TAPS Working Group B. Trammell, Ed. Internet-Draft Google Switzerland GmbH Intended status: Standards Track M. Welzl, Ed. Expires: 14 January 2021 University of Oslo T. Enghardt Netflix G. Fairhurst University of Aberdeen M. Kuehlewind Ericsson C. Perkins University of Glasgow P. Tiesel TU Berlin C.A. Wood Cloudflare T. Pauly Apple Inc. 13 July 2020 An Abstract Application Layer Interface to Transport Services draft-ietf-taps-interface-07 Abstract This document describes an abstract application programming interface, API, 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 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/. Trammell, et al. Expires 14 January 2021 [Page 1] Internet-Draft TAPS Interface July 2020 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 14 January 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology and Notation . . . . . . . . . . . . . . . . . . 5 3. Overview of Interface Design . . . . . . . . . . . . . . . . 7 4. API Summary . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Usage Examples . . . . . . . . . . . . . . . . . . . . . 8 4.1.1. Server Example . . . . . . . . . . . . . . . . . . . 9 4.1.2. Client Example . . . . . . . . . . . . . . . . . . . 10 4.1.3. Peer Example . . . . . . . . . . . . . . . . . . . . 10 4.2. Transport Properties . . . . . . . . . . . . . . . . . . 11 4.2.1. Transport Property Names . . . . . . . . . . . . . . 12 4.2.2. Transport Property Types . . . . . . . . . . . . . . 13 4.3. Scope of the Interface Definition . . . . . . . . . . . . 14 5. Pre-Establishment Phase . . . . . . . . . . . . . . . . . . . 15 5.1. Specifying Endpoints . . . . . . . . . . . . . . . . . . 15 5.2. Specifying Transport Properties . . . . . . . . . . . . . 18 5.2.1. Reliable Data Transfer (Connection) . . . . . . . . . 20 5.2.2. Preservation of Message Boundaries . . . . . . . . . 21 5.2.3. Configure Per-Message Reliability . . . . . . . . . . 21 5.2.4. Preservation of Data Ordering . . . . . . . . . . . . 21 5.2.5. Use 0-RTT Session Establishment with a Safely Replayable Message . . . . . . . . . . . . . . . . . 21 5.2.6. Multistream Connections in Group . . . . . . . . . . 22 5.2.7. Full Checksum Coverage on Sending . . . . . . . . . . 22 5.2.8. Full Checksum Coverage on Receiving . . . . . . . . . 22 5.2.9. Congestion control . . . . . . . . . . . . . . . . . 22 Trammell, et al. Expires 14 January 2021 [Page 2] Internet-Draft TAPS Interface July 2020 5.2.10. Interface Instance or Type . . . . . . . . . . . . . 23 5.2.11. Provisioning Domain Instance or Type . . . . . . . . 24 5.2.12. Use Temporary Local Address . . . . . . . . . . . . . 24 5.2.13. Multi-Paths Transport . . . . . . . . . . . . . . . . 25 5.2.14. Advertisement of Alternative Addresses . . . . . . . 26 5.2.15. Direction of communication . . . . . . . . . . . . . 26 5.2.16. Notification of excessive retransmissions . . . . . . 27 5.2.17. Notification of ICMP soft error message arrival . . . 27 5.2.18. Initiating side is not the first to write . . . . . . 27 5.3. Specifying Security Parameters and Callbacks . . . . . . 28 5.3.1. Pre-Connection Parameters . . . . . . . . . . . . . . 28 5.3.2. Connection Establishment Callbacks . . . . . . . . . 29 6. Establishing Connections . . . . . . . . . . . . . . . . . . 30 6.1. Active Open: Initiate . . . . . . . . . . . . . . . . . . 30 6.2. Passive Open: Listen . . . . . . . . . . . . . . . . . . 31 6.3. Peer-to-Peer Establishment: Rendezvous . . . . . . . . . 32 6.4. Connection Groups . . . . . . . . . . . . . . . . . . . . 34 7. Sending Data . . . . . . . . . . . . . . . . . . . . . . . . 35 7.1. Basic Sending . . . . . . . . . . . . . . . . . . . . . . 36 7.2. Sending Replies . . . . . . . . . . . . . . . . . . . . . 36 7.3. Send Events . . . . . . . . . . . . . . . . . . . . . . . 37 7.3.1. Sent . . . . . . . . . . . . . . . . . . . . . . . . 37 7.3.2. Expired . . . . . . . . . . . . . . . . . . . . . . . 37 7.3.3. SendError . . . . . . . . . . . . . . . . . . . . . . 38 7.4. Message Contexts . . . . . . . . . . . . . . . . . . . . 38 7.5. Message Properties . . . . . . . . . . . . . . . . . . . 38 7.5.1. Lifetime . . . . . . . . . . . . . . . . . . . . . . 40 7.5.2. Priority . . . . . . . . . . . . . . . . . . . . . . 40 7.5.3. Ordered . . . . . . . . . . . . . . . . . . . . . . . 40 7.5.4. Safely Replayable . . . . . . . . . . . . . . . . . . 41 7.5.5. Final . . . . . . . . . . . . . . . . . . . . . . . . 41 7.5.6. Corruption Protection Length . . . . . . . . . . . . 42 7.5.7. Reliable Data Transfer (Message) . . . . . . . . . . 42 7.5.8. Message Capacity Profile Override . . . . . . . . . . 43 7.5.9. No Fragmentation . . . . . . . . . . . . . . . . . . 43 7.6. Partial Sends . . . . . . . . . . . . . . . . . . . . . . 43 7.7. Batching Sends . . . . . . . . . . . . . . . . . . . . . 44 7.8. Send on Active Open: InitiateWithSend . . . . . . . . . . 44 8. Receiving Data . . . . . . . . . . . . . . . . . . . . . . . 45 8.1. Enqueuing Receives . . . . . . . . . . . . . . . . . . . 45 8.2. Receive Events . . . . . . . . . . . . . . . . . . . . . 46 8.2.1. Received . . . . . . . . . . . . . . . . . . . . . . 46 8.2.2. ReceivedPartial . . . . . . . . . . . . . . . . . . . 46 8.2.3. ReceiveError . . . . . . . . . . . . . . . . . . . . 