trammell-taps-interface-00.txt

Internet DRAFT - draft-trammell-taps-interface

draft-trammell-taps-interface

Last Version:	draft-trammell-taps-interface-00.txt	Tracker Entry
Date:	`02-Mar-2018`
Disposition:	expired

TAPS Working Group B. Trammell, Ed.
Internet-Draft ETH Zurich
Intended status: Informational M. Welzl, Ed.
Expires: September 2, 2018 University of Oslo
T. Enghardt
TU Berlin
G. Fairhurst
University of Aberdeen
M. Kuehlewind
ETH Zurich
C. Perkins
University of Glasgow
P. Tiesel
TU Berlin
C. Wood
Apple Inc.
March 01, 2018

An Abstract Application Layer Interface to Transport Services
draft-trammell-taps-interface-00

Abstract

This document describes an abstract programming interface to the
transport layer, following the Transport Services Architecture. It
supports the asynchronous, atomic transmission of messages over
transport protocols and network paths dynamically selected at
runtime. It is intended to replace the traditional BSD sockets API
as the lowest common denominator interface to the transport layer, in
an environment where endpoints have multiple interfaces and potential
transport protocols to select from.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

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This Internet-Draft will expire on September 2, 2018.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. Interface Design Principles . . . . . . . . . . . . . . . . . 4
4. API Summary . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Pre-Establishment Phase . . . . . . . . . . . . . . . . . . . 6
5.1. Specifying Endpoints . . . . . . . . . . . . . . . . . . 6
5.2. Specifying Transport Parameters . . . . . . . . . . . . . 7
5.2.1. Transport Parameters Object . . . . . . . . . . . . . 11
5.3. Specifying Security Parameters and Callbacks . . . . . . 12
6. Establishing Connections . . . . . . . . . . . . . . . . . . 13
6.1. Active Open: Initiate . . . . . . . . . . . . . . . . . . 13
6.2. Passive Open: Listen . . . . . . . . . . . . . . . . . . 14
6.3. Peer-to-Peer Establishment: Rendezvous . . . . . . . . . 15
6.4. Connection Groups . . . . . . . . . . . . . . . . . . . . 16
7. Sending Data . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Send Parameters . . . . . . . . . . . . . . . . . . . . . 19
7.1.1. Lifetime . . . . . . . . . . . . . . . . . . . . . . 19
7.1.2. Niceness . . . . . . . . . . . . . . . . . . . . . . 19
7.1.3. Ordered . . . . . . . . . . . . . . . . . . . . . . . 20
7.1.4. Idempotent . . . . . . . . . . . . . . . . . . . . . 20
7.1.5. Corruption Protection Length . . . . . . . . . . . . 20
7.1.6. Immediate Acknowledgement . . . . . . . . . . . . . . 20
7.1.7. Instantaneous Capacity Profile . . . . . . . . . . . 21
7.2. Sender-side Framing . . . . . . . . . . . . . . . . . . . 21
8. Receiving Data . . . . . . . . . . . . . . . . . . . . . . . 22
8.1. Receiver-side De-framing over Stream Protocols . . . . . 23
9. Setting and Querying of Connection Properties . . . . . . . . 24
9.1. Protocol Properties . . . . . . . . . . . . . . . . . . . 25
10. Connection Termination . . . . . . . . . . . . . . . . . . . 27

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11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
14.1. Normative References . . . . . . . . . . . . . . . . . . 28
14.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Additional Properties . . . . . . . . . . . . . . . 29
A.1. Protocol and Path Selection Properties . . . . . . . . . 29
A.1.1. Application Intents . . . . . . . . . . . . . . . . . 30
A.2. Protocol Properties . . . . . . . . . . . . . . . . . . . 32
A.3. Send Parameters . . . . . . . . . . . . . . . . . . . . . 32
Appendix B. Sample API definition in Go . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33

1. Introduction

The BSD Unix Sockets API's SOCK_STREAM abstraction, by bringing
network sockets into the UNIX programming model, allowing anyone who
knew how to write programs that dealt with sequential-access files to
also write network applications, was a revolution in simplicity. It
would not be an overstatement to say that this simple API is the
reason the Internet won the protocol wars of the 1980s. SOCK_STREAM
is tied to the Transmission Control Protocol (TCP), specified in 1981
[RFC0793]. TCP has scaled remarkably well over the past three and a
half decades, but its total ubiquity has hidden an uncomfortable
fact: the network is not really a file, and stream abstractions are
too simplistic for many modern application programming models.

In the meantime, the nature of Internet access, and the variety of
Internet transport protocols, is evolving. The challenges that new
protocols and access paradigms present to the sockets API and to
programming models based on them inspire the design principles of a
new approach, which we outline in Section 3.

As a first step to realizing this design, [TAPS-ARCH] describes a
high-level architecture for transport services. This document builds
a modern abstract programming interface atop this architecture,
deriving specific path and protocol selection properties and
supported transport features from the analysis provided in [RFC8095]
and [I-D.ietf-taps-minset].

2. Terminology and Notation

This API is described in terms of Objects, which an application can
interact with; Actions the application can perform on these Objects;
Events, which an Object can send to an application asynchronously;
and Parameters associated with these Actions and Events.

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The following notations, which can be combined, are used in this
document:

o An Action creates an Object: ~~~ Object := Action() ~~~

o An Action is performed on an Object: ~~~ Object.Action() ~~~

o An Object sends an Event: ~~~ Object -> Event<> ~~~

o An Action takes a set of Parameters; an Event contains a set of
Parameters: ~~~ Action(parameter, parameter, ...) /
Event<parameter, parameter, ...> ~~~

Actions associated with no Object are Actions on the abstract
interface itself; they are equivalent to Actions on a per-application
global context.

How these abstract concepts map into concrete implementations of this
API in a given language on a given platform is largely dependent on
the features of the language and the platform. Actions could be
implemented as functions or method calls, for instance, and Events
could be implemented via callback passing or other asynchronous
calling conventions. The method for registering callbacks and
handlers is left as an implementation detail, with the caveat that
the interface for receiving Messages must require the application to
invoke the Connection.Receive() Action once per Message to be
received (see Section 8).

