO) . . . 48
8.2.1. Advertised User Timeout . . . . . . . . . . . . . . . 48
8.2.2. User Timeout Enabled . . . . . . . . . . . . . . . . 48
8.2.3. Timeout Changeable . . . . . . . . . . . . . . . . . 49
8.3. Connection Lifecycle Events . . . . . . . . . . . . . . . 49
8.3.1. Soft Errors . . . . . . . . . . . . . . . . . . . . . 49
8.3.2. Path change . . . . . . . . . . . . . . . . . . . . . 49
9. Data Transfer . . . . . . . . . . . . . . . . . . . . . . . . 49
9.1. Messages and Framers . . . . . . . . . . . . . . . . . . 50
9.1.1. Message Contexts . . . . . . . . . . . . . . . . . . 50
9.1.2. Message Framers . . . . . . . . . . . . . . . . . . . 50
9.1.3. Message Properties . . . . . . . . . . . . . . . . . 53
9.2. Sending Data . . . . . . . . . . . . . . . . . . . . . . 58
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9.2.1. Basic Sending . . . . . . . . . . . . . . . . . . . . 59
9.2.2. Send Events . . . . . . . . . . . . . . . . . . . . . 59
9.2.3. Partial Sends . . . . . . . . . . . . . . . . . . . . 60
9.2.4. Batching Sends . . . . . . . . . . . . . . . . . . . 61
9.2.5. Send on Active Open: InitiateWithSend . . . . . . . . 61
9.2.6. Priority in TAPS . . . . . . . . . . . . . . . . . . 62
9.3. Receiving Data . . . . . . . . . . . . . . . . . . . . . 63
9.3.1. Enqueuing Receives . . . . . . . . . . . . . . . . . 63
9.3.2. Receive Events . . . . . . . . . . . . . . . . . . . 64
9.3.3. Receive Message Properties . . . . . . . . . . . . . 66
10. Connection Termination . . . . . . . . . . . . . . . . . . . 67
11. Connection State and Ordering of Operations and Events . . . 69
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70
13. Privacy and Security Considerations . . . . . . . . . . . . . 70
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 72
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 72
15.1. Normative References . . . . . . . . . . . . . . . . . . 72
15.2. Informative References . . . . . . . . . . . . . . . . . 73
Appendix A. Implementation Mapping . . . . . . . . . . . . . . . 76
A.1. Types . . . . . . . . . . . . . . . . . . . . . . . . . . 76
A.2. Events and Errors . . . . . . . . . . . . . . . . . . . . 77
A.3. Time Duration . . . . . . . . . . . . . . . . . . . . . . 77
Appendix B. Convenience Functions . . . . . . . . . . . . . . . 77
B.1. Adding Preference Properties . . . . . . . . . . . . . . 77
B.2. Transport Property Profiles . . . . . . . . . . . . . . . 78
B.2.1. reliable-inorder-stream . . . . . . . . . . . . . . . 78
B.2.2. reliable-message . . . . . . . . . . . . . . . . . . 78
B.2.3. unreliable-datagram . . . . . . . . . . . . . . . . . 79
Appendix C. Relationship to the Minimal Set of Transport Services
for End Systems . . . . . . . . . . . . . . . . . . . . . 80
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 83
1. Introduction
This document specifies a modern abstract application programming
interface (API) atop the high-level architecture for transport
services defined in [I-D.ietf-taps-arch]. The Transport Services
Architecture supports asynchronous, atomic transmission of messages
over transport protocols and network paths dynamically selected at
runtime, in environments where an endpoint selects from multiple
interfaces and potential transport protocols.
Applications that adopt this interface will benefit from a wide set
of transport features that can evolve over time. This protocol-
independent API ensures 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
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protocols, and can support applications by offering racing and
fallback mechanisms, which otherwise need to be separately
implemented in each application.
This API derives specific path and protocol selection properties and
supported transport features from the analysis provided in [RFC8095],
[RFC8923], and [RFC8922]. The design encourages implementations
underneath the interface to dynamically choose a transport protocol
depending on an application's choices rather than statically binding
applications to a protocol at compile time. Nevertheless, the
Transport Services API also provides applications with a way to
override transport selection and instantiate a specific stack, e.g.,
to support servers wishing to listen to a specific protocol.
However, forcing a specific transport stack choice is discouraged for
general use, because it can reduce portability.
1.1. Terminology and Notation
This API is described in terms of
* Objects with which an application can interact;
* Actions the application can perform on these Objects;
* Events, which an Object can send to an application to be processed
aynchronously; and
* Parameters associated with these Actions and Events.
The following notations, which can be combined, are used in this
document:
* An Action that creates an Object:
Object := Action()
* An Action that creates an array of Objects:
[]Object := Action()
* An Action that is performed on an Object:
Object.Action()
* An Object sends an Event:
Object -> Event<>
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* An Action takes a set of Parameters; an Event contains a set of
Parameters. Action and Event parameters whose names are suffixed
with a question mark are optional.
Action(param0, param1?, ...) / Event<param0, param1, ...>
Actions associated with no Object are Actions on the abstract
interface itself; they are equivalent to Actions on a per-application
global context.
Events are sent to the application or application-supplied code (e.g.
framers, see Section 9.1.2) for processing; the details of event
processing are platform- and implementation-specific.
We also make use of the following basic types:
* Boolean: Instances take the value "true" or "false".
* Integer: Instances take positive or negative numeric integer
values, or sometimes special non-numeric (symbolic) values.
* Numeric: Instances take positive or negative numeric values, or
sometimes special non-numeric (symbolic) values.
* Enumeration: A family of types in which each instance takes one of
a fixed, predefined set of values specific to a given enumerated
type.
* Tuple: An ordered grouping of multiple value types, represented as
a comma-separated list in parentheses, e.g., "(Enumeration,
Preference)". Instances take a sequence of values each valid for
the corresponding value type.
* Array: Denoted []Type, an instance takes a value for each of zero
or more elements in a sequence of the given Type. An array may be
of fixed or variable length.
* Collection: An unordered grouping of one or more values of the
same type.
For guidance on how these abstract concepts may be implemented in
languages in accordance with native design patterns and language and
platform features, see Appendix A.
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1.2. 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. Overview of Interface Design
The design of the interface specified in this document is based on a
set of principles, themselves an elaboration on the architectural
design principles defined in [I-D.ietf-taps-arch]. The interface
defined in this document provides:
* Access to a variety of transport protocols, independent of the
Protocol Stacks that will be used at runtime. All common features
of these protocol stacks are made available to the application in
a transport-independent way to the degree possible. This enables
applications written to a single API to make use of transport
protocols in terms of the features they provide.
* A unified interface to datagram and stream-oriented transports,
allowing use of a common API for connection establishment and
closing.
* Message-orientation, as opposed to stream-orientation, using
application-assisted framing and deframing where the underlying
transport does not provide these.
* Asynchronous Connection establishment, transmission, and
reception. This allows concurrent operations during establishment
and event-driven application interactions with the transport
layer, in line with developments in modern platforms and
programming languages;
* Selection between alternate network paths, using additional
information about the networks over which a connection can operate
(e.g. Provisioning Domain (PvD) information [RFC7556]) where
available.
* Explicit support for transport-specific features to be applied,
should that particular transport be part of a chosen Protocol
Stack.
* Explicit support for security properties as first-order transport
features.
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* Explicit support for configuration of cryptographic identities and
transport security parameters persistent across multiple
Connections.
* Explicit support for multistreaming and multipath transport
protocols, and the grouping of related Connections into Connection
Groups through cloning of Connections. This allows applications
to take full advantage of new transport protocols supporting these
features.
3. API Summary
The Transport Services API is the basic common abstract application
programming interface to the Transport Services Architecture defined
in the TAPS Architecture [I-D.ietf-taps-arch].
An application primarily interacts with this API through two Objects:
Preconnections and Connections. A Preconnection object (Section 6)
represents a set of properties and constraints on the selection and
configuration of paths and protocols to establish a Connection with
an Endpoint. A Connection object represents an instance of a
transport Protocol Stack on which data can be sent to and/or received
from a Remote Endpoint (i.e., a logical connection that, depending on
the kind of transport, can be bi-directional or unidirectional, and
that can use a stream protocol or a datagram protocol). Connections
are presented consistently to the application, irrespective of
whether the underlying transport is connection-less or connection-
oriented. Connections can be created from Preconnections in three
ways:
* by initiating the Preconnection (i.e., actively opening, as in a
client; Section 7.1),
* through listening on the Preconnection (i.e., passively opening,
as in a server Section 7.2),
* or rendezvousing on the Preconnection (i.e., peer to peer
establishment; Section 7.3).
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Once a Connection is established, data can be sent and received on it
in the form of Messages. The interface supports the preservation of
message boundaries both via explicit Protocol Stack support, and via
application support through a Message Framer that finds message
boundaries in a stream. Messages are received asynchronously through
event handlers registered by the application. Errors and other
notifications also happen asynchronously on the Connection. It is
not necessary for an application to handle all Events; some Events
may have implementation-specific default handlers. The application
should not assume that ignoring Events (e.g., Errors) is always safe.
Section 6, Section 7, Section 9.2, Section 9.3, and Section 10
describe the details of application interaction with Objects through
Actions and Events in each phase of a Connection, following the
phases (Pre-Establishment, Establishment, Data Transfer, and
Termination) described in Section 4.1 of [I-D.ietf-taps-arch].
3.1. Usage Examples
The following usage examples illustrate how an application might use
the Transport Services Interface to:
* Act as a server, by listening for incoming connections, receiving
requests, and sending responses, see Section 3.1.1.
* Act as a client, by connecting to a Remote Endpoint using
Initiate, sending requests, and receiving responses, see
Section 3.1.2.
* Act as a peer, by connecting to a Remote Endpoint using Rendezvous
while simultaneously waiting for incoming Connections, sending
Messages, and receiving Messages, see Section 3.1.3.
The examples in this section presume that a transport protocol is
available between the Local and Remote Endpoints that provides
Reliable Data Transfer, Preservation of data ordering, and
Preservation of Message Boundaries. In this case, the application
can choose to receive only complete messages.
If none of the available transport protocols provides Preservation of
Message Boundaries, but there is a transport protocol that provides a
reliable ordered byte stream, an application could receive this byte
stream as partial Messages and transform it into application-layer
Messages. Alternatively, an application might provide a Message
Framer, which can transform a sequence of Messages into a byte stream
and vice versa (Section 9.1.2).
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3.1.1. Server Example
This is an example of how an application might listen for incoming
Connections using the Transport Services Interface, and receive a
request, and send a response.
LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithInterface("any")
LocalSpecifier.WithService("https")
TransportProperties := NewTransportProperties()
TransportProperties.Require(preserve-msg-boundaries)
// Reliable Data Transfer and Preserve Order are Required by default
SecurityParameters := NewSecurityParameters()
SecurityParameters.Set(identity, myIdentity)
SecurityParameters.Set(key-pair, myPrivateKey, myPublicKey)
// Specifying a Remote Endpoint is optional when using Listen()
Preconnection := NewPreconnection(LocalSpecifier,
TransportProperties,
SecurityParameters)
Listener := Preconnection.Listen()
Listener -> ConnectionReceived<Connection>
// Only receive complete messages in a Conn.Received handler
Connection.Receive()
Connection -> Received<messageDataRequest, messageContext>
//---- Receive event handler begin ----
Connection.Send(messageDataResponse)
Connection.Close()
// Stop listening for incoming Connections
// (this example supports only one Connection)
Listener.Stop()
//---- Receive event handler end ----
3.1.2. Client Example
This is an example of how an application might open two Connections
to a remote application using the Transport Services Interface, and
send a request as well as receive a response on each of them.
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RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostname("example.com")
RemoteSpecifier.WithService("https")
TransportProperties := NewTransportProperties()
TransportProperties.Require(preserve-msg-boundaries)
// Reliable Data Transfer and Preserve Order are Required by default
SecurityParameters := NewSecurityParameters()
TrustCallback := NewCallback({
// Verify identity of the Remote Endpoint, return the result
})
SecurityParameters.SetTrustVerificationCallback(TrustCallback)
// Specifying a local endpoint is optional when using Initiate()
Preconnection := NewPreconnection(RemoteSpecifier,
TransportProperties,
SecurityParameters)
Connection := Preconnection.Initiate()
Connection2 := Connection.Clone()
Connection -> Ready<>
Connection2 -> Ready<>
//---- Ready event handler for any Connection C begin ----
C.Send(messageDataRequest)
// Only receive complete messages
C.Receive()
//---- Ready event handler for any Connection C end ----
Connection -> Received<messageDataResponse, messageContext>
Connection2 -> Received<messageDataResponse, messageContext>
// Close the Connection in a Receive event handler
Connection.Close()
Connection2.Close()
Preconnections are reusable after being used to initiate a
Connection. Hence, for example, after the Connections were closed,
the following would be correct: ~~~ //.. carry out adjustments to the
Preconnection, if desire Connection := Preconnection.Initiate() ~~~
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3.1.3. Peer Example
This is an example of how an application might establish a connection
with a peer using Rendezvous(), send a Message, and receive a
Message.
// Configure local candidates: a port on the Local Endpoint and via a STUN server
HostCandidate := NewLocalEndpoint()
HostCandidate.WithPort(9876)
StunCandidate := NewLocalEndpoint()
StunCandidate.WithStunServer(address, port, credentials)
LocalCandidates = [HostCandidate, StunCandidate]
// Configure transport and security properties
TransportProperties := ...
SecurityParameters := ...
Preconnection := NewPreconnection(LocalCandidates,
[], // No remote candidates yet
TransportProperties,
SecurityParameters)
// Resolve the LocalCandidates. The Preconnection.Resolve() call
// resolves both local and remote candidates but, since the remote
// candidates have not yet been specified, the ResolvedRemote list
// returned will be empty and is not used.
ResolvedLocal, ResolvedRemote = Preconnection.Resolve()
// ...Send the ResolvedLocal list to peer via signalling channel
// ...Receive a list of RemoteCandidates from peer via signalling channel
Preconnection.AddRemote(RemoteCandidates)
Preconnection.Rendezvous()
Preconnection -> RendezvousDone<Connection>
//---- RendezvousDone event handler begin ----
Connection.Send(messageDataRequest)
Connection.Receive()
//---- RendezvousDone event handler end ----
Connection -> Received<messageDataResponse, messageContext>
// Close the Connection in a Receive event handler
Connection.Close()
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4. Transport Properties
Each application using the Transport Services Interface declares its
preferences for how the transport service should operate using
properties at each stage of the lifetime of a connection using
Transport Properties, as defined in [I-D.ietf-taps-arch].
Transport Properties are divided into Selection, Connection, and
Message Properties. Selection Properties (see Section 6.2) can only
be set during pre-establishment. They are only used to specify which
paths and protocol stacks can be used and are preferred by the
application. Although Connection Properties (see Section 8.1) can be
set during pre-establishment, they may be changed later. They are
used to inform decisions made during establishment and to fine-tune
the established connection. Calling Initiate on a Preconnection
creates an outbound Connection or a Listener, and the Selection
Properties remain readable from the Connection or Listener, but
become immutable.
The behavior of the selected protocol stack(s) when sending Messages
is controlled by Message Properties (see Section 9.1.3).