47 8.3. Receive Message Properties . . . . . . . . . . . . . . . 48 8.3.1. UDP(-Lite)-specific Property: ECN . . . . . . . . . . 48 8.3.2. Early Data . . . . . . . . . . . . . . . . . . . . . 48 8.3.3. Receiving Final Messages . . . . . . . . . . . . . . 48 Trammell, et al. Expires 14 January 2021 [Page 3] Internet-Draft TAPS Interface July 2020 9. Message Framers . . . . . . . . . . . . . . . . . . . . . . . 49 9.1. Adding Message Framers to Connections . . . . . . . . . . 49 9.2. Framing Meta-Data . . . . . . . . . . . . . . . . . . . . 50 10. Managing Connections . . . . . . . . . . . . . . . . . . . . 50 10.1. Generic Connection Properties . . . . . . . . . . . . . 52 10.1.1. Retransmission Threshold Before Excessive Retransmission Notification . . . . . . . . . . . . . 52 10.1.2. Required Minimum Corruption Protection Coverage for Receiving . . . . . . . . . . . . . . . . . . . . . . 52 10.1.3. Priority (Connection) . . . . . . . . . . . . . . . 53 10.1.4. Timeout for Aborting Connection . . . . . . . . . . 53 10.1.5. Connection Group Transmission Scheduler . . . . . . 53 10.1.6. Capacity Profile . . . . . . . . . . . . . . . . . . 54 10.1.7. Policy for using Multi-Path Transports . . . . . . . 55 10.1.8. Bounds on Send or Receive Rate . . . . . . . . . . . 56 10.1.9. Read-only Connection Properties . . . . . . . . . . 56 10.2. TCP-specific Properties: User Timeout Option (UTO) . . . 57 10.2.1. Advertised User Timeout . . . . . . . . . . . . . . 57 10.2.2. User Timeout Enabled . . . . . . . . . . . . . . . . 58 10.2.3. Timeout Changeable . . . . . . . . . . . . . . . . . 58 10.3. Connection Lifecycle Events . . . . . . . . . . . . . . 58 10.3.1. Soft Errors . . . . . . . . . . . . . . . . . . . . 58 10.3.2. Excessive retransmissions . . . . . . . . . . . . . 59 11. Connection Termination . . . . . . . . . . . . . . . . . . . 59 12. Connection State and Ordering of Operations and Events . . . 59 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 61 14. Security Considerations . . . . . . . . . . . . . . . . . . . 61 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 62 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 63 16.1. Normative References . . . . . . . . . . . . . . . . . . 63 16.2. Informative References . . . . . . . . . . . . . . . . . 63 Appendix A. Convenience Functions . . . . . . . . . . . . . . . 66 A.1. Adding Preference Properties . . . . . . . . . . . . . . 66 A.2. Transport Property Profiles . . . . . . . . . . . . . . . 67 A.2.1. reliable-inorder-stream . . . . . . . . . . . . . . . 67 A.2.2. reliable-message . . . . . . . . . . . . . . . . . . 67 A.2.3. unreliable-datagram . . . . . . . . . . . . . . . . . 68 Appendix B. Relationship to the Minimal Set of Transport Services for End Systems . . . . . . . . . . . . . . . . . . . . . 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 72 Trammell, et al. Expires 14 January 2021 [Page 4] Internet-Draft TAPS Interface July 2020 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]. 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 common interface to the transport layer, in environments where endpoints select from multiple interfaces and potential transport protocols. As applications adopt this interface, they will benefit from a wide set of transport features that can evolve over time, and ensure that the system providing the interface can optimize its behavior based on the application requirements and network conditions, without requiring changes to the applications. This flexibility enables faster deployment of new features and protocols. It can also support applications by offering racing and fallback mechanisms, which otherwise need to be separately implemented in each application. It derives specific path and protocol selection properties and supported transport features from the analysis provided in [RFC8095], [I-D.ietf-taps-minset], and [I-D.ietf-taps-transport-security]. 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. The transport system implementations should provide 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. This specific transport stack choice is discouraged for general use, because it can reduce the portability. 2. 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 asynchronously; and Parameters associated with these Actions and Events. The following notations, which can be combined, are used in this document: * An Action creates an Object: Object := Action() * An Action creates an array of Objects: Trammell, et al. Expires 14 January 2021 [Page 5] Internet-Draft TAPS Interface July 2020 []Object := Action() * An Action is performed on an Object: Object.Action() * An Object sends an Event: Object -> Event<> * 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 Actions associated with no Object are Actions on the abstract interface itself; they are equivalent to Actions on a per-application global context. The way these abstract concepts map into concrete implementations of this API in a given language on a given platform largely depends 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 event queues, handler functions or classes, communicating sequential processes, or other asynchronous calling conventions. This specification treats Events and errors similarly. 