This specification treats Events and errors similarly, as errors,
just as any other Events, may occur asynchronously in network
applications. However, it is recommended that implementations of
this interface also return errors immediately, according to the error
handling idioms of the implementation platform, for errors which can
be immediately detected, such as inconsistency in transport
parameters.

3. Interface Design Principles

We begin with the architectural design principles defined in
[TAPS-ARCH]; from these, we derive and elaborate a set of principles
on which the design of the interface is based. The interface defined
in this document provides:

o A single interface to a variety of transport protocols to be used
in a variety of application design patterns, independent of the
properties of the application and the Protocol Stacks that will be
used at runtime, such that all common specialized features of
these protocol stacks are made available to the application as

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necessary in a transport-independent way, to enable applications
written to a single API to make use of transport protocols in
terms of the features they provide;

o Explicit support for security properties as first-order transport
features, and for long-term caching of cryptographic identities
and parameters for associations among endpoints;

o Asynchronous Connection establishment, transmission, and
reception, allowing most application interactions with the
transport layer to be Event-driven, in line with developments in
modern platforms and programming languages;

o Explicit support for multistreaming and multipath transport
protocols, and the grouping of related Connections into Connection
Groups through cloning of Connections, to allow applications to
take full advantage of new transport protocols supporting these
features; and

o Atomic transmission of data, using application-assisted framing
and deframing where the underlying transport does not provide
these.

4. API Summary

The Transport Services Interface is the basic common abstract
application programming interface to the Transport Services
Architecture defined in [TAPS-ARCH]. An application primarily
interacts with this interface through two Objects, Preconnections and
Connections. A Preconnection represents a set of parameters and
constraints on the selection and configuration of paths and protocols
to establish a Connection with a remote endpoint. A Connection
represents a transport Protocol Stack on which data can be sent to
and received from a remote endpoint. Connections can be created from
Preconnections in three ways: by initiating the Preconnection (i.e.,
actively opening, as in a client), through listening on the
Preconnection (i.e., passively opening, as in a server), or
rendezvousing on the Preconnection (i.e. peer to peer establishment).

Once a Connection is established, data can be sent 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 deframing callback which finds message
boundaries in a stream. Messages are received asynchronously through
a callback registered by the application. Errors and other
notifications also happen asynchronously on the Connection.

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In the following sections, we describe the details of application
interaction with Objects through Actions and Events in each phase of
a Connection, following the phases described in [TAPS-ARCH].

5. Pre-Establishment Phase

The pre-establishment phase allows applications to specify parameters
for the Connections they're about to make, or to query the API about
potential connections they could make.

A Preconnection Object represents a potential Connection. It has
state that describes parameters 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 5.1), the transport parameters (see Section 5.2), and the
security parameters (see Section 5.3):

Preconnection := NewPreconnection(LocalEndpoint,
RemoteEndpoint,
TransportParams,
SecurityParams)

The Local Endpoint MUST be specified if the Preconnection is used to
Listen() for incoming Connections, but is OPTIONAL if it is used to
Initiate() connections. The Remote Endpoint MUST be specified in the
Preconnection is used to Initiate() Connections, but is OPTIONAL if
it is used to Listen() for incoming Connections. The Local Endpoint
and the Remote Endpoint MUST both be specified if a peer-to-peer
Rendezvous is to occur based on the Preconnection.

Framers (see Section 7.2) and deframers (see Section 8.1), if
necessary, should be bound to the Preconnection during pre-
establishment.

Preconnections, as Connections, can be cloned, in order to establish
Connection groups before Connection initiation; see Section 6.4 for
details.

5.1. Specifying Endpoints

The transport services API uses the Local Endpoint and Remote
Endpoint types to refer to the endpoints of a transport connection.
Subtypes of these represent various different types of endpoint
identifiers, such as IP addresses, DNS names, and interface names, as
well as port numbers and service names.

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RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithHostname("example.com")
RemoteSpecifier.WithService("https")

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPv6Address(2001:db8:4920:e29d:a420:7461:7073:0a)
RemoteSpecifier.WithPort(443)

RemoteSpecifier := NewRemoteEndpoint()
RemoteSpecifier.WithIPv4Address(192.0.2.21)
RemoteSpecifier.WithPort(443)

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithInterface("en0")
LocalSpecifier.WithPort(443)

LocalSpecifier := NewLocalEndpoint()
LocalSpecifier.WithStunServer(address, port, credentials)

Implementations may also support additional endpoint representations
and provide a single NewEndpoint() call that takes different endpoint
representations.

Multiple endpoint identifiers can be specified for each Local
Endpoint and RemoteEndoint. For example, a Local Endpoint could be
configured with two interface names, or a Remote Endpoint could be
specified via both IPv4 and IPv6 addresses. The multiple identifiers
refer to the same endpoint.

The transport services API will resolve names internally, when the
Initiate(), Listen(), or Rendezvous() method is called establish a
Connection. The API does not need the application to resolve names,
and premature name resolution can damage performance by limiting the
scope for alternate path discovery during Connection establishment.
The Resolve() method is, however, provided to resolve a Local
Endpoint or a Remote Endpoint in cases where this is required, for
example with some NAT traversal protocols (see Section 6.3).

5.2. Specifying Transport Parameters

A Preconnection Object holds parameters reflecting the application's
requirements and preferences for the transport. These include
protocol and path selection parameters, as well as Generic and
Specific Protocol Properties for configuration of the detailed
operation of the selected Protocol Stacks.

All Transport Parameters are organized within a single namespace
shared with Send Parameters (see Section 7.1). All transport

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parameters take paremeter-specific values. Protocol and Path
Selection properties additionally take one of five preference levels,
though not all preference levels make sense with all such properties.
Note that it is possible for a set of specified transport parameters
to be internally inconsistent, or for preferences to be inconsistent
with the later use of the API by the application. Application
developers should reduce inconsistency by only using the most
stringent preference levels when failure to meet a preference would
break the application's functionality (e.g. the Reliable Data
Transfer preference, which is a core assumption of many application
protocols). Implementations of this interface should also raise
errors in configuration as early as possible, to help ensure these
inconsistencies are caught early in the development process.