Selection Properties can be set on Preconnections, and the effect of
Selection Properties can be queried on Connections and Messages.
Connection Properties can be set on Connections and Preconnections;
when set on Preconnections, they act as an initial default for the
resulting Connections. Message Properties can be set on Messages,
Connections, and Preconnections; when set on the latter two, they act
as an initial default for the Messages sent over those Connections,
Note that configuring Connection Properties and Message Properties on
Preconnections is preferred over setting them later. Early
specification of Connection Properties allows their use as additional
input to the selection process. Protocol Specific Properties, which
enable configuration of specialized features of a specific protocol,
see Section 3.2 of [I-D.ietf-taps-arch], are not used as an input to
the selection process, but only support configuration if the
respective protocol has been selected.
4.1. Transport Property Names
Transport Properties are referred to by property names. For the
purposes of this document, these names are alphanumeric strings in
which words may be separated by hyphens. These names serve two
purposes:
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* Allowing different components of a TAPS implementation to pass
Transport Properties, e.g., between a language frontend and a
policy manager, or as a representation of properties retrieved
from a file or other storage.
* Making the code of different TAPS implementations look similar.
While individual programming languages may preclude strict
adherence to the aforementioned naming convention (for instance,
by prohibiting the use of hyphens in symbols), users interacting
with multiple implementations will still benefit from the
consistency resulting from the use of visually similar symbols.
Transport Property Names are hierarchically organized in the form
[<Namespace>.]<PropertyName>.
* The Namespace component MUST be empty for well-known, generic
properties, i.e., for properties that are not specific to a
protocol and are defined in an RFC.
* Protocol Specific Properties MUST use the protocol acronym as the
Namespace, e.g., "tcp" for TCP specific Transport Properties. For
IETF protocols, property names under these namespaces SHOULD be
defined in an RFC.
* Vendor or implementation specific properties MUST use a string
identifying the vendor or implementation as the Namespace.
Namespaces for each of the keywords provided in the IANA protocol
numbers registry (see https://www.iana.org/assignments/protocol-
numbers/protocol-numbers.xhtml), reformatted where necessary to
conform to an implementation's naming conventions, are reserved for
Protocol Specific Properties and MUST NOT be used for vendor or
implementation-specific properties.
4.2. Transport Property Types
Each Transport Property has a one of the basic types described in
Section 1.1.
Most Selection Properties (see Section 6.2) are of the Enumeration
type, and use the Preference Enumeration, which takes one of five
possible values (Prohibit, Avoid, Ignore, Prefer, or Require)
denoting the level of preference for a given property during protocol
selection.
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5. Scope of the Interface Definition
This document defines a language- and platform-independent interface
to a Transport Services system. Given the wide variety of languages
and language conventions used to write applications that use the
transport layer to connect to other applications over the Internet,
this independence makes this interface necessarily abstract.
There is no interoperability benefit in tightly defining how the
interface is presented to application programmers across diverse
platforms. However, maintaining the "shape" of the abstract
interface across different platforms reduces the effort for
programmers who learn the transport services interface to then apply
their knowledge to another platform.
We therefore make the following recommendations:
* Actions, Events, and Errors in implementations of this interface
SHOULD use the names given for them in the document, subject to
capitalization, punctuation, and other typographic conventions in
the language of the implementation, unless the implementation
itself uses different names for substantially equivalent objects
for networking by convention.
* Implementations of this interface SHOULD implement each Selection
Property, Connection Property, and Message Context Property
specified in this document. Each interface SHOULD be implemented
even when in a specific implementation/platform it will always
result in no operation, e.g. there is no action when the API
specifies a Property that is not available in a transport protocol
implemented on a specific platform. For example, if TCP is the
only underlying transport protocol, the Message Property
"msgOrdered" can be implemented (trivially, as a no-op) as
disabling the requirement for ordering will not have any effect on
delivery order for Connections over TCP. Similarly, the "msg-
lifetime" Message Property can be implemented but ignored, as the
description of this Property states that "it is not guaranteed
that a Message will not be sent when its Lifetime has expired".
* Implementations may use other representations for Transport
Property Names, e.g., by providing constants, but should provide a
straight-forward mapping between their representation and the
property names specified here.
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6. Pre-Establishment Phase
The Pre-Establishment phase allows applications to specify properties
for the Connections that they are about to make, or to query the API
about potential Connections they could make.
A Preconnection Object represents a potential Connection. It is a
passive Object (a data structure) that merely maintains the state
that describes the properties of a Connection that might exist in the
future. This state comprises Local Endpoint and Remote Endpoint
Objects that denote the endpoints of the potential Connection (see
Section 6.1), the Selection Properties (see Section 6.2), any
preconfigured Connection Properties (Section 8.1), and the security
parameters (see Section 6.3):
Preconnection := NewPreconnection([]LocalEndpoint,
[]RemoteEndpoint,
TransportProperties,
SecurityParameters)
At least one Local Endpoint MUST be specified if the Preconnection is
used to Listen() for incoming Connections, but the list of Local
Endpoints MAY be empty if the Preconnection is used to Initiate()
connections. If no Local Endpoint is specified, the Transport
Services system will assign an ephemeral local port to the Connection
on the appropriate interface(s). At least one Remote Endpoint MUST
be specified if the Preconnection is used to Initiate() Connections,
but the list of Remote Endpoints MAY be empty if the Preconnection is
used to Listen() for incoming Connections. At least one Local
Endpoint and one Remote Endpoint MUST be specified if a peer-to-peer
Rendezvous() is to occur based on the Preconnection.
If more than one Local Endpoint is specified on a Preconnection, then
all the Local Endpoints on the Preconnection MUST represent the same
host. For example, they might correspond to different interfaces on
a multi-homed host, of they might correspond to local interfaces and
a STUN server that can be resolved to a server reflexive address for
a Preconnection used to make a peer-to-peer Rendezvous().
If more than one Remote Endpoint is specified on the Preconnection,
then all the Remote Endpoints on the Preconnection SHOULD represent
the same host. For example, the Remote Endpoints might represent
various network interfaces of a host, or a server reflexive address
that can be used to reach a host, or a set of hosts that provide
equivalent local balanced service.
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In most cases, it is expected that a single Remote Endpoint will be
specified by name, and a later call to Initiate() on the
Preconnection (see Section 7.1) will internally resolve that name to
a list of concrete endpoints. Specifying multiple Remote Endpoints
on a Preconnection allows applications to override this for more
detailed control.
If Message Framers are used (see Section 9.1.2), they MUST be added
to the Preconnection during pre-establishment.
6.1. Specifying Endpoints
The transport services API uses the Local Endpoint and Remote
Endpoint Objects to refer to the endpoints of a transport connection.
Endpoints can be created as either Remote or Local:
RemoteSpecifier := NewRemoteEndpoint()
LocalSpecifier := NewLocalEndpoint()
A single Endpoint Object represents the identity of a network host.
That endpoint can be more or less specific depending on which
identifiers are set. For example, an Endpoint that only specifies a
hostname may in fact end up corresponding to several different IP
addresses on different hosts.
An Endpoint Object can be configured with the following identifiers:
* Hostname (string):
RemoteSpecifier.WithHostname("example.com")
* Port (a 16-bit integer) or a Service (string) that maps to a port:
RemoteSpecifier.WithPort(443)
RemoteSpecifier.WithService("https")
* IP address (IPv4 or IPv6 address):
RemoteSpecifier.WithIPv4Address(192.0.2.21)
RemoteSpecifier.WithIPv6Address(2001:db8:4920:e29d:a420:7461:7073:0a)
* Interface name (string):
LocalSpecifier.WithInterface("en0")
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An Endpoint cannot have multiple identifiers of a same type set.
That is, an endpoint cannot have two IP addresses specified. Two
separate IP addresses are represented as two Endpoint Objects. If a
Preconnection specifies a Remote Endpoint with a specific IP address
set, it will only establish Connections to that IP address. If, on
the other hand, the Remote Endpoint specifies a hostname but no
addresses, the Connection can perform name resolution and attempt
using any address derived from the original hostname of the Remote
Endpoint.
The Transport Services API resolves names internally, when the
Initiate(), Listen(), or Rendezvous() method is called to establish a
Connection. Privacy considerations for the timing of this resolution
are given in Section 13.
The Resolve() action on a Preconnection can be used by the
application to force early binding when required, for example with
some Network Address Translator (NAT) traversal protocols (see
Section 7.3).
6.1.1. Using Multicast Endpoints
Specifying a multicast group address on a Local Endpoint will
indicate to the transport system that the resulting connection will
be used to receive multicast messages. The Remote Endpoint can be
used to filter incoming multicast from specific senders. Such a
Preconnection will only support calling Listen(), not Initiate().
The accepted Connections are receive-only.
Similarly, specifying a multicast group address on the Remote
Endpoint will indicate that the resulting connection will be used to
send multicast messages.
6.1.2. Endpoint Aliases
An Endpoint can have an alternative definition when using different
protocols. For example, a server that supports both TLS/TCP and QUIC
may be accessible on two different port numbers depending on which
protocol is used.
To support this, Endpoint Objects can specify "aliases". An Endpoint
can have multiple aliases set.
RemoteSpecifier.AddAlias(AlternateRemoteSpecifier)
In order to scope an alias to a specific transport protocol, an
Endpoint can specify a protocol identifier.
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RemoteSpecifier.WithProtocol(QUIC)
The following example shows a case where "example.com" has a server
running on port 443, with an alternate port of 8443 for QUIC.
RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostname("example.com")
RemoteSpecifier.WithPort(443)
QUICRemoteSpecifier := NewRemoteEndpoint()
QUICRemoteSpecifier.WithHostname("example.com")
QUICRemoteSpecifier.WithPort(8443)
QUICRemoteSpecifier.WithProtocol(QUIC)
RemoteSpecifier.AddAlias(QUICRemoteSpecifier)
6.1.3. Endpoint Examples
The following examples of Endpoints show common usage patterns.
Specify a Remote Endpoint using a hostname and service name:
RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostname("example.com")
RemoteSpecifier.WithService("https")
Specify a Remote Endpoint using an IPv6 address and remote port:
RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPv6Address(2001:db8:4920:e29d:a420:7461:7073:0a)
RemoteSpecifier.WithPort(443)
Specify a Remote Endpoint using an IPv4 address and remote port:
RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPv4Address(192.0.2.21)
RemoteSpecifier.WithPort(443)
Specify a Local Endpoint using a local interface name and local port:
LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithInterface("en0")
LocalSpecifier.WithPort(443)
As an alternative to specifying an interface name for the Local
Endpoint, an application can express more fine-grained preferences
using the "Interface Instance or Type" Selection Property, see
Section 6.2.11. However, if the application specifies Selection
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Properties that are inconsistent with the Local Endpoint, this will
result in an Error once the application attempts to open a
Connection.
Specify a Local Endpoint using a STUN server:
LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithStunServer(address, port, credentials)
Specify a Local Endpoint using an Any-Source Multicast group to join
on a named local interface:
LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithIPv4Address(233.252.0.0)
LocalSpecifier.WithInterface("en0")
Source-Specific Multicast requires setting both a Local and Remote
Endpoint:
LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithIPv4Address(232.1.1.1)
LocalSpecifier.WithInterface("en0")
RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPv4Address(192.0.2.22)
6.2. Specifying Transport Properties
A Preconnection Object holds properties reflecting the application's
requirements and preferences for the transport. These include
Selection Properties for selecting protocol stacks and paths, as well
as Connection Properties for configuration of the detailed operation
of the selected Protocol Stacks.
The protocol(s) and path(s) selected as candidates during
establishment are determined and configured using these properties.
Since there could be paths over which some transport protocols are
unable to operate, or remote endpoints that support only specific
network addresses or transports, transport protocol selection is
necessarily tied to path selection. This may involve choosing
between multiple local interfaces that are connected to different
access networks.
When additional information (such as Provisioning Domain (PvD)
information Path information can include network segment PMTU, set of
supported DSCPs, expected usage, cost, etc. The usage of this
information by the Transport Services System is generally independent
of the specific mechanism/protocol used to receive the information
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(e.g. zero-conf, DHCP, or IPv6 RA).[RFC7556]) is available about the
networks over which an endpoint can operate, this can inform the
selection between alternate network paths.
Most Selection Properties are represented as Preferences, which can
take one of five values:
+============+========================================+
| Preference | Effect |
+============+========================================+
| Require | Select only protocols/paths providing |
| | the property, fail otherwise |
+------------+----------------------------------------+
| Prefer | Prefer protocols/paths providing the |
| | property, proceed otherwise |
+------------+----------------------------------------+
| Ignore | No preference |
+------------+----------------------------------------+
| Avoid | Prefer protocols/paths not providing |
| | the property, proceed otherwise |
+------------+----------------------------------------+
| Prohibit | Select only protocols/paths not |
| | providing the property, fail otherwise |
+------------+----------------------------------------+
Table 1: Selection Property Preference Levels
The implementation MUST ensure an outcome that is consistent with all
application requirements expressed using Require and Prohibit. While
preferences expressed using Prefer and Avoid influence protocol and
path selection as well, outcomes can vary given the same Selection
Properties, because the available protocols and paths can differ
across systems and contexts. However, implementations are
RECOMMENDED to seek to provide a consistent outcome to an
application, given the same set of Selection Properties.
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Note that application preferences can conflict with each other. For
example, if an application indicates a preference for a specific path
by specifying an interface, but also a preference for a protocol, a
situation might occur in which the preferred protocol is not
available on the preferred path. In such cases, applications can
expect properties that determine path selection to be prioritized
over properties that determine protocol selection. The transport
system SHOULD determine the preferred path first, regardless of
protocol preferences. This ordering is chosen to provide consistency
across implementations, based on the fact that it is more common for
the use of a given network path to determine cost to the user (i.e.,
an interface type preference might be based on a user's preference to
avoid being charged more for a cellular data plan).
Selection and Connection Properties, as well as defaults for Message
Properties, can be added to a Preconnection to configure the
selection process and to further configure the eventually selected
protocol stack(s). They are collected into a TransportProperties
object to be passed into a Preconnection object:
TransportProperties := NewTransportProperties()
Individual properties are then set on the TransportProperties Object.
Setting a Transport Property to a value overrides the previous value
of this Transport Property.
TransportProperties.Set(property, value)
To aid readability, implementations MAY provide additional
convenience functions to simplify use of Selection Properties: see
Appendix B.1 for examples. In addition, implementations MAY provide
a mechanism to create TransportProperties objects that are
preconfigured for common use cases as outlined in Appendix B.2.
Transport Properties for an established connection can be queried via
the Connection object, as outlined in Section 8.
A Connection gets its Transport Properties either by being explicitly
configured via a Preconnection, by configuration after establishment,
or by inheriting them from an antecedent via cloning; see Section 7.4
for more.
Section 8.1 provides a list of Connection Properties, while Selection
Properties are listed in the subsections below. Many properties are
only considered during establishment, and can not be changed after a
Connection is established; however, they can still be queried. The
return type of a queried Selection Property is Boolean, where "true"
means that the Selection Property has been applied and "false" means
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that the Selection Property has not been applied. Note that "true"
does not mean that a request has been honored. For example, if
"Congestion control" was requested with preference level "Prefer",
but congestion control could not be supported, querying the
"congestionControl" property yields the value "false". If the
preference level "Avoid" was used for "Congestion control", and, as
requested, the Connection is not congestion controlled, querying the
"congestionControl" property also yields the value "false".