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 that can be immediately detected, such as inconsistency in Transport Properties. Errors can provide an optional reason to give the application further details as to why the error occurred. 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. Trammell, et al. Expires 14 January 2021 [Page 6] Internet-Draft TAPS Interface July 2020 3. 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: * 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 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; * 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, allowing concurrent operations during establishment and supporting event-driven application interactions with the transport layer, in line with developments in modern platforms and programming languages; * Explicit support for security properties as first-order transport features, and for configuration of cryptographic identities and transport security parameters persistent across multiple Connections; and * 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. 4. 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 represents a set of properties 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 Trammell, et al. Expires 14 January 2021 [Page 7] Internet-Draft TAPS Interface July 2020 be sent to and/or received from a remote Endpoint (i.e., depending on the kind of transport, connections can be bi-directional or unidirectional). 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 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 which 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 5, Section 6, Section 7, Section 8, and Section 11 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]. 4.1. Usage Examples The following usage examples illustrate how an application might use a Transport Services Interface to: * Act as a server, by listening for incoming connections, receiving requests, and sending responses, see Section 4.1.1. * Act as a client, by connecting to a remote endpoint using Initiate, sending requests, and receiving responses, see Section 4.1.2. * 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 4.1.3. The examples in this section presume that a transport protocol is available between the 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. Trammell, et al. Expires 14 January 2021 [Page 8] Internet-Draft TAPS Interface July 2020 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 may receive this byte stream as partial Messages and transform it into application-layer Messages. Alternatively, an application may provide a Message Framer, which can transform a byte stream into a sequence of Messages (Section 9). 4.1.1. Server Example This is an example of how an application might listen for incoming Connections using the Transport Services Interface, 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.AddIdentity(identity) SecurityParameters.AddPrivateKey(privateKey, publicKey) // Specifying a remote endpoint is optional when using Listen() Preconnection := NewPreconnection(LocalSpecifier, TransportProperties, SecurityParameters) Listener := Preconnection.Listen() Listener -> ConnectionReceived // Only receive complete messages in a Conn.Received handler Connection.Receive() Connection -> Received //---- 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 ---- Trammell, et al. Expires 14 January 2021 [Page 9] Internet-Draft TAPS Interface July 2020 4.1.2. Client Example This is an example of how an application might connect to a remote application using the Transport Services Interface, send a request, and receive a response. 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() Connection -> Ready<> //---- Ready event handler begin ---- Connection.Send(messageDataRequest) // Only receive complete messages Connection.Receive() //---- Ready event handler end ---- Connection -> Received // Close the Connection in a Receive event handler Connection.Close() 4.1.3. 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. Trammell, et al. Expires 14 January 2021 [Page 10] Internet-Draft TAPS Interface July 2020 LocalSpecifier := NewLocalEndpoint() LocalSpecifier.WithPort(9876) RemoteSpecifier := NewRemoteEndpoint() RemoteSpecifier.WithHostname("example.com") RemoteSpecifier.WithPort(9877) TransportProperties := NewTransportProperties() TransportProperties.Require(preserve-msg-boundaries) // Reliable Data Transfer and Preserve Order are Required by default SecurityParameters := NewSecurityParameters() SecurityParameters.AddIdentity(identity) SecurityParameters.AddPrivateKey(privateKey, publicKey) TrustCallback := New Callback({ // Verify identity of the remote endpoint, return the result }) SecurityParameters.SetTrustVerificationCallback(trustCallback) // Both local and remote endpoint must be specified Preconnection := NewPreconnection(LocalSpecifier, RemoteSpecifier, TransportProperties, SecurityParameters) Preconnection.Rendezvous() Preconnection -> RendezvousDone //---- Ready event handler begin ---- Connection.Send(messageDataRequest) // Only receive complete messages Connection.Receive() //---- Ready event handler end ---- Connection -> Received // Close the Connection in a Receive event handler Connection.Close() 4.2. 