The protocol(s) and path(s) selected as candidates during Connection
establishment are determined by a set of 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.

To reflect the needs of an individual Connection, they can be
specified with five different preference levels:

An implementation of this interface must provide sensible defaults
for protocol and path selection properties. The defaults given for
each property below represent a configuration that can be implemented
over TCP. An alternate set of default Protocol Selection Properties
would represent a configuration that can be implemented over UDP.

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The following properties can be used during Protocol and Path
selection:

o Reliable Data Transfer: This boolean property specifies whether
the application needs the transport protocol to ensure that data
is received completely and without corruption on the other side.
This also entails being notified when a Connection is closed or
aborted. This property applies to Connections and Connection
Groups. This is a strict requirement. The default is to enable
Reliable Data Transfer.

o Preservation of data ordering: This boolean property specifies
whether the application needs the transport protocol to assure
that data is received by the application on the other end in the
same order as it was sent. This property applies to Connections
and Connection Groups. This is a strict requirement. The default
is to preserve data ordering.

o Configure reliability on a per-Message basis: This boolean
property specifies whether an application considers it useful to
indicate its reliability requirements on a per-Message basis.
This property applies to Connections and Connection Groups. This
is not a strict requirement. The default is to not have this
option.

o Use 0-RTT session establishment with an idempotent Message: This
boolean property specifies whether an application would like to
supply a Message to the transport protocol before Connection
establishment, which will then be reliably transferred to the
other side before or during Connection establishment, potentially
multiple times. See also Section 7.1.4. This is a strict
requirement. The default is to not have this option.

o Multiplex Connections: This boolean property specifies that the
application would prefer multiple Connections between the same
endpoints within a Connection Group to be multiplexed onto a
single underlying transport connection where possible, for reasons
of efficiency. This is not a strict requirement. The default is
to not have this option.

o Notification of excessive retransmissions: This boolean property
specifies whether an application considers it useful to be
informed in case sent data was retransmitted more often than a
certain threshold. This property applies to Connections and
Connection Groups. This is not a strict requirement. The default
is to have this option.

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o Notification of ICMP error message arrival: This boolean property
specifies whether an application considers it useful to be
informed when an ICMP error message arrives. This property
applies to Connections and Connection Groups. This is not a
strict requirement. The default is to have this option.

o Control checksum coverage on sending or receiving: This boolean
property specifies whether the application considers it useful to
enable / disable / configure a checksum when sending data, or
decide whether to require a checksum or not when receiving data.
This property applies to Connections and Connection Groups. This
is not a strict requirement, as it signifies a reduction in
reliability. The default is full checksum coverage without being
able to change it, and requiring a checksum when receiving.

o Interface Type: This enumerated property specifies which kind of
access network interface, e.g., WiFi, Ethernet, or LTE, to prefer
over others for this Connection, in case they are available. In
general, Interface Types should be used only with the "Prefer" and
"Prohibit" preference level. Specifically, using the "Require"
preference level for Interface Type may limit path selection in a
way that is detrimental to connectivity. The default is to use
the default interface configured in the system policy.

o Capacity Profile: This enumerated property specifies the
application's expectation of the dominating traffic pattern for
this Connection. The Capacity Profile should only be used with
the "Prefer" preference level; other preference levels make no
sense for profiles. The following values are valid for Capacity
Profile:

Default: The application makes no representation about its
expected capacity profile. No special optimizations of the
tradeoff between delay, delay variation, and bandwidth
efficiency should be made when selecting and configuring
stacks.

Interactive/Low Latency: The application is interactive.
Response time (latency) should be optimized at the expense of
bandwidth efficiency and delay variation. This can be used by
the system to disable the coalescing of multiple small Messages
into larger packets (Nagle's algorithm), to prefer lower-
latency paths, signal a preference for lower-latency, higher-
loss treatment, and so on.

Constant Rate: The application expects to send/receive data at a
constant rate after Connection establishment. Delay and delay
variation should be optimized at the expense of bandwidth

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efficiency. This implies that the Connection may fail if the
desired rate cannot be maintained across the Path. A transport
may interpret this capacity profile as preferring a circuit
breaker [RFC8084] to a rate adaptive congestion controller.

Scavenger/Bulk: The application is not interactive. It expects
to send/receive a large amount of data, without any urgency.
This can be used to select protocol stacks with scavenger
transmission control, to signal a preference for less-than-
best-effort treatment, and so on.

In addition to protocol and path selection properties, the transport
parameters may also contain Generic and/or Specific Protocol
Properties (see Section 9.1). These properties will be passed to the
selected candidate Protocol Stack(s) to configure them before
candidate Connection establishment.

5.2.1. Transport Parameters Object

All transport parameters used in the pre-establishment phase are
collected in a TransportParameters Object that is passed to the
Preconnection Object.

TransportParameters := NewTransportParameters()

The Individual parameters are then added to the TransportParameters
Object. While Protocol Properties use the "add" call, Transport
Preferences use special calls for the levels defined in Section 5.2.

TransportParameters.Add(parameter, value)

TransportParameters.Require(preference)
TransportParameters.Prefer(preference)
TransportParameters.Ignore(preference)
TransportParameters.Avoid(preference)
TransportParameters.Prohibit(preference)

For an existing Connection, the Transport Parameters can be queried
any time by using the following call on the Connection Object:

TransportParameters := Connection.GetTransportParameters()

Note that most properties are only considered for Connection
establishment and can not be changed after a Connection is
established; however, they can be queried. See Section 9.

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A Connection gets its Transport Parameters either by being explicitly
configured via a Preconnection, or by inheriting them from an
antecedent via cloning; see Section 6.4 for more.

5.3. Specifying Security Parameters and Callbacks

Common parameters such as TLS ciphersuites are known to
implementations. Clients SHOULD use common safe defaults for these
values whenever possible. However, as discussed in
[I-D.pauly-taps-transport-security], 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 are created as follows

SecurityParameters := NewSecurityParameters()

Security configuration parameters and sample usage follow:

o 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 HSMs, handshake callbacks MUST be used. See below for
details.)