An implementation of this interface must provide sensible defaults
for Selection Properties. The default values for each property below
represent a configuration that can be implemented over TCP. If these
default values are used and TCP is not supported by a Transport
Services implementation, then an application using the default set of
Properties might not succeed in establishing a connection. Using the
same default values for independent Transport Services
implementations can be beneficial when applications are ported
between different implementations/platforms, even if this default
could lead to a connection failure when TCP is not available. If
default values other than those suggested below are used, it is
RECOMMENDED to clearly document any differences.
6.2.1. Reliable Data Transfer (Connection)
Name: reliability
Type: Preference
Default: Require
This property specifies whether the application needs to use a
transport protocol that ensures that all data is received at the
Remote Endpoint without corruption. When reliable data transfer is
enabled, this also entails being notified when a Connection is closed
or aborted.
6.2.2. Preservation of Message Boundaries
Name: preserveMsgBoundaries
Type: Preference
Default: Ignore
This property specifies whether the application needs or prefers to
use a transport protocol that preserves message boundaries.
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6.2.3. Configure Per-Message Reliability
Name: perMsgReliability
Type: Preference
Default: Ignore
This property specifies whether an application considers it useful to
specify different reliability requirements for individual Messages in
a Connection.
6.2.4. Preservation of Data Ordering
Name: preserveOrder
Type: Preference
Default: Require
This property specifies whether the application wishes to use a
transport protocol that can ensure that data is received by the
application on the other end in the same order as it was sent.
6.2.5. Use 0-RTT Session Establishment with a Safely Replayable Message
Name: zeroRttMsg
Type: Preference
Default: Ignore
This property specifies whether an application would like to supply a
Message to the transport protocol before Connection establishment
that will then be reliably transferred to the other side before or
during Connection establishment. This Message can potentially be
received multiple times (i.e., multiple copies of the message data
may be passed to the Remote Endpoint). See also Section 9.1.3.4.
6.2.6. Multistream Connections in Group
Name: multistreaming
Type: Preference
Default: Prefer
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This property specifies that the application would prefer multiple
Connections within a Connection Group to be provided by streams of a
single underlying transport connection where possible.
6.2.7. Full Checksum Coverage on Sending
Name: FullChecksumSend
Type: Preference
Default: Require
This property specifies the application's need for protection against
corruption for all data transmitted on this Connection. Disabling
this property could enable later control of the sender checksum
coverage (see Section 9.1.3.6).
6.2.8. Full Checksum Coverage on Receiving
Name: FullChecksumRecv
Type: Preference
Default: Require
This property specifies the application's need for protection against
corruption for all data received on this Connection. Disabling this
property could enable later control of the required minimum receiver
checksum coverage (see Section 8.1.1).
6.2.9. Congestion control
Name: congestionControl
Type: Preference
Default: Require
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This property specifies whether the application would like the
Connection to be congestion controlled or not. Note that if a
Connection is not congestion controlled, an application using such a
Connection SHOULD itself perform congestion control in accordance
with [RFC2914] or use a circuit breaker in accordance with [RFC8084],
whichever is appropriate. Also note that reliability is usually
combined with congestion control in protocol implementations,
rendering "reliable but not congestion controlled" a request that is
unlikely to succeed. If the Connection is congestion controlled,
performing additional congestion control in the application can have
negative performance implications.
6.2.10. Keep alive
Name: keepAlive
Type: Preference
Default: Ignore
This property specifies whether the application would like the
Connection to send keep-alive packets or not. Note that if a
Connection determines that keep-alive packets are being sent, the
applicaton should itself avoid generating additional keep alive
messages. Note that when supported, the system will use the default
period for generation of the keep alive-packets. (See also
Section 8.1.4).
6.2.11. Interface Instance or Type
Name: interface
Type: Collection of (Preference, Enumeration)
Default: Empty (not setting a preference for any interface)
This property allows the application to select any specific network
interfaces or categories of interfaces it wants to "Require",
"Prohibit", "Prefer", or "Avoid". Note that marking a specific
interface as "Require" strictly limits path selection to that single
interface, and often leads to less flexible and resilient connection
establishment.
In contrast to other Selection Properties, this property is a tuple
of an (Enumerated) interface identifier and a preference, and can
either be implemented directly as such, or for making one preference
available for each interface and interface type available on the
system.
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The set of valid interface types is implementation- and system-
specific. For example, on a mobile device, there may be "Wi-Fi" and
"Cellular" interface types available; whereas on a desktop computer,
"Wi-Fi" and "Wired Ethernet" interface types might be available. An
implementation should provide all types that are supported on the
local system, to allow applications to be written generically. For
example, if a single implementation is used on both mobile devices
and desktop devices, it should define the "Cellular" interface type
for both systems, since an application might wish to always prohibit
cellular.
The set of interface types is expected to change over time as new
access technologies become available. The taxonomy of interface
types on a given Transport Services system is implementation-
specific.
Interface types should not be treated as a proxy for properties of
interfaces such as metered or unmetered network access. If an
application needs to prohibit metered interfaces, this should be
specified via Provisioning Domain attributes (see Section 6.2.12) or
another specific property.
6.2.12. Provisioning Domain Instance or Type
Name: pvd
Type: Collection of (Preference, Enumeration)
Default: Empty (not setting a preference for any PvD)
Similar to interface instances and types (see Section 6.2.11), this
property allows the application to control path selection by
selecting which specific Provisioning Domain (PvD) or categories of
PVDs it wants to "Require", "Prohibit", "Prefer", or "Avoid".
Provisioning Domains define consistent sets of network properties
that may be more specific than network interfaces [RFC7556].
As with interface instances and types, this property is a tuple of an
(Enumerated) PvD identifier and a preference, and can either be
implemented directly as such, or for making one preference available
for each interface and interface type available on the system.
The identification of a specific PvD is implementation- and system-
specific, because there is currently no portable standard format for
a PvD identifier. For example, this identifier might be a string
name or an integer. As with requiring specific interfaces, requiring
a specific PvD strictly limits the path selection.
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Categories or types of PvDs are also defined to be implementation-
and system-specific. These can be useful to identify a service that
is provided by a PvD. For example, if an application wants to use a
PvD that provides a Voice-Over-IP service on a Cellular network, it
can use the relevant PvD type to require a PvD that provides this
service, without needing to look up a particular instance. While
this does restrict path selection, it is broader than requiring
specific PvD instances or interface instances, and should be
preferred over these options.
6.2.13. Use Temporary Local Address
Name: useTemporaryLocalAddress
Type: Preference
Default: Avoid for Listeners and Rendezvous Connections. Prefer for
other Connections.
This property allows the application to express a preference for the
use of temporary local addresses, sometimes called "privacy"
addresses [RFC4941]. Temporary addresses are generally used to
prevent linking connections over time when a stable address,
sometimes called "permanent" address, is not needed. There are some
caveats to note when specifying this property. First, if an
application Requires the use of temporary addresses, the resulting
Connection cannot use IPv4, because temporary addresses do not exist
in IPv4. Second, temporary local addresses might involve trading off
privacy for performance. For instance, temporary addresses can
interfere with resumption mechanisms that some protocols rely on to
reduce initial latency.
6.2.14. Multipath Transport
Name: multipath
Type: Enumeration
Default: Disabled for connections created through initiate and
rendezvous, Passive for listeners
This property specifies whether and how applications want to take
advantage of transferring data across multiple paths between the same
end hosts. Using multiple paths allows connections to migrate
between interfaces or aggregate bandwidth as availability and
performance properties change. Possible values are:
Disabled: The connection will not use multiple paths once
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established, even if the chosen transport supports using multiple
paths.
Active: The connection will negotiate the use of multiple paths if
the chosen transport supports this.
Passive: The connection will support the use of multiple paths if
the Remote Endpoint requests it.
The policy for using multiple paths is specified using the separate
"multipath-policy" property, see Section 8.1.7 below. To enable the
peer endpoint to initiate additional paths towards a local address
other than the one initially used, it is necessary to set the
Alternative Addresses property (see Section 6.2.15 below).
Setting this property to "Active", can have privacy implications: It
enables the transport to establish connectivity using alternate paths
that might result in users being linkable across the multiple paths,
even if the Advertisement of Alternative Addresses property (see
Section 6.2.15 below) is set to false.
Note that Multipath Transport has no corresponding Selection Property
of type Preference. Enumeration values other than "Disabled" are
interpreted as a preference for choosing protocols that can make use
of multiple paths. The "Disabled" value implies a requirement not to
use multiple paths in parallel but does not prevent choosing a
protocol that is capable of using multiple paths, e.g., it does not
prevent choosing TCP, but prevents sending the "MP_CAPABLE" option in
the TCP handshake.
6.2.15. Advertisement of Alternative Addresses
Name: advertises-altaddr
Type: Boolean
Default: False
This property specifies whether alternative addresses, e.g., of other
interfaces, should be advertised to the peer endpoint by the protocol
stack. Advertising these addresses enables the peer-endpoint to
establish additional connectivity, e.g., for connection migration or
using multiple paths.
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Note that this can have privacy implications because it might result
in users being linkable across the multiple paths. Also, note that
setting this to false does not prevent the local Transport Services
system from _establishing_ connectivity using alternate paths (see
Section 6.2.14 above); it only prevents _proactive advertisement_ of
addresses.
6.2.16. Direction of communication
Name: direction
Type: Enumeration
Default: Bidirectional
This property specifies whether an application wants to use the
connection for sending and/or receiving data. Possible values are:
Bidirectional: The connection must support sending and receiving
data
Unidirectional send: The connection must support sending data, and
the application cannot use the connection to receive any data
Unidirectional receive: The connection must support receiving data,
and the application cannot use the connection to send any data
Since unidirectional communication can be supported by transports
offering bidirectional communication, specifying unidirectional
communication may cause a transport stack that supports bidirectional
communication to be selected.
6.2.17. Notification of ICMP soft error message arrival
Name: softErrorNotify
Type: Preference
Default: Ignore
This property specifies whether an application considers it useful to
be informed when an ICMP error message arrives that does not force
termination of a connection. When set to true, received ICMP errors
are available as SoftErrors, see Section 8.3.1. Note that even if a
protocol supporting this property is selected, not all ICMP errors
will necessarily be delivered, so applications cannot rely upon
receiving them [RFC8085].
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6.2.18. Initiating side is not the first to write
Name: activeReadBeforeSend
Type: Preference
Default: Ignore
The most common client-server communication pattern involves the
client actively opening a connection, then sending data to the
server. The server listens (passive open), reads, and then answers.
This property specifies whether an application wants to diverge from
this pattern - either by actively opening with Initiate(),
immediately followed by reading, or passively opening with Listen(),
immediately followed by writing. This property is ignored when
establishing connections using Rendezvous(). Requiring this property
limits the choice of mappings to underlying protocols, which can
reduce efficiency. For example, it prevents the Transport Services
system from mapping Connections to SCTP streams, where the first
transmitted data takes the role of an active open signal
[I-D.ietf-taps-impl].
6.3. Specifying Security Parameters and Callbacks
Most security parameters, e.g., TLS ciphersuites, local identity and
private key, etc., may be configured statically. Others are
dynamically configured during connection establishment. Security
parameters and callbacks are partitioned based on their place in the
lifetime of connection establishment. Similar to Transport
Properties, both parameters and callbacks are inherited during
cloning (see Section 7.4).
6.3.1. Specifying Security Parameters on a Pre-Connection
Common security parameters such as TLS ciphersuites are known to
implementations. Clients should use common safe defaults for these
values whenever possible. However, as discussed in [RFC8922], many
transport security protocols require specific security parameters and
constraints from the client at the time of configuration and actively
during a handshake. These configuration parameters need to be
specified in the pre-connection phase and are created as follows:
SecurityParameters := NewSecurityParameters()
Security configuration parameters and sample usage follow:
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* Local identity and private keys: Used to perform private key
operations and prove one's identity to the Remote Endpoint.
(Note, if private keys are not available, e.g., since they are
stored in hardware security modules (HSMs), handshake callbacks
must be used. See below for details.)
SecurityParameters.Set(identity, myIdentity)
SecurityParameters.Set(key-pair, myPrivateKey, myPublicKey)
* Supported algorithms: Used to restrict what parameters are used by
underlying transport security protocols. When not specified,
these algorithms should use known and safe defaults for the
system. Parameters include: ciphersuites, supported groups, and
signature algorithms. These parameters take a collection of
supported algorithms as parameter.
SecurityParameters.Set(supported-group, "secp256k1")
SecurityParameters.Set(ciphersuite, "TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256")
SecurityParameters.Set(signature-algorithm, "ed25519")
* Pre-Shared Key import: Used to install pre-shared keying material
established out-of-band. Each pre-shared keying material is
associated with some identity that typically identifies its use or
has some protocol-specific meaning to the Remote Endpoint.
SecurityParameters.Set(pre-shared-key, key, identity)
* Session cache management: Used to tune session cache capacity,
lifetime, and other policies.
SecurityParameters.Set(max-cached-sessions, 16)
SecurityParameters.Set(cached-session-lifetime-seconds, 3600)
Connections that use Transport Services SHOULD use security in
general. However, for compatibility with endpoints that do not
support transport security protocols (such as a TCP endpoint that
does not support TLS), applications can initialize their security
parameters to indicate that security can be disabled, or can be
opportunistic. If security is disabled, the Transport Services
system will not attempt to add transport security automatically. If
security is opportunistic, it will allow Connections without
transport security, but will still attempt to use security if
available.
SecurityParameters := NewDisabledSecurityParameters()
SecurityParameters := NewOpportunisticSecurityParameters()
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Representation of Security Parameters in implementations should
parallel that chosen for Transport Property names as sugggested in
Section 5.
6.3.2. Connection Establishment Callbacks
Security decisions, especially pertaining to trust, are not static.
Once configured, parameters may also be supplied during connection
establishment. These are best handled as client-provided callbacks.
Callbacks block the progress of the connection establishment, which
distinguishes them from other Events in the transport system. How
callbacks and events are implemented is specific to each
implementation. Security handshake callbacks that may be invoked
during connection establishment include:
* Trust verification callback: Invoked when a Remote Endpoint's
trust must be validated before the handshake protocol can
continue.
TrustCallback := NewCallback({
// Handle trust, return the result
})
SecurityParameters.SetTrustVerificationCallback(trustCallback)
* Identity challenge callback: Invoked when a private key operation
is required, e.g., when local authentication is requested by a
Remote Endpoint.
ChallengeCallback := NewCallback({
// Handle challenge
})
SecurityParameters.SetIdentityChallengeCallback(challengeCallback)
7. Establishing Connections
Before a Connection can be used for data transfer, it needs to be
established. Establishment ends the pre-establishment phase; all
transport properties and cryptographic parameter specification must
be complete before establishment, as these will be used to select
candidate Paths and Protocol Stacks for the Connection.