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]. Trammell, et al. Expires 14 January 2021 [Page 11] Internet-Draft TAPS Interface July 2020 Transport Properties are divided into Selection, Connection, and Message Properties. Selection Properties (see The behavior of the selected protocol stack(s) when sending Messages is controlled by Message Properties (see Section 5.2) 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. Connection Properties (see Section 10.1) can also be set during pre- establishment but may be changed later. They are used to inform decisions made during establishment and to fine-tune the established connection.Section 7.5). All Transport Properties, regardless of the phase in which they are used, are organized within a single namespace. This enables setting them as defaults in earlier stages and querying them in later stages: * Connection Properties can be set on Preconnections * Message Properties can be set on Preconnections, Connections and Messages * The effect of Selection Properties can be queried on Connections and Messages 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. 4.2.1. 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: * 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. Trammell, et al. Expires 14 January 2021 [Page 12] Internet-Draft TAPS Interface July 2020 * Making 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 [.]. * 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 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 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. 4.2.2. Transport Property Types Transport Properties can have one of a set of data types: * Boolean: can take the values "true" and "false"; representation is implementation-dependent. * Integer: can take positive or negative numeric integer values; range and representation is implementation-dependent. * Numeric: can take positive or negative numeric values; range and representation is implementation-dependent. * Enumeration: can take one value of a finite set of values, dependent on the property itself. The representation is implementation dependent; however, implementations MUST provide a method for the application to determine the entire set of possible values for each property. Trammell, et al. Expires 14 January 2021 [Page 13] Internet-Draft TAPS Interface July 2020 * Preference: can take one of five values (Prohibit, Avoid, Ignore, Prefer, Require) for the level of preference of a given property during protocol selection; see Section 5.2. When querying, a Preference is of type Boolean, with "true" indicating that the Selection Property has been applied. For types Integer and Numeric, special values can be defined per property; it is up to implementations how these special values are represented (e.g., by using -1 for an otherwise non-negative value). 4.3. 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 these platforms reduces the effort for programmers who learn the transport services interface to then apply their knowledge across multiple platforms. 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 this 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 even if disabling ordering will not have any effect TCP because the API does not guarantee out-of-order delivery. 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". Trammell, et al. Expires 14 January 2021 [Page 14] Internet-Draft TAPS Interface July 2020 * 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. 5. Pre-Establishment Phase The Pre-Establishment phase allows applications to specify properties for the Connections they are 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 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 5.1), the Selection Properties (see Section 5.2), any preconfigured Connection Properties (Section 10.1), and the security parameters (see Section 5.3): Preconnection := NewPreconnection(LocalEndpoint?, RemoteEndpoint?, TransportProperties, 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. If no Local Endpoint is specified, the Transport System will assign an ephemeral local port to the Connection. The Remote Endpoint MUST be specified if 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. Transport Properties MUST always be specified while security parameters are OPTIONAL. If Message Framers are used (see Section 9), they MUST be added to the Preconnection during pre-establishment. 5.1. Specifying Endpoints The transport services API uses the Local Endpoint and Remote Endpoint Objects to refer to the endpoints of a transport connection. Actions on these Objects can be used to represent various different types of endpoint identifiers, such as IP addresses, DNS names, and interface names, as well as port numbers and service names. Trammell, et al. Expires 14 January 2021 [Page 15] Internet-Draft TAPS Interface July 2020 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 5.2.10. However, if the application specifies Selection Properties which 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 a 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: Trammell, et al. Expires 14 January 2021 [Page 16] Internet-Draft TAPS Interface July 2020 LocalSpecifier := NewLocalEndpoint() LocalSpecifier.WithIPv4Address(232.1.1.1) LocalSpecifier.WithInterface("en0") RemoteSpecifier := NewRemoteEndpoint() RemoteSpecifier.WithIPv4Address(192.0.2.22) 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 Remote Endpoint. 