SecurityParameters.AddIdentity(identity)
SecurityParameters.AddPrivateKey(privateKey, publicKey)

o Supported algorithms: Used to restrict what parameters are used by
underlying transport security protocols. When not specified,
these algorithms SHOULD default to known and safe defaults for the
system. Parameters include: ciphersuites, supported groups, and
signature algorithms.

SecurityParameters.AddSupportedGroup(22) // secp256k1
SecurityParameters.AddCiphersuite(0xCCA9) // TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
SecurityParameters.AddSignatureAlgorithm(7) // ed25519

o Session cache: Used to tune cache capacity, lifetime, re-use, and
eviction policies, e.g., LRU or FIFO.

SecurityParameters.SetSessionCacheCapacity(1024) // 1024 elements
SecurityParameters.SetSessionCacheLifetime(24*60*60) // 24 hours
SecurityParameters.SetSessionCacheReuse(1) // One-time use

o Pre-shared keying material: 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.

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SecurityParameters.AddPreSharedKey(key, identity)

Security decisions, especially pertaining to trust, are not static.
Thus, once configured, parameters must also be supplied during live
handshakes. These are best handled as client-provided callbacks.
Security handshake callbacks include:

o Trust verification callback: Invoked when a Remote Endpoint's
trust must be validated before the handshake protocol can proceed.

TrustCallback := NewCallback({
// Handle trust, return the result
})
SecurityParameters.SetTrustVerificationCallback(trustCallback)

o Identity challenge callback: Invoked when a private key operation
is required, e.g., when local authentication is requested by a
remote.

ChallengeCallback := NewCallback({
// Handle challenge
})
SecurityParameters.SetIdentityChallengeCallback(challengeCallback)

Like transport parameters, security parameters are inherited during
cloning (see Section 6.4).

6. Establishing Connections

Before a Connection can be used for data transfer, it must be
established. Establishment ends the pre-establishment phase; all
transport and cryptographic parameter specification must be complete
before establishment, as these parameters 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.

6.1. 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()

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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 parameters
necessary for candidate selection. After calling Initiate, no
further parameters may be bound to the Connection. The Initiate()
call consumes the Preconnection and creates a Connection Object. A
Preconnection can only be initiated once.

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 7) after calling
Initate(); note that any idempotent data 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 8) will
occur before the Ready Event for Connections established using
Initiate.

Connection -> InitiateError<>

An InitiateError occurs either when the set of transport and
cryptographic 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 to the remote
endpoint (e.g. because the remote endpoint is not accepting
connections, or the application is prohibited from opening a
Connection by the operating system).

6.2. 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:

Preconnection.Listen()

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Before calling Listen, the caller must have initialized the
Preconnection during the pre-establishment phase with a Local
Endpoint specifier, as well as all parameters necessary for Protocol
Stack selection. A Remote Endpoint may optionally be specified, to
constrain what Connections are accepted. The Listen() Action
consumes the Preconnection. Once Listen() has been called, no
further parameters may be bound to the Preconnection, and no
subsequent establishment call may be made on the Preconnection.

Preconnection -> ConnectionReceived<Connection>

The ConnectionReceived Event occurs when a Remote Endpoint has
established a transport-layer connection to this Preconnection (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.

Preconnection -> ListenError<>

A ListenError occurs either when the Preconnection cannot be
fulfilled for listening, when the Local Endpoint (or Remote Endpoint,
if specified) cannot be resolved, or when the application is
prohibited from listening by policy.

6.3. Peer-to-Peer Establishment: Rendezvous

Simultaneous peer-to-peer Connection establishment is supported by
the Rendezvous() Action:

Preconnection.Rendezvous()

The Preconnection Object must be specified with both a Local Endpoint
and a Remote Endpoint, and also the transport and security parameters
needed for Protocol Stack selection. The Rendezvous() Action causes
the Preconnection to listen on the Local Endpoint for an incoming
Connection from the Remote Endpoint, while simultaneously trying to
establish a Connection from the Local Endpoint to the Remote
Endpoint. This corresponds to a TCP simultaneous open, for example.

The Rendezvous() Action consumes the Preconnection. Once
Rendezvous() has been called, no further parameters may be bound to
the Preconnection, and no subsequent establishment call may be made
on the Preconnection.

Preconnection -> RendezvousDone<Connection>

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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.

Preconnection -> RendezvousError<msgRef, error>

An RendezvousError occurs either when the Preconnection cannot be
fulfilled for listening, 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.

When using some NAT traversal protocols, e.g., ICE [RFC5245], it is
expected that the Local Endpoint will be configured with some method
of discovering NAT bindings, e.g., a STUN server. In this case, the
Local Endpoint may resolve to a mixture of local and server reflexive
addresses. The Resolve() method on the Preconnection can be used to
discover these bindings:

PreconnectionBindings := Preconnection.Resolve()

The Resolve() call returns a list of Preconnection Objects, that
represent the concrete addresses, local and server reflexive, on
which a Rendezvous() for the Preconnection will listen for incoming
Connections. This list can be passed to a peer via a signalling
protocol, such as SIP or WebRTC, to configure the remote.

6.4. Connection Groups

Groups of Preconnections or Connections can be created using the
Clone Action:

Preconnection := Preconnection.Clone()

Connection := Connection.Clone()

Calling Clone on a Connection yields a group of two Connections: the
parent Connection on which Clone was called, and the resulting clone
Connection. These connections are "entangled" with each other, and
become part of a Connection group. Calling Clone on any of these two
Connections adds a third Connection to the group, and so on.
Connections in a Connection Group share all their properties, and
changing the properties on one Connection in the group changes the
property for all others.

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Calling Clone on a Preconnection yields a Preconnection with the same
parameters, which is entangled with the parent Preconnection: all the
Connections created from entangled Preconnections will be entangled
as if they had been cloned, and will belong to the same Connection
Group.

Establishing a Connection from a cloned Preconnection will not cause
Connections for other entangled Preconnections to be established;
each such Connection must be established separately. Changes to the
parameters of a Preconnection entangled with a Preconnection from
which a Connection has already been established will fail. Calling
Clone on a Preconnection may be taken by the system an implicit
signal that Protocol Stacks supporting multiplexed Connections for
efficient Connection Grouping are preferred by the application.