Establishment may be active, using the Initiate() Action; passive,
using the Listen() Action; or simultaneous for peer-to-peer, using
the Rendezvous() Action. These Actions are described in the
subsections below.
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7.1. Active Open: Initiate
Active open is the Action of establishing a Connection to a Remote
Endpoint presumed to be listening for incoming Connection requests.
Active open is used by clients in client-server interactions. Active
open is supported by this interface through the Initiate Action:
Connection := Preconnection.Initiate(timeout?)
The timeout parameter specifies how long to wait before aborting
Active open. Before calling Initiate, the caller must have populated
a Preconnection Object with a Remote Endpoint specifier, optionally a
Local Endpoint specifier (if not specified, the system will attempt
to determine a suitable Local Endpoint), as well as all properties
necessary for candidate selection.
The Initiate() Action returns a Connection object. Once Initiate()
has been called, any changes to the Preconnection MUST NOT have any
effect on the Connection. However, the Preconnection can be reused,
e.g., to Initiate another Connection.
Once Initiate is called, the candidate Protocol Stack(s) may cause
one or more candidate transport-layer connections to be created to
the specified Remote Endpoint. The caller may immediately begin
sending Messages on the Connection (see Section 9.2) after calling
Initiate(); note that any data marked "Safely Replayable" that is
sent while the Connection is being established may be sent multiple
times or on multiple candidates.
The following Events may be sent by the Connection after Initiate()
is called:
Connection -> Ready<>
The Ready Event occurs after Initiate has established a transport-
layer connection on at least one usable candidate Protocol Stack over
at least one candidate Path. No Receive Events (see Section 9.3)
will occur before the Ready Event for Connections established using
Initiate.
Connection -> EstablishmentError<reason?>
An EstablishmentError occurs either when the set of transport
properties and security parameters cannot be fulfilled on a
Connection for initiation (e.g., the set of available Paths and/or
Protocol Stacks meeting the constraints is empty) or reconciled with
the Local and/or Remote Endpoints; when the remote specifier cannot
be resolved; or when no transport-layer connection can be established
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to the Remote Endpoint (e.g., because the Remote Endpoint is not
accepting connections, the application is prohibited from opening a
Connection by the operating system, or the establishment attempt has
timed out for any other reason).
Connection establishment and transmission of the first message can be
combined in a single action Section 9.2.5.
7.2. Passive Open: Listen
Passive open is the Action of waiting for Connections from Remote
Endpoints, commonly used by servers in client-server interactions.
Passive open is supported by this interface through the Listen Action
and returns a Listener object:
Listener := Preconnection.Listen()
Before calling Listen, the caller must have initialized the
Preconnection during the pre-establishment phase with a Local
Endpoint specifier, as well as all properties necessary for Protocol
Stack selection. A Remote Endpoint may optionally be specified, to
constrain what Connections are accepted.
The Listen() Action returns a Listener object. Once Listen() has
been called, any changes to the Preconnection MUST NOT have any
effect on the Listener. The Preconnection can be disposed of or
reused, e.g., to create another Listener.
Listener.Stop()
Listening continues until the global context shuts down, or until the
Stop action is performed on the Listener object.
Listener -> ConnectionReceived<Connection>
The ConnectionReceived Event occurs when a Remote Endpoint has
established a transport-layer connection to this Listener (for
Connection-oriented transport protocols), or when the first Message
has been received from the Remote Endpoint (for Connectionless
protocols), causing a new Connection to be created. The resulting
Connection is contained within the ConnectionReceived Event, and is
ready to use as soon as it is passed to the application via the
event.
Listener.SetNewConnectionLimit(value)
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If the caller wants to rate-limit the number of inbound Connections
that will be delivered, it can set a cap using
SetNewConnectionLimit(). This mechanism allows a server to protect
itself from being drained of resources. Each time a new Connection
is delivered by the ConnectionReceived Event, the value is
automatically decremented. Once the value reaches zero, no further
Connections will be delivered until the caller sets the limit to a
higher value. By default, this value is Infinite. The caller is
also able to reset the value to Infinite at any point.
Listener -> EstablishmentError<reason?>
An EstablishmentError occurs either when the Properties and Security
Parameters of the Preconnection cannot be fulfilled for listening or
cannot be reconciled with the Local Endpoint (and/or Remote Endpoint,
if specified), when the Local Endpoint (or Remote Endpoint, if
specified) cannot be resolved, or when the application is prohibited
from listening by policy.
Listener -> Stopped<>
A Stopped Event occurs after the Listener has stopped listening.
7.3. Peer-to-Peer Establishment: Rendezvous
Simultaneous peer-to-peer Connection establishment is supported by
the Rendezvous() Action:
Preconnection.Rendezvous()
A Preconnection Object used in a Rendezvous() MUST have both the
Local Endpoint candidates and the Remote Endpoint candidates
specified, along with the transport properties and security
parameters needed for Protocol Stack selection, before the
Rendezvous() Action is initiated.
The Rendezvous() Action listens on the Local Endpoint candidates for
an incoming Connection from the Remote Endpoint candidates, while
also simultaneously trying to establish a Connection from the Local
Endpoint candidates to the Remote Endpoint candidates.
If there are multiple Local Endpoints or Remote Endpoints configured,
then initiating a Rendezvous() action will systematically probe the
reachability of those endpoint candidates following an approach such
as that used in Interactive Connectivity Establishment (ICE)
[RFC5245].
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If the endpoints are suspected to be behind a NAT, Rendezvous() can
be initiated using Local Endpoints that support a method of
discovering NAT bindings such as Session Traversal Utilities for NAT
(STUN) [RFC8489] or Traversal Using Relays around NAT (TURN)
[RFC5766]. In this case, the Local Endpoint will resolve to a
mixture of local and server reflexive addresses. The Resolve()
action on the Preconnection can be used to discover these bindings:
[]LocalEndpoint, []RemoteEndpoint := Preconnection.Resolve()
The Resolve() call returns lists of Local Endpoints and Remote
Endpoints, that represent the concrete addresses, local and server
reflexive, on which a Rendezvous() for the Preconnection will listen
for incoming Connections, and to which it will attempt to establish
connections.
An application that uses Rendezvous() to establish a peer-to-peer
connection in the presence of NATs will configure the Preconnection
object with at least one a Local Endpoint that supports NAT binding
discovery. It will then Resolve() the Preconnection, and pass the
resulting list of Local Endpoint candidates to the peer via a
signalling protocol, for example as part of an ICE [RFC5245] exchange
within SIP [RFC3261] or WebRTC [RFC7478]. The peer will then, via
the same signalling channel, return the Remote Endpoint candidates.
The set of Remote Endpoint candidates are then configured onto the
Preconnection:
Preconnection.AddRemote([]RemoteEndpoint)
The Rendezvous() Action can be initiated once both the Local Endpoint
candidates and the Remote Endpoint candidates retrieved from the peer
via the signalling channel have been added to the Preconnection.
If successful, the Rendezvous() Action returns a Connection object
via a RendezvousDone<> Event:
Preconnection -> RendezvousDone<Connection>
The RendezvousDone<> Event occurs when a Connection is established
with the Remote Endpoint. For Connection-oriented transports, this
occurs when the transport-layer connection is established; for
Connectionless transports, it occurs when the first Message is
received from the Remote Endpoint. The resulting Connection is
contained within the RendezvousDone<> Event, and is ready to use as
soon as it is passed to the application via the Event. Changes made
to a Preconnection after Rendezvous() has been called do not have any
effect on existing Connections.
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An EstablishmentError occurs either when the Properties and Security
Parameters of the Preconnection cannot be fulfilled for rendezvous or
cannot be reconciled with the Local and/or Remote Endpoints, when the
Local Endpoint or Remote Endpoint cannot be resolved, when no
transport-layer connection can be established to the Remote Endpoint,
or when the application is prohibited from rendezvous by policy:
Preconnection -> EstablishmentError<reason?>
7.4. Connection Groups
Connection Groups can be created using the Clone Action:
Connection := Connection.Clone(framer?)
Calling Clone on a Connection yields a Connection Group containing
two Connections: the parent Connection on which Clone was called, and
a resulting cloned Connection. The new Connection is actively
openend, and it will send a Ready Event or an EstablishmentError
Event. Calling Clone on any of these Connections adds another
Connection to the Connection Group. Connections in a Connection
Group share all Connection Properties except "Connection Priority"
(see Section 8.1.2), and these Connection Properties are entangled:
Changing one of the Connection Properties on one Connection in the
Connection Group automatically changes the Connection Property for
all others. For example, changing "Timeout for aborting Connection"
(see Section 8.1.3) on one Connection in a Connection Group will
automatically make the same change to this Connection Property for
all other Connections in the Connection Group. Like all other
Properties, "Connection Priority" is copied to the new Connection
when calling Clone(), but in this case, a later change to the
"Connection Priority" on one Connection does not change it on the
other Connections in the same Connection Group.
Message Properties are also not entangled. For example, changing
"Lifetime" (see Section 9.1.3.1) of a Message will only affect a
single Message on a single Connection.
A new Connection created by Clone can have a Message Framer assigned
via the optional "framer" parameter of the Clone Action. If this
parameter is not supplied, the stack of Message Framers associated
with a Connection is copied to the cloned Connection when calling
Clone. Then, a cloned Connection has the same stack of Message
Framers as the Connection from which they are Cloned, but these
Framers may internally maintain per-Connection state.
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It is also possible to check which Connections belong to the same
Connection Group. Calling GroupedConnections() on a specific
Connection returns a set of all Connections in the same group.
[]Connection := Connection.GroupedConnections()
Connections will belong to the same group if the application
previously called Clone. Passive Connections can also be added to
the same group - e.g., when a Listener receives a new Connection that
is just a new stream of an already active multi-streaming protocol
instance.
If the underlying protocol supports multi-streaming, it is natural to
use this functionality to implement Clone. In that case, Connections
in a Connection Group are multiplexed together, giving them similar
treatment not only inside endpoints, but also across the end-to-end
Internet path.
Note that calling Clone() can result in on-the-wire signaling, e.g.,
to open a new transport connection, depending on the underlying
Protocol Stack. When Clone() leads to the opening of multiple such
connections, the Transport Services system will ensure consistency of
Connection Properties by uniformly applying them to all underlying
connections in a group. Even in such a case, there are possibilities
for a Transport Services system to implement prioritization within a
Connection Group [TCP-COUPLING] [RFC8699].
Attempts to clone a Connection can result in a CloneError:
Connection -> CloneError<reason?>
The "Connection Priority" Connection Property operates on Connections
in a Connection Group using the same approach as in Section 9.1.3.2:
when allocating available network capacity among Connections in a
Connection Group, sends on Connections with lower Priority values
will be prioritized over sends on Connections with higher Priority
values. Capacity will be shared among these Connections according to
the Connection Group Transmission Scheduler property (Section 8.1.5).
See Section 9.2.6 for more.
8. Managing Connections
During pre-establishment and after establishment, connections can be
configured and queried using Connection Properties, and asynchronous
information may be available about the state of the connection via
Soft Errors.
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Connection Properties represent the configuration and state of the
selected Protocol Stack(s) backing a Connection. These Connection
Properties may be Generic, applying regardless of transport protocol,
or Specific, applicable to a single implementation of a single
transport protocol stack. Generic Connection Properties are defined
in Section 8.1 below.
Protocol Specific Properties are defined in a transport- and
implementation-specific way, and MUST NOT apply across different
protocols. Too much reliance by an application on Protocol Specific
Properties can significantly reduce the flexibility of a transport
services implementation.
The application can set and query Connection Properties on a per-
Connection basis. Connection Properties that are not read-only can
be set during pre-establishment (see Section 6.2), as well as on
connections directly using the SetProperty action:
Connection.SetProperty(property, value)
Note that changing one of the Connection Properties on one Connection
in a Connection Group will also change it for all other Connections
of that group; see further Section 7.4.
At any point, the application can query Connection Properties.
ConnectionProperties := Connection.GetProperties()
value := ConnectionProperties.Get(property)
if ConnectionProperties.Has(boolean_or_preference_property) then ...
Depending on the status of the connection, the queried Connection
Properties will include different information:
* The connection state, which can be one of the following:
Establishing, Established, Closing, or Closed.
* Whether the connection can be used to send data. A connection can
not be used for sending if the connection was created with the
Selection Property "Direction of Communication" set to
"unidirectional receive" or if a Message marked as "Final" was
sent over this connection. See also Section 9.1.3.5.
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* Whether the connection can be used to receive data. A connection
cannot be used for reading if the connection was created with the
Selection Property "Direction of Communication" set to
"unidirectional send" or if a Message marked as "Final" was
received. See Section 9.3.3.3. The latter is only supported by
certain transport protocols, e.g., by TCP as half-closed
connection.
* For Connections that are Established, Closing, or Closed:
Connection Properties (Section 8.1) of the actual protocols that
were selected and instantiated, and Selection Properties that the
application specified on the Preconnection. Selection Properties
of type "Preference" will be exposed as boolean values indicating
whether or not the property applies to the selected transport.
Note that the instantiated protocol stack might not match all
Protocol Selection Properties that the application specified on
the Preconnection.
* For Connections that are Established, additional properties of the
path(s) in use. These properties can be derived from the local
provisioning domain [RFC7556], measurements by the Protocol Stack,
or other sources.
8.1. Generic Connection Properties
Generic Connection Properties are defined independent of the chosen
protocol stack and therefore available on all Connections.
Many Connection Properties have a corresponding Selection Property
that enables applications to express their preference for protocols
providing a supporting transport feature.
8.1.1. Required Minimum Corruption Protection Coverage for Receiving
Name: recvChecksumLen
Type: Integer (non-negative with special value "Full Coverage")
Default: Full Coverage
This property specifies the minimum number of bytes in a received
message that need to be covered by a checksum. A special value of 0
means that a received packet does not need to have a non-zero
checksum field. A receiving endpoint will not forward messages that
have less coverage to the application. The application is
responsible for handling any corruption within the non-protected part
of the message [RFC8085].
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8.1.2. Connection Priority
Name: connPrio
Type: Integer
Default: 100
This Property is a non-negative integer representing the relative
inverse priority (i.e., a lower value reflects a higher priority) of
this Connection relative to other Connections in the same Connection
Group. It has no effect on Connections not part of a Connection
Group. As noted in Section 7.4, this property is not entangled when
Connections are cloned, i.e., changing the Priority on one Connection
in a Connection Group does not change it on the other Connections in
the same Connection Group. No guarantees of a specific behavior
regarding Connection Priority are given; a Transport Services system
may ignore this property. See Section 9.2.6 for more details.
8.1.3. Timeout for Aborting Connection
Name: connTimeout
Type: Numeric, with special value "Disabled"
Default: Disabled
This property specifies how long to wait before deciding that an
active Connection has failed when trying to reliably deliver data to
the Remote Endpoint. Adjusting this Property will only take effect
when the underlying stack supports reliability. The special value
"Disabled" means that no timeout is scheduled.