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. These multiple identifiers refer to the same transport endpoint. The transport services API resolves names internally, when the Initiate(), Listen(), or Rendezvous() method is called to establish a Connection. The API 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. The Resolve() action on 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 6.3). Specifying a multicast group address on the 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 by specific senders. This will restrict the application to establishing the Preconnection by calling Listen(). 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. Trammell, et al. Expires 14 January 2021 [Page 17] Internet-Draft TAPS Interface July 2020 5.2. 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. Most Selection Properties are represented as preferences, which can have one of five preference levels: +============+========================================+ | 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 In addition, the pseudo-level "Default" can be used to reset the property to the default level used by the implementation. This level will never show up when queuing the value of a preference - the effective preference must be returned instead. The implementation MUST ensure an outcome that is consistent with application requirements as expressed using Require and Prohibit. While preferences expressed using Prefer and Avoid influence protocol Trammell, et al. Expires 14 January 2021 [Page 18] Internet-Draft TAPS Interface July 2020 and path selection as well, outcomes may vary given the same Selection Properties, as the available protocols and paths may vary across systems and contexts. However, implementations are RECOMMENDED to aim to provide a consistent outcome to an application, given the same Selection Properties. Note that application preferences may 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, implementations SHOULD prioritize Selection Properties that select paths over those that select protocols. Therefore, the transport system SHOULD race the path first, ignoring the protocol preference if the protocol does not work on the path. 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 added to the TransportProperties Object: TransportProperties.Add(property, value) Selection Properties of type "Preference" can be frequently used. Implementations MAY therefore provide additional convenience functions, see Appendix A.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 A.2. For an existing Connection, the Transport Properties can be queried any time by using the following call on the Connection Object: TransportProperties := Connection.GetTransportProperties() 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 6.4 for more. Trammell, et al. Expires 14 January 2021 [Page 19] Internet-Draft TAPS Interface July 2020 Section 10.1 provides a list of Connection Properties, while Selection Properties are listed in the subsections below. Note that many properties are only considered during establishment, and can not be changed after a Connection is established; however, they can 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 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 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 recommended 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 beneficical when application are ported between different implementations, even if this default could lead to a connection failure, as, for example, an application needs to be explicitly designed to support a connectionless mode. In this case the application can regonize the failure and explicitly specify a different set of Protocol Selection Properties that result in a usable protocol. If default values other than those recommended below are used, it is recommended to clearly document the differences. 5.2.1. 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 on the other side without corruption. This also entails being notified when a Connection is closed or aborted when reliable data transfer is enabled. Trammell, et al. Expires 14 January 2021 [Page 20] Internet-Draft TAPS Interface July 2020 5.2.2. Preservation of Message Boundaries Name: preserveMsgBoundaries Type: Preference Default: Prefer This property specifies whether the application needs or prefers to use a transport protocol that preserves message boundaries. 5.2.3. Configure Per-Message Reliability Name: perMsgReliability Type: Preference Default: Ignore This 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. 5.2.4. 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. 5.2.5. 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, which will then be reliably transferred to the other side before or during Connection establishment, potentially multiple times (i.e., multiple copies of the message data may be passed to the Remote Trammell, et al. Expires 14 January 2021 [Page 21] Internet-Draft TAPS Interface July 2020 Endpoint). See also Section 7.5.4. Note that disabling this property has no effect for protocols that are not connection-oriented and do not protect against duplicated messages, e.g., UDP. 5.2.6. Multistream Connections in Group Name: multistreaming Type: Preference Default: Prefer 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. 