There is only one Protocol Property that is not entangled, i.e., it
is a separate per-Connection Property for individual Connections in
the group: niceness. Niceness works as in Section 7.1.2: when
allocating available network capacity among Connections in a
Connection Group, sends on Connections with higher Niceness values
will be prioritized over sends on Connections with lower Niceness
values. An ideal transport system implementation would assign the
Connection the capacity share (M-N) x C / M, where N is the
Connection's Niceness value, M is the maximum Niceness value used by
all Connections in the group and C is the total available capacity.
However, the niceness setting is purely advisory, and no guarantees
are given about capacity allocation and each implementation is free
to implement exact capacity allocation as it sees fit.

7. Sending Data

Once a Connection has been established, it can be used for sending
data. Data is sent by passing a Message Object and additional
parameters Section 7.1 to the Send Action on an established
Connection:

Connection.Send(Message, sendParameters)

The type of the Message to be passed is dependent on the
implementation, and on the constraints on the Protocol Stacks implied
by the Connection's transport parameters. It may itself contain an
array of octets to be transmitted in the transport protocol payload,
or be transformable to an array of octets by a sender-side framer
(see Section 7.2).

Some transport protocols can deliver arbitrarily sized Messages, but
other protocols constrain the maximum Message size. Applications can

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query the protocol property Maximum Message Size on Send to determine
the maximum size.

There may also be system and Protocol Stack dependent limits on the
size of a Message which can be transmitted atomically. For that
reason, the Message object passed to the Send action may also be a
partial Message, either representing the whole data object and
information about the range of bytes to send from it, or an object
referring back to the larger whole Message. The details of partial
Message sending are implementation-dependent.

If Send is called on a Connection which has not yet been established,
an Initiate Action will be implicitly performed simultaneously with
the Send. Used together with the Idempotent property (see
Section 7.1.4), this can be used to send data during establishment
for 0-RTT session resumption on Protocol Stacks that support it.

Like all Actions in this interface, the Send Action is asynchronous.

Connection -> Sent<msgRef>

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 the implementation of this interface. The exact
disposition of the Message when the Sent Event occurs is specific to
the implementation and the constraints on the Protocol Stacks implied
by the Connection's transport parameters. The Sent Event contains an
implementation-specific reference to 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 system than an application
that only issues a Send after this Event fires.

Connection -> Expired<msgRef>

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 7.1.1) expired. This is separate from SendError, as it
is an expected behavior for partially reliable transports. The
Expired Event contains an implementation-specific reference to the
Message to which it applies.

Connection -> SendError<msgRef>

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A SendError occurs when a Message could not be 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 send parameters not consistent with the
Connection's transport parameters. The SendError contains an
implementation-specific reference to the Message to which it applies.

7.1. Send Parameters

The Send Action takes per-Message send parameters which control how
the contents will be sent down to the underlying Protocol Stack and
transmitted.

If Send Parameters should be overridden for a specific Message, an
empty sent parameter Object can be acquired and all desired Send
Parameters can be added to that Object. A sendParameters Object can
be reused for sending multiple contents with the same properties.

SendParameters := NewSendParameters()
SendParameters.Add(parameter, value)

The Send Parameters share a single namespace with the Transport
Parameters (see Section 5.2). This allows the specification of
Protocol Properties that can be overridden on a per-Message basis.

Send Parameters may be inconsistent with the properties of the
Protocol Stacks underlying the Connection on which a given Message is
sent. For example, infinite Lifetime is not possible on a Message
over a Connection not providing reliability. Sending a Message with
Send Properties inconsistent with the Transport Preferences on the
Connection yields an error.

The following send parameters are supported:

7.1.1. Lifetime

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. When a Message's Lifetime is infinite, it must
be transmitted reliably. The type and units of Lifetime are
implementation-specific.

7.1.2. Niceness

Niceness represents an unbounded hierarchy of priorities of Messages,
relative to other Messages sent over the same Connection and/or
Connection Group (see Section 6.4). It is most naturally represented
as a non-negative integer. A Message with Niceness 0 will yield to a

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Message with Niceness 1, which will yield to a Message with Niceness
2, and so on. Niceness may be used as a sender-side scheduling
construct only, or be used to specify priorities on the wire for
Protocol Stacks supporting prioritization.

Note that this inversion of normal schemes for expressing priority
has a convenient property: priority increases as both Niceness and
Lifetime decrease.

7.1.3. Ordered

Ordered is a boolean property. If true, this Message should be
delivered after the last Message passed to the same Connection via
the Send Action; if false, this Message may be delivered out of
order.

7.1.4. Idempotent

Idempotent is a boolean property. If true, the application-layer
entity in the Message is safe to send to the remote endpoint more
than once for a single Send Action. It is used to mark data 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.

7.1.5. Corruption Protection Length

This numeric property specifies the length of the section of the
Message, starting from byte 0, that the application assumes will be
received without corruption due to lower layer errors. It is used to
specify options for simple integrity protection via checksums. By
default, the entire Message is protected by checksum. A value of 0
means that no checksum is required, and a special value (e.g. -1) can
be used to indicate the default. Only full coverage is guaranteed,
any other requests are advisory.

7.1.6. Immediate Acknowledgement

This boolean property specifies, if true, that an application wants
this Message to be acknowledged immediately by the receiver. In case
of reliable transmission, this informs the transport protocol on the
sender side faster that it can remove the Message from its buffer;
therefore this property can be useful for latency-critical
applications that maintain tight control over the send buffer (see
Section 7).

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7.1.7. Instantaneous Capacity Profile

This enumerated property specifies the application's preferred
tradeoffs for sending this Message; it is a per-Message override of
the Capacity Profile protocol and path selection property (see
Section 5.2).

The following values are valid for Instantaneous Capacity Profile:

Default: No special optimizations of the tradeoff between delay,
delay variation, and bandwidth efficiency should be made when
sending this message.

Interactive/Low Latency: Response time (latency) should be optimized
at the expense of bandwidth efficiency and delay variation when
sending this message. This can be used by the system to disable
the coalescing of multiple small Messages into larger packets
(Nagle's algorithm), to signal a preference for lower-latency,
higher-loss treatment, and so on.