8.1.4. Timeout for keep alive packets
Name: keepAliveTimeout
Type: Numeric, with special value "Disabled"
Default: Implementation-defined
A Transport Services system can request a protocol that supports
sending keep alive packets Section 6.2.10. This property specifies
the maximum length of time an idle connection (one for which no
transport packets have been sent) should wait before the Local
Endpoint sends a keep-alive packet to the Remote Endpoint. Adjusting
this Property will only take effect when the underlying stack
supports sending keep-alive packets. Guidance on setting this value
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for datagram transports is provided in [RFC8085]. A value greater
than the connection timeout (Section 8.1.3), or the special value
"Disabled", will disable the sending of keep-alive packets.
8.1.5. Connection Group Transmission Scheduler
Name: connScheduler
Type: Enumeration
Default: Weighted Fair Queueing (see Section 3.6 in [RFC8260])
This property specifies which scheduler should be used among
Connections within a Connection Group, see Section 7.4. The set of
schedulers can be taken from [RFC8260].
8.1.6. Capacity Profile
Name: connCapacityProfile
This property specifies the desired network treatment for traffic
sent by the application and the tradeoffs the application is prepared
to make in path and protocol selection to receive that desired
treatment. When the capacity profile is set to a value other than
Default, the Transport Services system SHOULD select paths and
configure protocols to optimize the tradeoff between delay, delay
variation, and efficient use of the available capacity based on the
capacity profile specified. How this is realized is implementation-
specific. The Capacity Profile MAY also be used to set markings on
the wire for Protocol Stacks supporting this. Recommendations for
use with DSCP are provided below for each profile; note that when a
Connection is multiplexed, the guidelines in Section 6 of [RFC7657]
apply.
The following values are valid for the Capacity Profile:
Default: The application provides no information about its expected
capacity profile. Transport Services system implementations that
map the requested capacity profile onto per-connection DSCP
signaling SHOULD assign the DSCP Default Forwarding [RFC2474] Per
Hop Behaviour (PHB).
Scavenger: The application is not interactive. It expects to send
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and/or receive data without any urgency. This can, for example,
be used to select protocol stacks with scavenger transmission
control and/or to assign the traffic to a lower-effort service.
Transport Services system implementations that map the requested
capacity profile onto per-connection DSCP signaling SHOULD assign
the DSCP Less than Best Effort [RFC8622] PHB.
Low Latency/Interactive: The application is interactive, and prefers
loss to latency. Response time should be optimized at the expense
of delay variation and efficient use of the available capacity
when sending on this connection. This can be used by the system
to disable the coalescing of multiple small Messages into larger
packets (Nagle's algorithm); to prefer immediate acknowledgment
from the peer endpoint when supported by the underlying transport;
and so on. Transport Services system implementations that map the
requested capacity profile onto per-connection DSCP signaling
without multiplexing SHOULD assign a DSCP Assured Forwarding
(AF41,AF42,AF43,AF44) [RFC2597] PHB. Inelastic traffic that is
expected to conform to the configured network service rate could
be mapped to the DSCP Expedited Forwarding [RFC3246] or [RFC5865]
PHBs.
Low Latency/Non-Interactive: The application prefers loss to
latency, but is not interactive. Response time should be
optimized at the expense of delay variation and efficient use of
the available capacity when sending on this connection. Transport
system implementations that map the requested capacity profile
onto per-connection DSCP signaling without multiplexing SHOULD
assign a DSCP Assured Forwarding (AF21,AF22,AF23,AF24) [RFC2597]
PHB.
Constant-Rate Streaming: The application expects to send/receive
data at a constant rate after Connection establishment. Delay and
delay variation should be minimized at the expense of efficient
use of the available capacity. This implies that the Connection
might fail if the Path is unable to maintain the desired rate. A
transport can interpret this capacity profile as preferring a
circuit breaker [RFC8084] to a rate-adaptive congestion
controller. Transport system implementations that map the
requested capacity profile onto per-connection DSCP signaling
without multiplexing SHOULD assign a DSCP Assured Forwarding
(AF31,AF32,AF33,AF34) [RFC2597] PHB.
Capacity-Seeking: The application expects to send/receive data at
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the maximum rate allowed by its congestion controller over a
relatively long period of time. Transport Services system
implementations that map the requested capacity profile onto per-
connection DSCP signaling without multiplexing SHOULD assign a
DSCP Assured Forwarding (AF11,AF12,AF13,AF14) [RFC2597] PHB per
Section 4.8 of [RFC4594].
The Capacity Profile for a selected protocol stack may be modified on
a per-Message basis using the Transmission Profile Message Property;
see Section 9.1.3.8.
8.1.7. Policy for using Multipath Transports
Name: multipath-policy
Type: Enumeration
Default: Handover
This property specifies the local policy for transferring data across
multiple paths between the same end hosts if Multipath Transport is
not set to Disabled (see Section 6.2.14). Possible values are:
Handover: The connection ought only to attempt to migrate between
different paths when the original path is lost or becomes
unusable. The thresholds used to declare a path unusable are
implementation specific.
Interactive: The connection ought only to attempt to minimize the
latency for interactive traffic patterns by transmitting data
across multiple paths when this is beneficial. The goal of
minimizing the latency will be balanced against the cost of each
of these paths. Depending on the cost of the lower-latency path,
the scheduling might choose to use a higher-latency path. Traffic
can be scheduled such that data may be transmitted on multiple
paths in parallel to achieve a lower latency. The specific
scheduling algorithm is implementation-specific.
Aggregate: The connection ought to attempt to use multiple paths in
parallel to maximize available capacity and possibly overcome the
capacity limitations of the individual paths. The actual strategy
is implementation specific.
Note that this is a local choice - the Remote Endpoint can choose a
different policy.
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8.1.8. Bounds on Send or Receive Rate
Name: minSendRate / minRecvRate / maxSendRate / maxRecvRate
Type: Numeric (with special value "Unlimited") / Numeric (with
special value "Unlimited") / Numeric (with special value
"Unlimited") / Numeric (with special value "Unlimited")
Default: Unlimited / Unlimited / Unlimited / Unlimited
This property specifies an upper-bound rate that a transfer is not
expected to exceed (even if flow control and congestion control allow
higher rates), and/or a lower-bound rate below which the application
does not deem it will be useful. These are specified in bits per
second. The special value "Unlimited" indicates that no bound is
specified.
8.1.9. Group Connection Limit
Name: groupConnLimit
Type: Numeric (with special value "Unlimited")
Default: Unlimited
This property controls the number of Connections that can be accepted
from a peer as new members of the Connection's group. Similar to
SetNewConnectionLimit(), this limits the number of ConnectionReceived
Events that will occur, but constrained to the group of the
Connection associated with this property. For a multi-streaming
transport, this limits the number of allowed streams.
8.1.10. Isolate Session
Name: isolateSession
Type: Boolean
Default: false
When set to true, this property will initiate new Connections using
as little cached information (such as session tickets or cookies) as
possible from previous connections that are not in the same
Connection Group. Any state generated by this Connection will only
be shared with Connections in the same Connection Group. Cloned
Connections will use saved state from within the Connection Group.
This is used for separating Connection Contexts as specified in
[I-D.ietf-taps-arch].
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Note that this does not guarantee no leakage of information, as
implementations may not be able to fully isolate all caches (e.g.
RTT estimates). Note that this property may degrade connection
performance.
8.1.11. Read-only Connection Properties
The following generic Connection Properties are read-only, i.e. they
cannot be changed by an application.
8.1.11.1. Maximum Message Size Concurrent with Connection Establishment
Name: zeroRttMsgMaxLen
Type: Integer
This property represents the maximum Message size that can be sent
before or during Connection establishment, see also Section 9.1.3.4.
It is given in Bytes.
8.1.11.2. Maximum Message Size Before Fragmentation or Segmentation
Name: singularTransmissionMsgMaxLen
Type: Integer
This property, if applicable, represents the maximum Message size
that can be sent without incurring network-layer fragmentation at the
sender. It exposes a value to the application based on the Maximum
Packet Size (MPS) as described in Datagram PLPMTUD [RFC8899]. This
can allow a sending stack to avoid unwanted fragmentation at the
network-layer or segmentation by the transport layer.
8.1.11.3. Maximum Message Size on Send
Name: sendMsgMaxLen
Type: Integer
This property represents the maximum Message size that an application
can send.
8.1.11.4. Maximum Message Size on Receive
Name: recvMsgMaxLen
Type: Integer
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This numeric property represents the maximum Message size that an
application can receive.
8.2. TCP-specific Properties: User Timeout Option (UTO)
These properties specify configurations for the User Timeout Option
(UTO), in the case that TCP becomes the chosen transport protocol.
Implementation is optional and useful only if TCP is implemented in
the Transport Services system.
These TCP-specific properties are included here because the feature
"Suggest timeout to the peer" is part of the minimal set of transport
services [RFC8923], where this feature was categorized as
"functional". This means that when an implementation offers this
feature, it has to expose an interface to it to the application.
Otherwise, the implementation might violate assumptions by the
application, which could cause the application to fail.
All of the below properties are optional (e.g., it is possible to
specify "User Timeout Enabled" as true, but not specify an Advertised
User Timeout value; in this case, the TCP default will be used).
These properties reflect the API extension specified in Section 3 of
[RFC5482].
8.2.1. Advertised User Timeout
Name: tcp.userTimeoutValue
Type: Integer
Default: the TCP default
This time value is advertised via the TCP User Timeout Option (UTO)
[RFC5482] at the Remote Endpoint to adapt its own "Timeout for
aborting Connection" (see Section 8.1.3) value.
8.2.2. User Timeout Enabled
Name: tcp.userTimeout
Type: Boolean
Default: false
This property controls whether the UTO option is enabled for a
connection. This applies to both sending and receiving.
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8.2.3. Timeout Changeable
Name: tcp.userTimeoutRecv
Type: Boolean
Default: true
This property controls whether the "Timeout for aborting Connection"
(see Section 8.1.3) may be changed based on a UTO option received
from the remote peer. This boolean becomes false when "Timeout for
aborting Connection" (see Section 8.1.3) is used.
8.3. Connection Lifecycle Events
During the lifetime of a connection there are events that can occur
when configured.
8.3.1. Soft Errors
Asynchronous introspection is also possible, via the SoftError Event.
This event informs the application about the receipt and contents of
an ICMP error message related to the Connection. This will only
happen if the underlying protocol stack supports access to soft
errors; however, even if the underlying stack supports it, there is
no guarantee that a soft error will be signaled.
Connection -> SoftError<>
8.3.2. Path change
This event notifies the application when at least one of the paths
underlying a Connection has changed. Changes occur on a single path
when the PMTU changes as well as when multiple paths are used and
paths are added or removed, or a handover has been performed.
Connection -> PathChange<>
9. Data Transfer
Data is sent and received as Messages, which allows the application
to communicate the boundaries of the data being transferred.
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9.1. Messages and Framers
Each Message has an optional Message Context, which allows to add
Message Properties, identify Send Events related to a specific
Message or to inspect meta-data related to the Message sent. Framers
can be used to extend or modify the message data with additional
information that can be processed at the receiver to detect message
boundaries.
9.1.1. Message Contexts
Using the MessageContext object, the application can set and retrieve
meta-data of the message, including Message Properties (see
Section 9.1.3) and framing meta-data (see Section 9.1.2.2).
Therefore, a MessageContext object can be passed to the Send action
and is returned by each Send and Receive related event.
Message Properties can be set and queried using the Message Context:
MessageContext.add(scope?, parameter, value)
PropertyValue := MessageContext.get(scope?, property)
To get or set Message Properties, the optional scope parameter is
left empty. To get or set meta-data for a Framer, the application
has to pass a reference to this Framer as the scope parameter.
For MessageContexts returned by send Events (see Section 9.2.2) and
receive Events (see Section 9.3.2), the application can query
information about the Local and Remote Endpoint:
RemoteEndpoint := MessageContext.GetRemoteEndpoint()
LocalEndpoint := MessageContext.GetLocalEndpoint()
9.1.2. Message Framers
Although most applications communicate over a network using well-
formed Messages, the boundaries and metadata of the Messages are
often not directly communicated by the transport protocol itself.
For example, HTTP applications send and receive HTTP messages over a
byte-stream transport, requiring that the boundaries of HTTP messages
be parsed from the stream of bytes.
Message Framers allow extending a Connection's Protocol Stack to
define how to encapsulate or encode outbound Messages, and how to
decapsulate or decode inbound data into Messages. Message Framers
allow message boundaries to be preserved when using a Connection
object, even when using byte-stream transports. This is designed
based on the fact that many of the current application protocols
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evolved over TCP, which does not provide message boundary
preservation, and since many of these protocols require message
boundaries to function, each application layer protocol has defined
its own framing.
To use a Message Framer, the application adds it to its Preconnection
object. Then, the Message Framer can intercept all calls to Send()
or Receive() on a Connection to add Message semantics, in addition to
interacting with the setup and teardown of the Connection. A Framer
can start sending data before the application sends data if the
framing protocol requires a prefix or handshake (see [RFC8229] for an
example of such a framing protocol).
Initiate() Send() Receive() Close()
| | ^ |
| | | |
+----v----------v---------+----------v-----+
| Connection |
+----+----------+---------^----------+-----+
| | | |
| +-----------------+ |
| | Messages | |
| +-----------------+ |
| | | |
+----v----------v---------+----------v-----+
| Framer(s) |
+----+----------+---------^----------+-----+
| | | |
| +-----------------+ |
| | Byte-stream | |
| +-----------------+ |
| | | |
+----v----------v---------+----------v-----+
| Transport Protocol Stack |
+------------------------------------------+
Figure 1: Protocol Stack showing a Message Framer
Note that while Message Framers add the most value when placed above
a protocol that otherwise does not preserve message boundaries, they
can also be used with datagram- or message-based protocols. In these
cases, they add an additional transformation to further encode or
encapsulate, and can potentially support packing multiple
application-layer Messages into individual transport datagrams.
The API to implement a Message Framer can vary depending on the
implementation; guidance on implementing Message Framers can be found
in [I-D.ietf-taps-impl].
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9.1.2.1. Adding Message Framers to Pre-Connections
The Message Framer object can be added to one or more Preconnections
to run on top of transport protocols. Multiple Framers may be added
to a Preconnection; in this case, the Framers operate as a framing
stack, i.e. the last one added runs first when framing outbound
messages, and last when parsing inbound data.
The following example adds a basic HTTP Message Framer to a
Preconnection:
framer := NewHTTPMessageFramer()
Preconnection.AddFramer(framer)
Since Message Framers pass from Preconnection to Listener or
Connection, addition of Framers must happen before any operation that
may result in the creation of a Connection.
9.1.2.2. Framing Meta-Data
When sending Messages, applications can add Framer-specific key/value
pairs to a MessageContext (Section 9.1.1). This mechanism can be
used, for example, to set the type of a Message for a TLV format.
The namespace of values is custom for each unique Message Framer.
messageContext := NewMessageContext()
messageContext.add(framer, key, value)
Connection.Send(messageData, messageContext)
When an application receives a MessageContext in a Receive event, it
can also look to see if a value was set by a specific Message Framer.
messageContext.get(framer, key) -> value
For example, if an HTTP Message Framer is used, the values could
correspond to HTTP headers:
httpFramer := NewHTTPMessageFramer()
...
messageContext := NewMessageContext()
messageContext.add(httpFramer, "accept", "text/html")
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9.1.3. Message Properties
Applications needing to annotate the Messages they send with extra
information (for example, to control how data is scheduled and
processed by the transport protocols supporting the Connection) can
include this information in the Message Context passed to the Send
Action. For other uses of the message context, see Section 9.1.1.