5.2.7. Full Checksum Coverage on Sending Name: perMsgChecksumLenSend Type: Preference Default: Require This property specifies whether the application desires protection against corruption for all data transmitted on this Connection. Disabling this property may enable to control checksum coverage later (see Section 7.5.6). 5.2.8. Full Checksum Coverage on Receiving Name: perMsgChecksumLenRecv Type: Preference Default: Require This property specifies whether the application desires protection against corruption for all data received on this Connection. 5.2.9. Congestion control Name: congestionControl Type: Preference Default: Require Trammell, et al. Expires 14 January 2021 [Page 22] Internet-Draft TAPS Interface July 2020 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]. 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. 5.2.10. Interface Instance or Type Name: interface Type: Set (Preference, Enumeration) Default: Empty set (not setting a preference for any interface) This property allows the application to select which 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 a single interface, and may often lead 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. 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, there may be "Wi-Fi" and "Wired Ethernet" interface types available. An implementation should provide all types that are supported on the local system to all remote systems, 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 may want 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. Trammell, et al. Expires 14 January 2021 [Page 23] Internet-Draft TAPS Interface July 2020 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 5.2.11) or another specific property. 5.2.11. Provisioning Domain Instance or Type Name: pvd Type: Set (Preference, Enumeration) Default: Empty set (not setting a preference for any PvD) Similar to interface instances and types (see Section 5.2.10), this property allows the application to control path selection by selecting which specific Provisioning Domains or categories of Provisioning Domains 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 Provisioning Domain (PvD) is defined to be implementation- and system-specific, since there is not a portable standard format for a PvD identifier. For example, this identifier may be a string name or an integer. As with requiring specific interfaces, requiring a specific PvD strictly limits path selection. Categories or types of PvDs are also defined to be implementation- and system-specific. These may 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 some 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. 5.2.12. Use Temporary Local Address Name: useTemporaryLocalAddress Trammell, et al. Expires 14 January 2021 [Page 24] Internet-Draft TAPS Interface July 2020 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. Note that if an application Requires the use of temporary addresses, the resulting Connection cannot use IPv4, as temporary addresses do not exist in IPv4. 5.2.13. Multi-Paths 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 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 10.1.7 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 5.2.14 below). Trammell, et al. Expires 14 January 2021 [Page 25] Internet-Draft TAPS Interface July 2020 Setting this property to "Active", may have privacy implications: It enables the transport to establish connectivity using alternate paths that may make users linkable across multiple paths, even if the Advertisement of Alternative Addresses property (see Section 5.2.14 below) is set to false. 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. 5.2.14. 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. Note that this may have privacy implications because it may make users linkable across multiple paths. Also, note that setting this to false does not prevent the local transport system from _establishing_ connectivity using alternate paths (see Section 5.2.13 above); it only prevents _procative advertisement_ of addresses. 5.2.15. 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 Trammell, et al. Expires 14 January 2021 [Page 26] Internet-Draft TAPS Interface July 2020 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. 5.2.16. Notification of excessive retransmissions Name: retransmitNotify Type: Preference Default: Ignore This property specifies whether an application considers it useful to be informed in case sent data was retransmitted more often than a certain threshold (see Section 10.1.1 for configuration of this threshold). 5.2.17. 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 will be available as SoftErrors, see Section 10.3.1. Note that even if a protocol supporting this property is selected, not all ICMP errors will necessarily be delivered, so applications cannot rely on receiving them. 5.2.18. Initiating side is not the first to write Name: activeReadBeforeSend Type: Preference Default: Ignore Trammell, et al. Expires 14 January 2021 [Page 27] Internet-Draft TAPS Interface July 2020 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 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]. 5.3. 