Constant Rate: Delay and delay variation should be optimized at the
expense of bandwidth efficiency.

Scavenger/Bulk: This Message may be sent at the system's leisure.
This can be used to signal a preference for less-than-best-effort
treatment, to delay sending until lower-cost paths are available,
and so on.

7.2. Sender-side Framing

Sender-side framing allows a caller to provide the interface with a
function that takes a Message of an appropriate application-layer
type and returns an array of octets, the on-the-wire representation
of the Message to be handed down to the Protocol Stack. It consists
of a Framer Object with a single Action, Frame. Since the Framer
depends on the protocol used at the application layer, it is bound to
the Preconnection during the pre-establishment phase:

Preconnection.FrameWith(Framer)

OctetArray := Framer.Frame(Message)

Sender-side framing is a convenience feature of the interface, for
parity with receiver-side framing (see Section 8.1).

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8. Receiving Data

Once a Connection is established, Messages may be received on it.
The application can indicate that it is ready to receive Messages by
calling Receive() on the Connection.

Connection.Receive(ReceiveHandler, maxLength)

Receive takes a ReceiveHandler, which can handle the Received Event
and the ReceiveError error. Each call to Receive will result in at
most one Received event being sent to the handler, though
implementations may provide convenience functions to indicate
readiness to receive a larger but finite number of Messages with a
single call. This allows an application to provide backpressure to
the transport stack when it is temporarily not ready to receive
messages.

Receive also takes an optional maxLength argument, the maximum size
(in bytes of data) Message 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 received as a partial Message. Note that maxLength does
not guarantee that the application will receive that many bytes if
they are available; the interface may return partial Messages smaller
than maxLength according to implementation constraints.

Connection -> Received<Message>

As with sending, the type of the Message to be passed is dependent on
the implementation, and on the constraints on the Protocol Stacks
implied by the Connection's transport parameters. The Message may
also contain metadata from protocols in the Protocol Stack; which
metadata is available is Protocol Stack dependent. In particular,
when this information is available, the value of the Explicit
Congestion Notification (ECN) field is contained in such metadata.
This information can be used for logging and debugging purposes, and
for building applications which need access to information about the
transport internals for their own operation.

The Message Object must provide some method to retrieve an octet
array containing application data, corresponding to a single message
within the underlying Protocol Stack's framing. See Section 8.1 for
handling framing in situations where the Protocol Stack provides
octet-stream transport only.

The Message Object passed to Received is complete and atomic, unless
one of the following conditions holds:

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o the underlying Protocol Stack supports message boundary
preservation, and the size of the Message is larger than the
buffers available for a single message;

o the underlying Protocol Stack does not support message boundary
preservation, and the deframer (see Section 8.1) cannot determine
the end of the message using the buffer space it has available; or

o the underlying Protocol Stack does not support message boundary
preservation, and no deframer was supplied by the application

The Message Object passed to Received will indicate one of the
following:

1. this is a complete message;

2. this is a partial message containing a section of a message with
a known message boundary (made partial for local buffering
reasons, either by the underlying Protocol Stack or the
deframer). In this case, the Message Object passed to Received
may contain the byte offset of the data in the partial Message
within the full Message, an indication whether this is the last
(highest-offset) partial Message in the full Message, and an
optional reference to the full Message it belongs to; or

3. this is a partial message containing data with no definite
message boundary, i.e. the only known message boundary is given
by termination of the Connection

Note that in the absence of message boundary preservation and without
deframing, the entire Connection is represented as one large message
of indeterminate length.

Connection -> ReceiveError<>

A ReceiveError occurs when data is received by the underlying
Protocol Stack that cannot be fully retrieved or deframed, or when
some other indication is received that reception has failed. Such
conditions that irrevocably lead the the termination of the
Connection are signaled using ConnectionError instead (see
Section 10).

8.1. Receiver-side De-framing over Stream Protocols

The Receive Event is intended to be fired once per application-layer
Message sent by the remote endpoint; i.e., it is a desired property
of this interface that a Send at one end of a Connection maps to
exactly one Receive on the other end. This is possible with Protocol

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Stacks that provide message boundary preservation, but is not the
case over Protocol Stacks that provide a simple octet stream
transport.

For preserving message boundaries over stream transports, this
interface provides receiver-side de-framing. This facility is based
on the observation that, since many of our current application
protocols evolved over TCP, which does not provide message boundary
preservation, and since many of these protocols require message
boundaries to function, each application layer protocol has defined
its own framing. A Deframer allows an application to push this de-
framing down into the interface, in order to transform an octet
stream into a sequence of Messages.

Concretely, receiver-side de-framing allows a caller to provide the
interface with a function that takes an octet stream, as provided by
the underlying Protocol Stack, reads and returns a single Message of
an appropriate type for the application and platform, and leaves the
octet stream at the start of the next Message to deframe. It
consists of a Deframer Object with a single Action, Deframe. Since
the Deframer depends on the protocol used at the application layer,
it is bound to the Preconnection during the pre-establishment phase:

Preconnection.DeframeWith(Deframer)

Message := Deframer.Deframe(OctetStream, ...)

9. Setting and Querying of Connection Properties

At any point, the application can set and query the properties of a
Connection. Depending on the phase the Connection is in, the
Connection properties will include different information.

ConnectionProperties := Connection.GetProperties()

Connection.SetProperties()

Connection properties include:

o The status of the Connection, which can be one of the following:
Establishing, Established, Closing, or Closed.

o Transport Features of the protocols that conform to the Required
and Prohibited Transport Preferences, which might be selected by
the transport system during Establishment. These features
correspond to the properties given in Section 5.2 and can only be
queried.

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o Transport Features of the Protocol Stacks that were selected and
instantiated, once the Connection has been established. These
features correspond to the properties given in Section 5.2 and can
only be queried. Instead of preference levels, these features
have boolean values indicating whether or not they were selected.
Note that these transport features may not fully reflect the
specified parameters given in the pre-establishment phase. For
example, a certain Protocol Selection Property that an application
specified as Preferred may not actually be present in the chosen
Protocol Stack Instances because none of the currently available
transport protocols had this feature.

o Protocol Properties of the Protocol Stack in use (see Section 9.1
below). These can be set or queried. Certain specific procotol
queries may be read-only, on a protocol- and property-specific
basis.

o Path Properties of the path(s) in use, once the Connection has
been established. These properties can be derived from the local
provisioning domain, measurements by the Protocol Stack, or other
sources. They can only be queried.