Message Properties are per-Message, not per-Send if partial Messages
are sent (Section 9.2.3). All data blocks associated with a single
Message share properties specified in the Message Contexts. For
example, it would not make sense to have the beginning of a Message
expire, but allow the end of a Message to still be sent.
A MessageContext object contains metadata for the Messages to be sent
or received.
messageData := "hello"
messageContext := NewMessageContext()
messageContext.add(parameter, value)
Connection.Send(messageData, messageContext)
The simpler form of Send, which does not take any messageContext, is
equivalent to passing a default MessageContext without adding any
Message Properties.
If an application wants to override Message Properties for a specific
message, it can acquire an empty MessageContext Object and add all
desired Message Properties to that Object. It can then reuse the
same messageContext Object for sending multiple Messages with the
same properties.
Properties can be added to a MessageContext object only before the
context is used for sending. Once a messageContext has been used
with a Send call, it is invalid to modify any of its properties.
The Message Properties could be inconsistent with the properties of
the Protocol Stacks underlying the Connection on which a given
Message is sent. For example, a Protocol Stack must be able to
provide ordering if the msgOrdered property of a Message is enabled.
Sending a Message with Message Properties inconsistent with the
Selection Properties of the Connection yields an error.
Connection Properties describe the default behavior for all Messages
on a Connection. If a Message Property contradicts a Connection
Property, and if this per-Message behavior can be supported, it
overrides the Connection Property for the specific Message. For
example, if "Reliable Data Transfer (Connection)" is set to "Require"
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and a protocol with configurable per-Message reliability is used,
setting "Reliable Data Transfer (Message)" to "false" for a
particular Message will allow this Message to be sent without any
reliability guarantees. Changing the Reliable Data Transfer property
on Messages is only possible for Connections that were established
enabling the Selection Property "Configure Per-Message Reliability".
The following Message Properties are supported:
9.1.3.1. Lifetime
Name: msgLifetime
Type: Numeric
Default: infinite
The Lifetime specifies how long a particular Message can wait to be
sent to the Remote Endpoint before it is irrelevant and no longer
needs to be (re-)transmitted. This is a hint to the Transport
Services system - it is not guaranteed that a Message will not be
sent when its Lifetime has expired.
Setting a Message's Lifetime to infinite indicates that the
application does not wish to apply a time constraint on the
transmission of the Message, but it does not express a need for
reliable delivery; reliability is adjustable per Message via the
"Reliable Data Transfer (Message)" property (see Section 9.1.3.7).
The type and units of Lifetime are implementation-specific.
9.1.3.2. Priority
Name: msgPrio
Type: Integer (non-negative)
Default: 100
This property specifies the priority of a Message, relative to other
Messages sent over the same Connection.
A Message with Priority 0 will yield to a Message with Priority 1,
which will yield to a Message with Priority 2, and so on. Priorities
may be used as a sender-side scheduling construct only, or be used to
specify priorities on the wire for Protocol Stacks supporting
prioritization.
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Note that this property is not a per-message override of the
Connection Priority - see Section 8.1.2. The Priority properties may
interact, but can be used independently and be realized by different
mechanisms; see Section 9.2.6.
9.1.3.3. Ordered
Name: msgOrdered
Type: Boolean
Default: the queried Boolean value of the Selection Property
"preserveOrder" (Section 6.2.4)
The order in which Messages were submitted for transmission via the
Send Action will be preserved on delivery via Receive<> events for
all Messages on a Connection that have this Message Property set to
true.
If false, the Message is delivered to the receiving application
without preserving the ordering. This property is used for protocols
that support preservation of data ordering, see Section 6.2.4, but
allow out-of-order delivery for certain messages, e.g., by
multiplexing independent messages onto different streams.
9.1.3.4. Safely Replayable
Name: safelyReplayable
Type: Boolean
Default: false
If true, Safely Replayable specifies that a Message is safe to send
to the Remote Endpoint more than once for a single Send Action. It
marks the data as safe for certain 0-RTT establishment techniques,
where retransmission of the 0-RTT data may cause the remote
application to receive the Message multiple times.
For protocols that do not protect against duplicated messages, e.g.,
UDP, all messages need to be marked as "Safely Replayable". To
enable protocol selection to choose such a protocol, "Safely
Replayable" needs to be added to the TransportProperties passed to
the Preconnection. If such a protocol was chosen, disabling "Safely
Replayable" on individual messages MUST result in a SendError.
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9.1.3.5. Final
Name: final
Type: Boolean
Default: false
If true, this indicates a Message is the last that the application
will send on a Connection. This allows underlying protocols to
indicate to the Remote Endpoint that the Connection has been
effectively closed in the sending direction. For example, TCP-based
Connections can send a FIN once a Message marked as Final has been
completely sent, indicated by marking endOfMessage. Protocols that
do not support signalling the end of a Connection in a given
direction will ignore this property.
A Final Message must always be sorted to the end of a list of
Messages. The Final property overrides Priority and any other
property that would re-order Messages. If another Message is sent
after a Message marked as Final has already been sent on a
Connection, the Send Action for the new Message will cause a
SendError Event.
9.1.3.6. Sending Corruption Protection Length
Name: msgChecksumLen
Type: Integer (non-negative with special value "Full Coverage")
Default: Full Coverage
This property specifies the minimum length of the section of a sent
Message, starting from byte 0, that the application requires to be
delivered without corruption due to lower layer errors. It is used
to specify options for simple integrity protection via checksums. A
value of 0 means that no checksum needs to be calculated, and "Full
Coverage" means that the entire Message needs to be protected by a
checksum. Only "Full Coverage" is guaranteed, any other requests are
advisory, which may result in "Full Coverage" being applied.
9.1.3.7. Reliable Data Transfer (Message)
Name: msgReliable
Type: Boolean
Default: the queried Boolean value of the Selection Property
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"reliability" (Section 6.2.1)
When true, this property specifies that a Message should be sent in
such a way that the transport protocol ensures all data is received
on the other side without corruption. Changing the "Reliable Data
Transfer" property on Messages is only possible for Connections that
were established enabling the Selection Property "Configure Per-
Message Reliability". When this is not the case, changing
"msgReliable" will generate an error.
Disabling this property indicates that the Transport Services system
may disable retransmissions or other reliability mechanisms for this
particular Message, but such disabling is not guaranteed.
9.1.3.8. Message Capacity Profile Override
Name: msgCapacityProfile
Type: Enumeration
Default: inherited from the Connection Property
"connCapacityProfile" (Section 8.1.6)
This enumerated property specifies the application's preferred
tradeoffs for sending this Message; it is a per-Message override of
the Capacity Profile connection property (see Section 8.1.6).
9.1.3.9. No Network-Layer Fragmentation
Name: noFragmentation
Type: Boolean
Default: false
This property specifies that a message should be sent and received
without network-layer fragmentation, if possible. It can be used to
avoid network layer fragmentation when transport segmentation is
prefered.
This only takes effect when the transport uses a network layer that
supports this functionality. When it does take effect, setting this
property to true will cause the sender to avoid network-layer source
frgementation. When using IPv4, this will result in the Don't
Fragment bit being set in the IP header.
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Attempts to send a message with this property that result in a size
greater than the transport's current estimate of its maximum packet
size ("singularTransmissionMsgMaxLen") can result in transport
segmentation when permitted, or in a "SendError".
Note: noSegmentation should be used when it is desired to only send a
message within a single network packet.
9.1.3.10. No Segmentation
Name: noSegmentation
Type: Boolean
Default: false
When set to true, this property requests the transport layer to not
provide segmentation of messages larger than the maximum size
permitted by the network layer, and also to avoid network-layer
source fragmentation of messages. When running over IPv4, setting
this property to true can result in a sending endpount setting the
Don't Fragment bit in the IPv4 header of packets generated by the
transport layer. An attempt to send a message that results in a size
greater than the transport's current estimate of its maximum packet
size (singularTransmissionMsgMaxLen) will result in a SendError.
This only takes effect when the transport and network layer support
this functionality.
9.2. Sending Data
Once a Connection has been established, it can be used for sending
Messages. By default, Send enqueues a complete Message, and takes
optional per-Message properties (see Section 9.2.1). All Send
actions are asynchronous, and deliver Events (see Section 9.2.2).
Sending partial Messages for streaming large data is also supported
(see Section 9.2.3).
Messages are sent on a Connection using the Send action:
Connection.Send(messageData, messageContext?, endOfMessage?)
where messageData is the data object to send, and messageContext
allows adding Message Properties, identifying Send Events related to
a specific Message or inspecting meta-data related to the Message
sent (see Section 9.1.1).
The optional endOfMessage parameter supports partial sending and is
described in Section 9.2.3.
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9.2.1. Basic Sending
The most basic form of sending on a connection involves enqueuing a
single Data block as a complete Message with default Message
Properties.
messageData := "hello"
Connection.Send(messageData)
The interpretation of a Message to be sent is dependent on the
implementation, and on the constraints on the Protocol Stacks implied
by the Connection's transport properties. For example, a Message may
be a single datagram for UDP Connections; or an HTTP Request for HTTP
Connections.
Some transport protocols can deliver arbitrarily sized Messages, but
other protocols constrain the maximum Message size. Applications can
query the Connection Property "Maximum Message size on send"
(Section 8.1.11.3) to determine the maximum size allowed for a single
Message. If a Message is too large to fit in the Maximum Message
Size for the Connection, the Send will fail with a SendError event
(Section 9.2.2.3). For example, it is invalid to send a Message over
a UDP connection that is larger than the available datagram sending
size.
9.2.2. Send Events
Like all Actions in this interface, the Send Action is asynchronous.
There are several Events that can be delivered in response to Sending
a Message. Exactly one Event (Sent, Expired, or SendError) will be
delivered in response to each call to Send.
Note that if partial Sends are used (Section 9.2.3), there will still
be exactly one Send Event delivered for each call to Send. For
example, if a Message expired while two requests to Send data for
that Message are outstanding, there will be two Expired events
delivered.
The interface should allow the application to correlate which Send
Action resulted in a particular Send Event. The manner in which this
correlation is indicated is implementation-specific.
9.2.2.1. Sent
Connection -> Sent<messageContext>
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The Sent Event occurs when a previous Send Action has completed,
i.e., when the data derived from the Message has been passed down or
through the underlying Protocol Stack and is no longer the
responsibility of this interface. The exact disposition of the
Message (i.e., whether it has actually been transmitted, moved into a
buffer on the network interface, moved into a kernel buffer, and so
on) when the Sent Event occurs is implementation-specific. The Sent
Event contains a reference to the Message Context of the Message to
which it applies.
Sent Events allow an application to obtain an understanding of the
amount of buffering it creates. That is, if an application calls the
Send Action multiple times without waiting for a Sent Event, it has
created more buffer inside the Transport Services system than an
application that always waits for the Sent Event before calling the
next Send Action.
9.2.2.2. Expired
Connection -> Expired<messageContext>
The Expired Event occurs when a previous Send Action expired before
completion; i.e. when the Message was not sent before its Lifetime
(see Section 9.1.3.1) expired. This is separate from SendError, as
it is an expected behavior for partially reliable transports. The
Expired Event contains a reference to the Message Context of the
Message to which it applies.
9.2.2.3. SendError
Connection -> SendError<messageContext, reason?>
A SendError occurs when a Message was not sent due to an error
condition: an attempt to send a Message which is too large for the
system and Protocol Stack to handle, some failure of the underlying
Protocol Stack, or a set of Message Properties not consistent with
the Connection's transport properties. The SendError contains a
reference to the Message Context of the Message to which it applies.
9.2.3. Partial Sends
It is not always possible for an application to send all data
associated with a Message in a single Send Action. The Message data
may be too large for the application to hold in memory at one time,
or the length of the Message may be unknown or unbounded.
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Partial Message sending is supported by passing an endOfMessage
boolean parameter to the Send Action. This value is always true by
default, and the simpler forms of Send are equivalent to passing true
for endOfMessage.
The following example sends a Message in two separate calls to Send.
messageContext := NewMessageContext()
messageContext.add(parameter, value)
messageData := "hel"
endOfMessage := false
Connection.Send(messageData, messageContext, endOfMessage)
messageData := "lo"
endOfMessage := true
Connection.Send(messageData, messageContext, endOfMessage)
All data sent with the same MessageContext object will be treated as
belonging to the same Message, and will constitute an in-order series
until the endOfMessage is marked.
9.2.4. Batching Sends
To reduce the overhead of sending multiple small Messages on a
Connection, the application could batch several Send Actions
together. This provides a hint to the system that the sending of
these Messages ought to be coalesced when possible, and that sending
any of the batched Messages can be delayed until the last Message in
the batch is enqueued.
The semantics for starting and ending a batch can be implementation-
specific, but need to allow multiple Send Actions to be enqueued.
Connection.StartBatch()
Connection.Send(messageData)
Connection.Send(messageData)
Connection.EndBatch()
9.2.5. Send on Active Open: InitiateWithSend
For application-layer protocols where the Connection initiator also
sends the first message, the InitiateWithSend() action combines
Connection initiation with a first Message sent:
Connection := Preconnection.InitiateWithSend(messageData, messageContext?, timeout?)
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Whenever possible, a messageContext should be provided to declare the
Message passed to InitiateWithSend as "Safely Replayable". This
allows the Transport Services system to make use of 0-RTT
establishment in case this is supported by the available protocol
stacks. When the selected stack(s) do not support transmitting data
upon connection establishment, InitiateWithSend is identical to
Initiate() followed by Send().
Neither partial sends nor send batching are supported by
InitiateWithSend().
The Events that may be sent after InitiateWithSend() are equivalent
to those that would be sent by an invocation of Initiate() followed
immediately by an invocation of Send(), with the caveat that a send
failure that occurs because the Connection could not be established
will not result in a SendError separate from the EstablishmentError
signaling the failure of Connection establishment.
9.2.6. Priority in TAPS
The Transport Services interface provides two properties to allow a
sender to signal the relative priority of data transmission: the
Priority Message Property Section 9.1.3.2, and the Connection
Priority Connection Property Section 8.1.2. These properties are
designed to allow the expression and implementation of a wide variety
of approaches to transmission priority in the transport and
application layer, including those which do not appear on the wire
(affecting only sender-side transmission scheduling) as well as those
that do (e.g. [I-D.ietf-httpbis-priority].
A Transport Services system gives no guarantees about how its
expression of relative priorities will be realized. However, the
Transport Services system will seek to ensure that performance of
relatively-prioritized connections and messages is not worse with
respect to those connections and messages than an equivalent
configuration in which all prioritization properties are left at
their defaults.
The Transport Services interface does order Connection Priority over
the Priority Message Property. In the absense of other externalities
(e.g., transport-layer flow control), a priority 1 Message on a
priority 0 Connection will be sent before a priority 0 Message on a
priority 1 Connection in the same group.