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. Thus, we partition security parameters and callbacks based on their place in the lifetime of connection establishment. Similar to Transport Properties, both parameters and callbacks are inherited during cloning (see Section 6.4). 5.3.1. Pre-Connection Parameters 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.ietf-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 need to be specified in the pre-connection phase and are created as follows: SecurityParameters := NewSecurityParameters() Security configuration parameters and sample usage follow: * 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.Add('identity', identity) SecurityParameters.Add('keypair', privateKey, publicKey) Trammell, et al. Expires 14 January 2021 [Page 28] Internet-Draft TAPS Interface July 2020 * 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. SecurityParameters.Add('supported-group', 'secp256k1') SecurityParameters.Add('ciphersuite, 'TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256') SecurityParameters.Add('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.Add('pre-shared-key', key, identity) * Session cache management: Used to tune cache capacity, lifetime, re-use, and eviction policies, e.g., LRU or FIFO.may also me changed, but are implementation-specific. 5.3.2. 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. 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. ChallengeCallback := NewCallback({ // Handle challenge }) SecurityParameters.SetIdentityChallengeCallback(challengeCallback) Trammell, et al. Expires 14 January 2021 [Page 29] Internet-Draft TAPS Interface July 2020 6. Establishing Connections Before a Connection can be used for data transfer, it must 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. 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(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 7) 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<> Trammell, et al. Expires 14 January 2021 [Page 30] Internet-Draft TAPS Interface July 2020 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 -> EstablishmentError 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 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). See also Section 7.8 to combine Connection establishment and transmission of the first message in a single action. 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 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. Listening continues until the global context shuts down, or until the Stop action is performed on the Listener object: Listener.Stop() After Stop() is called, the Listener can be disposed of. Trammell, et al. Expires 14 January 2021 [Page 31] Internet-Draft TAPS Interface July 2020 Listener -> ConnectionReceived 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) 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 A 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. 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 properties and security parameters needed for Protocol Stack selection. Trammell, et al. Expires 14 January 2021 [Page 32] Internet-Draft TAPS Interface July 2020 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 returns a Connection object. Once Rendezvous() has been called, any changes to the Preconnection MUST NOT have any effect on the Connection. However, the Preconnection can be reused, e.g., for Rendezvous of another Connection. Preconnection -> RendezvousDone 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 -> EstablishmentError 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. When using some NAT traversal protocols, e.g., Interactive Connectivity Establishment (ICE) [RFC5245], it is expected that the Local Endpoint will be configured with some method of discovering NAT bindings, e.g., a Session Traversal Utilities for NAT (STUN) server. In this case, the Local Endpoint may resolve to a mixture of local and server reflexive addresses. The Resolve() action on the Preconnection can be used to discover these bindings: []Preconnection := 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. These resolved Preconnections will share all other Properties with the Preconnection from which they are derived, though some Properties may be made more-specific by the resolution process. This list can be passed to a peer via a signalling protocol, such as SIP [RFC3261] or WebRTC [RFC7478], to configure the remote. Trammell, et al. Expires 14 January 2021 [Page 33] Internet-Draft TAPS Interface July 2020 6.4. Connection Groups Entangled Connections can be created using the Clone Action: 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 cloned 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 Connection Group, and so on. Connections in a Connection Group generally share Connection Properties. However, there may be exceptions, such as "Priority (Connection)", see Section 10.1.3. Like all other Properties, Priority is copied to the new Connection when calling Clone(), but it is not entangled: Changing Priority on one Connection does not change it on the other Connections in the same Connection Group. In addition, incoming entangled Connections can be received by creating a Listener on an existing connection: Listener := Connection.ListenClone() ListenClone() creates a Listener that