9.1. Protocol Properties

Protocol Properties represent the configuration of the selected
Protocol Stacks backing a Connection. Some properties apply
generically across multiple transport protocols, while other
properties only apply to specific protocols. The default settings of
these properties will vary based on the specific protocols being used
and the system's configuration.

Note that Protocol Properties are also set during pre-establishment,
as transport parameters, to preconfigure Protocol Stacks during
establishment.

Generic Protocol Properties include:

o Relative niceness: This numeric property is similar to the
Niceness send property (see Section 7.1.2), a non-negative integer
representing the relative inverse 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 6.4, this property is not entangled when
Connections are cloned.

o Timeout for aborting Connection: This numeric property specifies
how long to wait before aborting a Connection during

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establishment, or before deciding that a Connection has failed
after establishment. It is given in seconds.

o Retransmission threshold before excessive retransmission
notification: This numeric property specifies after how many
retransmissions to inform the application about "Excessive
Retransmissions".

o Required minimum coverage of the checksum for receiving: This
numeric property specifies the part of the received data that
needs to be covered by a checksum. It is given in Bytes. A value
of 0 means that no checksum is required, and a special value
(e.g., -1) indicates full checksum coverage.

o Connection group transmission scheduler: This enumerated property
specifies which scheduler should be used among Connections within
a Connection Group. It applies to Connection Groups; the set of
schedulers can be taken from [I-D.ietf-tsvwg-sctp-ndata].

o Maximum message size concurrent with Connection establishment:
This numeric property represents the maximum Message size that can
be sent before or during Connection establishment, see also
Section 7.1.4. It is given in Bytes. This property is read-only.

o Maximum Message size before fragmentation or segmentation: This
numeric property, if applicable, represents the maximum Message
size that can be sent without incurring network-layer
fragmentation and/or transport layer segmentation at the sender.
This property is read-only.

o Maximum Message size on send: This numeric property represents the
maximum Message size that can be sent. This property is read-
only.

o Maximum Message size on receive: This numeric property represents
the maximum Message size that can be received. This property is
read-only.

In order to specify Specific Protocol Properties, Transport System
implementations may offer applications to attach a set of options to
the Preconnection Object, associated with a specific protocol. For
example, an application could specify a set of TCP Options to use if
and only if TCP is selected by the system. Such properties must not
be assumed to apply across different protocols. Attempts to set
specific protocol properties on a Protocol Stack not containing that
specific protocol are simply ignored, and do not raise an error.

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10. Connection Termination

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 system. For example,
if reliable delivery was requested for a Message handed over before
calling Close, the transport system will ensure that this Message is
indeed delivered. If the Remote Endpoint still has data to send, it
cannot be received after this call.

Connection.Close()

The Closed Event can inform the application that the Remote Endpoint
has closed the Connection; however, there is no guarantee that a
remote close will be signaled.

Connection -> Closed<>

Abort terminates a Connection without delivering remaining data:

Connection.Abort()

A ConnectionError can inform the application that the other side has
aborted the Connection; however, there is no guarantee that an abort
will be signaled:

Connection -> ConnectionError<>

11. IANA Considerations

RFC-EDITOR: Please remove this section before publication.

This document has no Actions for IANA.

12. 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 Section 5.3. It does not recommend use (or disuse) of
specific algorithms or protocols. Any API-compatible transport
security protocol should work in a TAPS system.

13. 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).

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This work has been supported by Leibniz Prize project funds of DFG -
German Research Foundation: Gottfried Wilhelm Leibniz-Preis 2011 (FKZ
FE 570/4-1).

This work has been supported by the UK Engineering and Physical
Sciences Research Council under grant EP/R04144X/1.

Thanks to Stuart Cheshire, Josh Graessley, David Schinazi, and Eric
Kinnear for their implementation and design efforts, including Happy
Eyeballs, that heavily influenced this work. Thanks to Laurent Chuat
and Jason Lee for initial work on the Post Sockets interface, from
which this work has evolved.

14. References

14.1. Normative References

[I-D.ietf-taps-minset]
Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for TAPS Systems", draft-ietf-taps-minset-02
(work in progress), February 2018.

[I-D.ietf-tsvwg-rtcweb-qos]
Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
qos-18 (work in progress), August 2016.

[I-D.ietf-tsvwg-sctp-ndata]
Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", draft-ietf-tsvwg-
sctp-ndata-13 (work in progress), September 2017.

[TAPS-ARCH]
Pauly, T., Ed., Trammell, B., Ed., Brunstrom, A.,
Fairhurst, G., Perkins, C., Tiesel, P., and C. Wood, "An
Architecture for Transport Services", n.d..

14.2. Informative References

[I-D.pauly-taps-transport-security]
Pauly, T., Rose, K., and C. Wood, "A Survey of Transport
Security Protocols", draft-pauly-taps-transport-
security-01 (work in progress), January 2018.

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

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[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>.

[RFC8084] Fairhurst, G., "Network Transport Circuit Breakers",
BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
<https://www.rfc-editor.org/info/rfc8084>.

[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>.

Appendix A. Additional Properties

The interface specified by this document represents the minimal
common interface to an endpoint in the transport services
architecture [TAPS-ARCH], based upon that architecture and on the
minimal set of transport service features elaborated in
[I-D.ietf-taps-minset]. However, the interface has been designed
with extension points to allow the implementation of features beyond
those in the minimal common interface: Protocol Selection Properties,
Path Selection Properties, and options on Message send are open sets.
Implementations of the interface are free to extend these sets to
provide additional expressiveness to applications written on top of
them.

This appendix enumerates a few additional parameters and properties
that could be used to enhance transport protocol and/or path
selection, or the transmission of messages given a Protocol Stack
that implements them. These are not part of the interface, and may
be removed from the final document, but are presented here to support
discussion within the TAPS working group as to whether they should be
added to a future revision of the base specification.