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9.3. Receiving Data
Once a Connection is established, it can be used for receiving data
(unless the "Direction of Communication" property is set to
"unidirectional send"). As with sending, the data is received in
Messages. Receiving is an asynchronous operation, in which each call
to Receive enqueues a request to receive new data from the
connection. Once data has been received, or an error is encountered,
an event will be delivered to complete any pending Receive requests
(see Section 9.3.2). If Messages arrive at the Transport Services
system before Receive requests are issued, ensuing Receive requests
will first operate on these Messages before awaiting any further
Messages.
9.3.1. Enqueuing Receives
Receive takes two parameters to specify the length of data that an
application is willing to receive, both of which are optional and
have default values if not specified.
Connection.Receive(minIncompleteLength?, maxLength?)
By default, Receive will try to deliver complete Messages in a single
event (Section 9.3.2.1).
The application can set a minIncompleteLength value to indicate the
smallest partial Message data size in bytes that should be delivered
in response to this Receive. By default, this value is infinite,
which means that only complete Messages should be delivered (see
Section 9.3.2.2 and Section 9.1.2 for more information on how this is
accomplished). If this value is set to some smaller value, the
associated receive event will be triggered only when at least that
many bytes are available, or the Message is complete with fewer
bytes, or the system needs to free up memory. Applications should
always check the length of the data delivered to the receive event
and not assume it will be as long as minIncompleteLength in the case
of shorter complete Messages or memory issues.
The maxLength argument indicates the maximum size of a Message in
bytes that the application is currently prepared to receive. The
default value for maxLength is infinite. If an incoming Message is
larger than the minimum of this size and the maximum Message size on
receive for the Connection's Protocol Stack, it will be delivered via
ReceivedPartial events (Section 9.3.2.2).
Note that maxLength does not guarantee that the application will
receive that many bytes if they are available; the interface could
return ReceivedPartial events with less data than maxLength according
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to implementation constraints. Note also that maxLength and
minIncompleteLength are intended only to manage buffering, and are
not interpreted as a receiver preference for message reordering.
9.3.2. Receive Events
Each call to Receive will be paired with a single Receive Event,
which can be a success or an error. This allows an application to
provide backpressure to the transport stack when it is temporarily
not ready to receive messages.
The interface should allow the application to correlate which call to
Receive resulted in a particular Receive Event. The manner in which
this correlation is indicated is implementation-specific.
9.3.2.1. Received
Connection -> Received<messageData, messageContext>
A Received event indicates the delivery of a complete Message. It
contains two objects, the received bytes as messageData, and the
metadata and properties of the received Message as messageContext.
The messageData object provides access to the bytes that were
received for this Message, along with the length of the byte array.
The messageContext is provided to enable retrieving metadata about
the message and referring to the message, e.g., to send replies and
map responses to their requests. See Section 9.1.1 for details.
See Section 9.1.2 for handling Message framing in situations where
the Protocol Stack only provides a byte-stream transport.
9.3.2.2. ReceivedPartial
Connection -> ReceivedPartial<messageData, messageContext, endOfMessage>
If a complete Message cannot be delivered in one event, one part of
the Message can be delivered with a ReceivedPartial event. To
continue to receive more of the same Message, the application must
invoke Receive again.
Multiple invocations of ReceivedPartial deliver data for the same
Message by passing the same MessageContext, until the endOfMessage
flag is delivered or a ReceiveError occurs. All partial blocks of a
single Message are delivered in order without gaps. This event does
not support delivering discontiguous partial Messages. If, for
example, Message A is divided into three pieces (A1, A2, A3) and
Message B is divided into three pieces (B1, B2, B3), and
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preserveOrder is not Required, the ReceivedPartial may deliver them
in a sequence like this: A1, B1, B2, A2, A3, B3, because the
messageContext allows the application to identify the pieces as
belonging to Message A and B, respectively. However, a sequence
like: A1, A3 will never occur.
If the minIncompleteLength in the Receive request was set to be
infinite (indicating a request to receive only complete Messages),
the ReceivedPartial event may still be delivered if one of the
following conditions is true:
* the underlying Protocol Stack supports message boundary
preservation, and the size of the Message is larger than the
buffers available for a single message;
* the underlying Protocol Stack does not support message boundary
preservation, and the Message Framer (see Section 9.1.2) cannot
determine the end of the message using the buffer space it has
available; or
* the underlying Protocol Stack does not support message boundary
preservation, and no Message Framer was supplied by the
application
Note that in the absence of message boundary preservation or a
Message Framer, all bytes received on the Connection will be
represented as one large Message of indeterminate length.
9.3.2.3. ReceiveError
Connection -> ReceiveError<messageContext, reason?>
A ReceiveError occurs when data is received by the underlying
Protocol Stack that cannot be fully retrieved or parsed, and when it
is useful for the application to be notified of such errors. For
example, a ReceiveError can indicate that a Message (identified via
the MessageContext) that was being partially received previously, but
had not completed, encountered an error and will not be completed.
This can be useful for an application, which may want to use this
error as a hint to remove previously received Message parts from
memory. As another example, if an incoming Message does not fulfill
the Required Minimum Corruption Protection Coverage for Receiving
property (see Section 8.1.1), an application can use this error as a
hint to inform the peer application to adjust the Sending Corruption
Protection Length property (see Section 9.1.3.6).
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In contrast, internal protocol reception errors (e.g., loss causing
retransmissions in TCP) are not signalled by this Event. Conditions
that irrevocably lead to the termination of the Connection are
signaled using ConnectionError (see Section 10).
9.3.3. Receive Message Properties
Each Message Context may contain metadata from protocols in the
Protocol Stack; which metadata is available is Protocol Stack
dependent. These are exposed through additional read-only Message
Properties that can be queried from the MessageContext object (see
Section 9.1.1) passed by the receive event. The following metadata
values are supported:
9.3.3.1. UDP(-Lite)-specific Property: ECN
When available, Message metadata carries the value of the Explicit
Congestion Notification (ECN) field. This information can be used
for logging and debugging, and for building applications that need
access to information about the transport internals for their own
operation. This property is specific to UDP and UDP-Lite because
these protocols do not implement congestion control, and hence expose
this functionality to the application (see [RFC8293], following the
guidance in [RFC8085])
9.3.3.2. Early Data
In some cases it can be valuable to know whether data was read as
part of early data transfer (before connection establishment has
finished). This is useful if applications need to treat early data
separately, e.g., if early data has different security properties
than data sent after connection establishment. In the case of TLS
1.3, client early data can be replayed maliciously (see [RFC8446]).
Thus, receivers might wish to perform additional checks for early
data to ensure it is safely replayable. If TLS 1.3 is available and
the recipient Message was sent as part of early data, the
corresponding metadata carries a flag indicating as such. If early
data is enabled, applications should check this metadata field for
Messages received during connection establishment and respond
accordingly.
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9.3.3.3. Receiving Final Messages
The Message Context can indicate whether or not this Message is the
Final Message on a Connection. For any Message that is marked as
Final, the application can assume that there will be no more Messages
received on the Connection once the Message has been completely
delivered. This corresponds to the Final property that may be marked
on a sent Message, see Section 9.1.3.5.
Some transport protocols and peers do not support signaling of the
Final property. Applications therefore should not rely on receiving
a Message marked Final to know that the sending endpoint is done
sending on a connection.
Any calls to Receive once the Final Message has been delivered will
result in errors.
10. Connection Termination
A Connection can be terminated i) by the Local Endpoint (i.e., the
application calls the Close, CloseGroup, Abort or AbortGroup Action),
ii) by the Remote Endpoint (i.e., the remote application calls the
Close, CloseGroup, Abort or AbortGroup Action), or iii) because of an
error (e.g., a timeout). A local call of the Close Action will cause
the Connection to either send a Closed Event or a ConnectionError
Event, and a local call of the CloseGroup Action will cause all of
the Connections in the group to either send a Closed Event or a
ConnectionError Event. A local call of the Abort Action will cause
the Connection to send a ConnectionError Event, indicating local
Abort as a reason, and a local call of the AbortGroup Action will
cause all of the Connections in the group to send a ConnectionError
Event, indicating local Abort as a reason.
Remote Action calls map to Events similar to local calls (e.g., a
remote Close causes the Connection to either send a Closed Event or a
ConnectionError Event), but, different from local Action calls, it is
not guaranteed that such Events will indeed be invoked. When an
application needs to free resources associated with a Connection, it
should therefore not rely on the invocation of such Events due to
termination calls from the Remote Endpoint, but instead use the local
termination Actions.
Close terminates a Connection after satisfying all the requirements
that were specified regarding the delivery of Messages that the
application has already given to the Transport Services system. Upon
successfully satisfying all these requirements, the Connection will
send the Closed Event. For example, if reliable delivery was
requested for a Message handed over before calling Close, the Closed
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Event will signify that this Message has indeed been delivered. This
Action does not affect any other Connection in the same Connection
Group.
Connection.Close()
The Closed Event informs the application that a Close Action has
successfully completed, or that the Remote Endpoint has closed the
Connection. There is no guarantee that a remote Close will be
signaled.
Connection -> Closed<>
Abort terminates a Connection without delivering any remaining
Messages. This action does not affect any other Connection that is
entangled with this one in a Connection Group. When the Abort Action
has finished, the Connection will send a ConnectionError Event,
indicating local Abort as a reason.
Connection.Abort()
CloseGroup gracefully terminates a Connection and any other
Connections in the same Connection Group. For example, all of the
Connections in a group might be streams of a single session for a
multistreaming protocol; closing the entire group will close the
underlying session. See also Section 7.4. All Connections in the
group will send a Closed Event when the CloseGroup Action was
successful. As with Close, any Messages remaining to be processed on
a Connection will be handled prior to closing.
Connection.CloseGroup()
AbortGroup terminates a Connection and any other Connections that are
in the same Connection Group without delivering any remaining
Messages. When the AbortGroup Action has finished, all Connections
in the group will send a ConnectionError Event, indicating local
Abort as a reason.
Connection.AbortGroup()
A ConnectionError informs the application that: 1) data could not be
delivered to the peer after a timeout, or 2) the Connection has been
aborted (e.g., because the peer has called Abort). There is no
guarantee that an Abort from the peer will be signaled.
Connection -> ConnectionError<reason?>
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11. Connection State and Ordering of Operations and Events
This interface is designed to be independent of an implementation's
concurrency model. The details of how exactly actions are handled,
and how events are dispatched, are implementation dependent.
Each transition of connection state is associated with one of more
events:
* Ready<> occurs when a Connection created with Initiate() or
InitiateWithSend() transitions to Established state.
* ConnectionReceived<> occurs when a Connection created with
Listen() transitions to Established state.
* RendezvousDone<> occurs when a Connection created with
Rendezvous() transitions to Established state.
* Closed<> occurs when a Connection transitions to Closed state
without error.
* EstablishmentError<> occurs when a Connection created with
Initiate() transitions from Establishing state to Closed state due
to an error.
* ConnectionError<> occurs when a Connection transitions to Closed
state due to an error in all other circumstances.
The following diagram shows the possible states of a Connection and
the events that occur upon a transition from one state to another.
(*) (**)
Establishing -----> Established -----> Closing ------> Closed
| ^
| |
+---------------------------------------------------+
EstablishmentError<>
(*) Ready<>, ConnectionReceived<>, RendezvousDone<>
(**) Closed<>, ConnectionError<>
Figure 2: Connection State Diagram
The interface provides the following guarantees about the ordering of
operations:
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* Sent<> events will occur on a Connection in the order in which the
Messages were sent (i.e., delivered to the kernel or to the
network interface, depending on implementation).
* Received<> will never occur on a Connection before it is
Established; i.e. before a Ready<> event on that Connection, or a
ConnectionReceived<> or RendezvousDone<> containing that
Connection.
* No events will occur on a Connection after it is Closed; i.e.,
after a Closed<> event, an EstablishmentError<> or
ConnectionError<> will not occur on that connection. To ensure
this ordering, Closed<> will not occur on a Connection while other
events on the Connection are still locally outstanding (i.e.,
known to the interface and waiting to be dealt with by the
application).
12. IANA Considerations
RFC-EDITOR: Please remove this section before publication.
This document has no Actions for IANA. Later versions of this
document may create IANA registries for generic transport property
names and transport property namespaces (see Section 4.1).
13. Privacy and Security Considerations
This document describes a generic API for interacting with a
transport services (TAPS) system. Part of this API includes
configuration details for transport security protocols, as discussed
in Section 6.3. It does not recommend use (or disuse) of specific
algorithms or protocols. Any API-compatible transport security
protocol ought to work in a TAPS system. Security considerations for
these protocols are discussed in the respective specifications.
The described API is used to exchange information between an
application and the Transport Services system. While it is not
necessarily expected that both systems are implemented by the same
authority, it is expected that the Transport Services system
implementation is either provided as a library that is selected by
the application from a trusted party, or that it is part of the
operating system that the application also relies on for other tasks.
In either case, the Transport Services API is an internal interface
that is used to change information locally between two systems.
However, as the Transport Services system is responsible for network
communication, it is in the position to potentially share any
information provided by the application with the network or another
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communication peer. Most of the information provided over the
Transport Services API are useful to configure and select protocols
and paths and are not necessarily privacy sensitive. Still, some
information could be privacy sensitive because it might reveal usage
characteristics and habits of the user of an application.
Of course any communication over a network reveals usage
characteristics, as all packets, as well as their timing and size,
are part of the network-visible wire image [RFC8546]. However, the
selection of a protocol and its configuration also impacts which
information is visible, potentially in clear text, and which other
entities can access it. In most cases, information provided for
protocol and path selection should not directly translate to
information that can be observed by network devices on the path.
However, there might be specific configuration information that is
intended for path exposure, e.g., a DiffServ codepoint setting, that
is either provided directly by the application or indirectly
configured for a traffic profile.
Applications should be aware that communication attempts can lead to
more than one connection establishment. This is the case, for
example, when the Transport Services system also executes name
resolution, when support mechanisms such as TURN or ICE are used to
establish connectivity, if protocols or paths are raised, or if a
path fails and fallback or re-establishment is supported in the
Transport Services system.
Applications should also take care to not assume that all data
received using the Transport Services API is always complete or well-
formed. Specifically, messages that are received partially
Section 9.3.2.2 could be a source of truncation attacks if
applications do not distinguish between partial messages and complete
messages.
The interface explicitly does not require the application to resolve
names, though there is a tradeoff between early and late binding of
addresses to names. Early binding allows the API implementation to
reduce connection setup latency, at the cost of potentially limited
scope for alternate path discovery during Connection establishment,
as well as potential additional information leakage about application
interest when used with a resolution method (such as DNS without TLS)
which does not protect query confidentiality.
These communication activities are not different from what is used
today. However, the goal of a Transport Services system is to
support such mechanisms as a generic service within the transport
layer. This enables applications to more dynamically benefit from
innovations and new protocols in the transport, although it reduces
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transparency of the underlying communication actions to the
application itself. The TAPS API is designed such that protocol and
path selection can be limited to a small and controlled set if
required by the application for functional or security purposes.
Further, TAPS implementations should provide an interface to poll
information about which protocol and path is currently in use as well
as provide logging about the communication events of each connection.
14. 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.
This work has been supported by the Research Council of Norway under
its "Toppforsk" programme through the "OCARINA" project.
Thanks to Stuart Cheshire, Josh Graessley, David Schinazi, and Eric
Kinnear for their implementation and design efforts, including Happy
Eyeballs, that heavily influenced this work. Thanks to Laurent Chuat
and Jason Lee for initial work on the Post Sockets interface, from
which this work has evolved. Thanks to Maximilian Franke for asking
good questions based on implementation experience and for
contributing text, e.g., on multicast.
15. References
15.1. Normative References
[I-D.ietf-taps-arch]
Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
Perkins, C., Tiesel, P. S., and C. A. Wood, "An
Architecture for Transport Services", Work in Progress,
Internet-Draft, draft-ietf-taps-arch-10, 30 April 2021,
<https://www.ietf.org/archive/id/draft-ietf-taps-arch-
10.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>.
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[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[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>.
[RFC8303] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
Transport Features Provided by IETF Transport Protocols",
RFC 8303, DOI 10.17487/RFC8303, February 2018,
<https://www.rfc-editor.org/info/rfc8303>.
[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>.
15.2. Informative References
[I-D.ietf-httpbis-priority]
Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-priority-03, 11 January 2021,
<https://www.ietf.org/archive/id/draft-ietf-httpbis-
priority-03.txt>.
[I-D.ietf-taps-impl]
Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K.,
Jones, T., Tiesel, P. S., Perkins, C., and M. Welzl,
"Implementing Interfaces to Transport Services", Work in
Progress, Internet-Draft, draft-ietf-taps-impl-09, 30
April 2021, <https://www.ietf.org/archive/id/draft-ietf-
taps-impl-09.txt>.
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[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.
[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<https://www.rfc-editor.org/info/rfc5245>.
[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
RFC 5482, DOI 10.17487/RFC5482, March 2009,
<https://www.rfc-editor.org/info/rfc5482>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.
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[RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use Cases and Requirements", RFC 7478,
DOI 10.17487/RFC7478, March 2015,
<https://www.rfc-editor.org/info/rfc7478>.
[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>.
[RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>.
[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>.
[RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
August 2017, <https://www.rfc-editor.org/info/rfc8229>.
[RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", RFC 8260,
DOI 10.17487/RFC8260, November 2017,
<https://www.rfc-editor.org/info/rfc8260>.
[RFC8293] Ghanwani, A., Dunbar, L., McBride, M., Bannai, V., and R.
Krishnan, "A Framework for Multicast in Network
Virtualization over Layer 3", RFC 8293,
DOI 10.17487/RFC8293, January 2018,
<https://www.rfc-editor.org/info/rfc8293>.
[RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
February 2020, <https://www.rfc-editor.org/info/rfc8489>.
[RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
[RFC8622] Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
June 2019, <https://www.rfc-editor.org/info/rfc8622>.
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[RFC8699] Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion
Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699,
January 2020, <https://www.rfc-editor.org/info/rfc8699>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
[RFC8922] Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C.
Wood, "A Survey of the Interaction between Security
Protocols and Transport Services", RFC 8922,
DOI 10.17487/RFC8922, October 2020,
<https://www.rfc-editor.org/info/rfc8922>.
[RFC8923] Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for End Systems", RFC 8923, DOI 10.17487/RFC8923,
October 2020, <https://www.rfc-editor.org/info/rfc8923>.
[TCP-COUPLING]
Islam, S., Welzl, M., Hiorth, K., Hayes, D., Armitage, G.,
and S. Gjessing, "ctrlTCP: Reducing Latency through
Coupled, Heterogeneous Multi-Flow TCP Congestion Control",
IEEE INFOCOM Global Internet Symposium (GI) workshop (GI
2018) , 2018.
Appendix A. Implementation Mapping
The way the concepts from this abstract interface map into concrete
APIs in a given language on a given platform largely depends on the
features and norms of the language and the platform. Actions could
be implemented as functions or method calls, for instance, and Events
could be implemented via event queues, handler functions or classes,
communicating sequential processes, or other asynchronous calling
conventions.
A.1. Types
The basic types mentioned in Section 1.1 typically have natural
correspondences in practical programming languages, perhaps
constrained by implementation-specific limitations. For example:
* An Integer can typically be represented in C by an "int" or
"long", subject to the underlying platform's ranges for each. To
accommodate special values, a C function that returns a non-
negative "int" on success may return -1 on failure. In Python,
such a function might return "None" or raise an exception.
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* In C, a Tuple may be represented as a "struct" with one member for
each of the value types in the ordered grouping. In Python, by
contrast, a Tuple may be represented natively as a "tuple", a
sequence of dynamically-typed elements.
* A Collection may be represented as a "std::set" in C++ or as a
"set" in Python. In C, it may be represented as an array or as a
higher-level data structure with appropriate accessors defined.
The objects described in Section 1.1 can similarly be represented in
different ways depending on which programming language is used.
Objects like Preconnections, Connections, and Listeners can be long-
lived, and benefit from using object-oriented constructs. Note that
in C, these objects may need to provide a way to release or free
their underlying memory when the application is done using them. For
example, since a Preconnection can be used to initiate multiple
Connections, it is the responsibility of the application to clean up
the Preconnection memory if necessary.
A.2. Events and Errors
This specification treats Events and Errors similarly. Errors, just
as any other Events, may occur asynchronously in network
applications. However, implementations of this interface may report
Errors synchronously, according to the error handling idioms of the
implementation platform, where they can be immediately detected, such
as by generating an exception when attempting to initiate a
connection with inconsistent Transport Properties. An error can
provide an optional reason to the application with further details
about why the error occurred.
A.3. Time Duration
Time duration types are implementation-specific. For instance, it
could be a number of seconds, number of milliseconds, or a "struct
timeval" in C or a user-defined "Duration" class in C++.
Appendix B. Convenience Functions
B.1. Adding Preference Properties
As Selection Properties of type "Preference" will be set on a
TransportProperties object quite frequently, implementations can
provide special actions for adding each preference level i.e,
"TransportProperties.Set(some_property, avoid)" is equivalent to
"TransportProperties.Avoid(some_property)":
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TransportProperties.Require(property)
TransportProperties.Prefer(property)
TransportProperties.Ignore(property)
TransportProperties.Avoid(property)
TransportProperties.Prohibit(property)
B.2. Transport Property Profiles
To ease the use of the interface specified by this document,
implementations can provide a mechanism to create Transport Property
objects (see Section 6.2) that are pre-configured with frequently
used sets of properties; the following are in common use in current
applications:
B.2.1. reliable-inorder-stream
This profile provides reliable, in-order transport service with
congestion control. TCP is an example of a protocol that provides
this service. It should consist of the following properties:
+=======================+=========+
| Property | Value |
+=======================+=========+
| reliability | require |
+-----------------------+---------+
| preserveOrder | require |
+-----------------------+---------+
| congestionControl | require |
+-----------------------+---------+
| preserveMsgBoundaries | ignore |
+-----------------------+---------+
Table 2: reliable-inorder-
stream preferences
B.2.2. reliable-message
This profile provides message-preserving, reliable, in-order
transport service with congestion control. SCTP is an example of a
protocol that provides this service. It should consist of the
following properties:
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+=======================+=========+
| Property | Value |
+=======================+=========+
| reliability | require |
+-----------------------+---------+
| preserveOrder | require |
+-----------------------+---------+
| congestionControl | require |
+-----------------------+---------+
| preserveMsgBoundaries | require |
+-----------------------+---------+
Table 3: reliable-message
preferences
B.2.3. unreliable-datagram
This profile provides a datagram transport service without any
reliability guarantee. An example of a protocol that provides this
service is UDP. It consists of the following properties:
+=======================+=========+
| Property | Value |
+=======================+=========+
| reliability | avoid |
+-----------------------+---------+
| preserveOrder | avoid |
+-----------------------+---------+
| congestionControl | ignore |
+-----------------------+---------+
| preserveMsgBoundaries | require |
+-----------------------+---------+
| safely replayable | true |
+-----------------------+---------+
Table 4: unreliable-datagram
preferences
Applications that choose this Transport Property Profile would avoid
the additional latency that could be introduced by retransmission or
reordering in a transport protocol.
Applications that choose this Transport Property Profile to reduce
latency should also consider setting an appropriate Capacity Profile
Property, see Section 8.1.6 and might benefit from controlling
checksum coverage, see Section 6.2.7 and Section 6.2.8.
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Appendix C. Relationship to the Minimal Set of Transport Services for
End Systems
[RFC8923] identifies a minimal set of transport services that end
systems should offer. These services make all non-security-related
transport features of TCP, MPTCP, UDP, UDP-Lite, SCTP and LEDBAT
available that 1) require interaction with the application, and 2) do
not get in the way of a possible implementation over TCP (or, with
limitations, UDP). The following text explains how this minimal set
is reflected in the present API. For brevity, it is based on the
list in Section 4.1 of [RFC8923], updated according to the discussion
in Section 5 of [RFC8923]. The present API covers all elements of
this section except "Notification of Excessive Retransmissions (early
warning below abortion threshold)". This list is a subset of the
transport features in Appendix A of [RFC8923], which refers to the
primitives in "pass 2" (Section 4) of [RFC8303] for further details
on the implementation with TCP, MPTCP, UDP, UDP-Lite, SCTP and
LEDBAT.
* Connect: "Initiate" Action (Section 7.1).
* Listen: "Listen" Action (Section 7.2).
* Specify number of attempts and/or timeout for the first
establishment message: "timeout" parameter of "Initiate"
(Section 7.1) or "InitiateWithSend" Action (Section 9.2.5).
* Disable MPTCP: "Multipath Transport" Property (Section 6.2.14).
* Hand over a message to reliably transfer (possibly multiple times)
before connection establishment: "InitiateWithSend" Action
(Section 9.2.5).
* Change timeout for aborting connection (using retransmit limit or
time value): "Timeout for Aborting Connection" property, using a
time value (Section 8.1.3).
* Timeout event when data could not be delivered for too long:
"ConnectionError" Event (Section 10).
* Suggest timeout to the peer: "TCP-specific Properties: User
Timeout Option (UTO)" (Section 8.2).
* Notification of ICMP error message arrival: "Notification of ICMP
soft error message arrival" property (Section 6.2.17).
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* Choose a scheduler to operate between streams of an association:
"Connection Group Transmission Scheduler" property
(Section 8.1.5).
* Configure priority or weight for a scheduler: "Connection
Priority" property (Section 8.1.2).
* "Specify checksum coverage used by the sender" and "Disable
checksum when sending": "Sending Corruption Protection Length"
property (Section 9.1.3.6) and "Full Checksum Coverage on Sending"
property (Section 6.2.7).
* "Specify minimum checksum coverage required by receiver" and
"Disable checksum requirement when receiving": "Required Minimum
Corruption Protection Coverage for Receiving" property
(Section 8.1.1) and "Full Checksum Coverage on Receiving" property
(Section 6.2.8).
* "Specify DF field": "No Network-Layer Fragmentation" property
(Section 9.1.3.9).
* Get max. transport-message size that may be sent using a non-
fragmented IP packet from the configured interface: "Maximum
Message Size Before Fragmentation or Segmentation" property
(Section 8.1.11.2).
* Get max. transport-message size that may be received from the
configured interface: "Maximum Message Size on Receive" property
(Section 8.1.11.4).
* Obtain ECN field: "UDP(-Lite)-specific Property: ECN" is a read-
only Message Property of the MessageContext object
(Section 9.3.3.1).
* "Specify DSCP field", "Disable Nagle algorithm", "Enable and
configure a "Low Extra Delay Background Transfer"": as suggested
in Section 5.5 of [RFC8923], these transport features are
collectively offered via the "Capacity Profile" property
(Section 8.1.6). Per-Message control ("Request not to bundle
messages") is offered via the "Message Capacity Profile Override"
property (Section 9.1.3.8).
* Close after reliably delivering all remaining data, causing an
event informing the application on the other side: this is offered
by the "Close" Action with slightly changed semantics in line with
the discussion in Section 5.2 of [RFC8923] (Section 10).
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* "Abort without delivering remaining data, causing an event
informing the application on the other side" and "Abort without
delivering remaining data, not causing an event informing the
application on the other side": this is offered by the "Abort"
action without promising that this is signaled to the other side.
If it is, a "ConnectionError" Event will fire at the peer
(Section 10).
* "Reliably transfer data, with congestion control", "Reliably
transfer a message, with congestion control" and "Unreliably
transfer a message": data is transferred via the "Send" action
(Section 9.2). Reliability is controlled via the "Reliable Data
Transfer (Connection)" (Section 6.2.1) property and the "Reliable
Data Transfer (Message)" Message Property (Section 9.1.3.7).
Transmitting data as a message or without delimiters is controlled
via Message Framers (Section 9.1.2). The choice of congestion
control is provided via the "Congestion control" property
(Section 6.2.9).
* Configurable Message Reliability: the "Lifetime" Message Property
implements a time-based way to configure message reliability
(Section 9.1.3.1).
* "Ordered message delivery (potentially slower than unordered)" and
"Unordered message delivery (potentially faster than ordered)":
these two transport features are controlled via the Message
Property "Ordered" (Section 9.1.3.3).
* Request not to delay the acknowledgement (SACK) of a message:
should the protocol support it, this is one of the transport
features the Transport Services system can apply when an
application uses the "Capacity Profile" Property (Section 8.1.6)
or the "Message Capacity Profile Override" Message Property
(Section 9.1.3.8) with value "Low Latency/Interactive".
* Receive data (with no message delimiting): "Receive" Action
(Section 9.3) and "Received" Event (Section 9.3.2.1).
* Receive a message: "Receive" Action (Section 9.3) and "Received"
Event (Section 9.3.2.1), using Message Framers (Section 9.1.2).
* Information about partial message arrival: "Receive" Action
(Section 9.3) and "ReceivedPartial" Event (Section 9.3.2.2).
* Notification of send failures: "Expired" Event (Section 9.2.2.2)
and "SendError" Event (Section 9.2.2.3).
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* Notification that the stack has no more user data to send:
applications can obtain this information via the "Sent" Event
(Section 9.2.2.1).
* Notification to a receiver that a partial message delivery has
been aborted: "ReceiveError" Event (Section 9.3.2.3).
Authors' Addresses
Brian Trammell (editor)
Google Switzerland GmbH
Gustav-Gull-Platz 1
CH- 8004 Zurich
Switzerland
Email: ietf@trammell.ch
Michael Welzl (editor)
University of Oslo
PO Box 1080 Blindern
0316 Oslo
Norway
Email: michawe@ifi.uio.no
Theresa Enghardt
Netflix
121 Albright Way
Los Gatos, CA 95032,
United States of America
Email: ietf@tenghardt.net
Godred Fairhurst
University of Aberdeen
Fraser Noble Building
Aberdeen, AB24 3UE
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Mirja Kuehlewind
Ericsson
Ericsson-Allee 1
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Herzogenrath
Germany
Email: mirja.kuehlewind@ericsson.com
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
Philipp S. Tiesel
SAP SE
Konrad-Zuse-Ring 10
14469 Potsdam
Germany
Email: philipp@tiesel.net
Christopher A. Wood
Cloudflare
101 Townsend St
San Francisco,
United States of America
Email: caw@heapingbits.net
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: tpauly@apple.com
Kyle Rose
Akamai Technologies, Inc.
145 Broadway
Cambridge, MA,
United States of America
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Email: krose@krose.org
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