A.1. Protocol and Path Selection Properties

The following protocol and path selection properties might be made
available in addition to those specified in Section 5.2:

o Suggest a timeout to the Remote Endpoint: This boolean property
specifies whether an application considers it useful to propose a
timeout until the Connection is assumed to be lost. This property
applies to Connections and Connection Groups. This is not a
strict requirement. The default is to have this option.

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[EDITOR'S NOTE: For discussion of this option, see
https://github.com/taps-api/drafts/issues/109]

o Request not to delay acknowledgment of Message: This boolean
property specifies whether an application considers it useful to
request for Message that its acknowledgment be sent out as early
as possible instead of potentially being bundled with other
acknowledgments. This property applies to Connections and
Connection groups. This is not a strict requirement. The default
is to not have this option. [EDITOR'S NOTE: For discussion of
this option, see https://github.com/taps-api/drafts/issues/90]

A.1.1. Application Intents

Application Intents are a group of transport properties expressing
what an application wants to achieve, knows, assumes or prefers
regarding its communication. They are not strict requirements. In
particular, they should not be used to express any Quality of Service
expectations that an application might have. Instead, an application
should express its intentions and its expected traffic
characteristics in order to help the transport system make decisions
that best match it, but on a best-effort basis. Even though
Application Intents do not represent Quality of Service requirements,
a transport system may use them to determine a DSCP value, e.g.
similar to Table 1 in [I-D.ietf-tsvwg-rtcweb-qos].

Application Intents can influence protocol selection, protocol
configuration, path selection, and endpoint selection. For example,
setting the "Timeliness" Intent to "Interactive" may lead the
transport system to disable the Nagle algorithm for a Connection,
while setting the "Timeliness" to "Background" may lead it to setting
the DSCP value to "scavenger". If the "Size to be Sent" Intent is
set on an individual Message, it may influence path selection.

Specifying Application Intents is not mandatory. An application can
specify any combination of Application Intents. If specified,
Application Intents are defined as parameters passed to the
Preconnection Object, and may influence the Connection established
from that Preconnection. If a Connection is cloned to form a
Connection Group, and associated Application Intents are cloned along
with the other transport parameters. Some Intents have also
corresponding Message Properties, similar to the properties in
Section 7.1.

Application Intents can be added to this interface as Transport
Preferences with the "Prefer" preference level.

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A.1.1.1. Traffic Category

This Intent specifies what the application expect the dominating
traffic pattern to be.

Possible Category values are:

Query: Single request / response style workload, latency bound

Control: Long lasting low bandwidth control channel, not bandwidth
bound

Stream: Stream of data with steady data rate

Bulk: Bulk transfer of large Messages, presumably bandwidth bound

The default is to not assume any particular traffic pattern. Most
categories suggest the use of other intents to further describe the
traffic pattern anticipated, e.g., the bulk category suggesting the
use of the Message Size intents or the stream category suggesting the
Stream Bitrate and Duration intents.

A.1.1.2. Size to be Sent / Received

This Intent specifies what the application expects the size of a
transfer to be. It is a numeric property and given in Bytes.

A.1.1.3. Duration

This Intent specifies what the application expects the lifetime of a
transfer to be. It is a numeric property and given in milliseconds.

A.1.1.4. Send / Receive Bit-rate

This Intent specifies what the application expects the bit-rate of a
transfer to be. It is a numeric property and given in Bytes per
second.

A.1.1.5. Cost Preferences

This Intent describes what an application prefers regarding monetary
costs, e.g., whether it considers it acceptable to utilize limited
data volume. It provides hints to the transport system on how to
handle trade-offs between cost and performance or reliability. This
Intent can also apply to an individual Messages.

No Expense: Avoid transports associated with monetary cost

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Optimize Cost: Prefer inexpensive transports and accept service
degradation

Balance Cost: Use system policy to balance cost and other criteria

Ignore Cost: Ignore cost, choose transport solely based on other
criteria

The default is "Balance Cost".

A.2. Protocol Properties

The following protocol properties might be made available in addition
to those in Section 9.1:

o Abort timeout to suggest to the Remote Endpoint: This numeric
property specifies the timeout to propose to the Remote Endpoint.
It is given in seconds. [EDITOR'S NOTE: For discussion of this
property, see https://github.com/taps-api/drafts/issues/109]

A.3. Send Parameters

The following send parameters might be made available in addition to
those specified in Section 7.1:

o Immediate: Immediate is a boolean property. If true, the caller
prefers immediacy to efficient capacity usage for this Message.
For example, this means that the Message should not be bundled
with other Message into the same transmission by the underlying
Protocol Stack.

o Send Bitrate: This numeric property in Bytes per second specifies
at what bitrate the application wishes the Message to be sent. A
transport supporting this feature will not exceed the requested
Send Bitrate even if flow-control and congestion control allow
higher bitrates. This helps to avid bursty traffic pattern on
busy video streaming servers.

Appendix B. Sample API definition in Go

This document defines an abstract interface. To illustrate how this
would map concretely into a programming language, an API interface
definition in Go is available online at https://github.com/mami-
project/postsocket. Documentation for this API - an illustration of
the documentation an application developer would see for an instance
of this interface - is available online at
https://godoc.org/github.com/mami-project/postsocket. This API

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definition will be kept largely in sync with the development of this
abstract interface definition.

Authors' Addresses

Brian Trammell (editor)
ETH Zurich
Gloriastrasse 35
8092 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
TU Berlin
Marchstrasse 23
10587 Berlin
Germany

Email: theresa@inet.tu-berlin.de

Godred Fairhurst
University of Aberdeen
Fraser Noble Building
Aberdeen, AB24 3UE
Scotland

Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/

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Mirja Kuehlewind
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland

Email: mirja.kuehlewind@tik.ee.ethz.ch

Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom

Email: csp@csperkins.org

Philipp S. Tiesel
TU Berlin
Marchstrasse 23
10587 Berlin
Germany

Email: philipp@inet.tu-berlin.de

Chris Wood
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
United States of America

Email: cawood@apple.com

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