listens on the same LocalEndpoint as the one the cloned Connection is using. Any new Connection received by this Listener will be entangled with the cloned Connection. Changing one of the Connection Properties on one Connection in the group changes it for all others. Message Properties, however, are not entangled. For example, changing "Timeout for aborting Connection" (see Section 10.1.4) on one Connection in a group will automatically change this Connection Property for all Connections in the group in the same way. However, changing "Lifetime" (see Section 7.5.1) of a Message will only affect a single Message on a single Connection, entangled or not. If the underlying protocol supports multi-streaming, it is natural to use this functionality to implement Clone. In that case, entangled Connections are multiplexed together, giving them similar treatment not only inside endpoints but also across the end-to-end Internet path. Trammell, et al. Expires 14 January 2021 [Page 34] Internet-Draft TAPS Interface July 2020 Note that calling Clone() may result in on-the-wire signaling, e.g., to open a new connection, depending on the underlying Protocol Stack. When Clone() leads to multiple connections being opened instead of multi-streaming, the transport 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 system to implement prioritization within a Connection Group [TCP-COUPLING] [RFC8699]. Attempts to clone a Connection can result in a CloneError: Connection -> CloneError The Connection Property "Priority" operates on entangled Connections as in Section 7.5.2: when allocating available network capacity among Connections in a Connection Group, sends on Connections with higher Priority values will be prioritized over sends on Connections with lower Priority values. A transport system implementation should, if possible, assign each Connection the capacity share (M-N) x C / M, where N is the Connection's Priority value, M is the maximum Priority value used by all Connections in the group and C is the total available capacity. However, the Priority setting is purely advisory, and no guarantees are given about the way capacity is shared. Each implementation is free to implement a way to share capacity that it sees fit. 7. Sending Data Once a Connection has been established, it can be used for sending data. Data is sent as Messages, which allow the application to communicate the boundaries of the data being transferred. By default, Send enqueues a complete Message, and takes optional per- Message properties (see Section 7.1). All Send actions are asynchronous, and deliver events (see Section 7.3). Sending partial Messages for streaming large data is also supported (see Section 7.6). Messages are sent on a Connection using the Send action: Connection.Send(messageData, messageContext?, endOfMessage?) where messageData is the data object to send. The optional messageContext parameter allows adding Message Properties as described in Section 7.5. Moreover, the messageContext can be used to identify Send Events related to a specific Message (see Section 7.3) or to inspect meta-data related to the Message sent (see Section 7.4). Trammell, et al. Expires 14 January 2021 [Page 35] Internet-Draft TAPS Interface July 2020 The optional endOfMessage parameter supports partial sending and is described in Section 7.6. 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. This is further decribed in Section 9. 7.1. 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 10.1.9.3) 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 7.3.3). For example, it is invalid to send a Message over a UDP connection that is larger than the available datagram sending size. 7.2. Sending Replies When a message is sent in response to a message received, the application may use the Message Context of the received Message to construct a Message Context for the reply. replyMessageContext := requestMessageContext.reply() By using the "replyMessageContext", the transport system is informed that the message to be sent is a response and can map the response to the same underlying transport connection or stream the request was received from. The concept of Message Contexts is described in Section 7.4. Trammell, et al. Expires 14 January 2021 [Page 36] Internet-Draft TAPS Interface July 2020 7.3. 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 7.6), 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. 7.3.1. Sent Connection -> Sent 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 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 always waits for the Sent Event before calling the next Send Action. 7.3.2. Expired Connection -> Expired 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 7.5.1) 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 to which it applies. Trammell, et al. Expires 14 January 2021 [Page 37] Internet-Draft TAPS Interface July 2020 7.3.3. SendError Connection -> SendError 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 Message Properties not consistent with the Connection's transport properties. The SendError contains a reference to the Message to which it applies. 7.4. Message Contexts Using the MessageContext object, the application can set and retrieve meta-data of the message, including Message Properties (see Section 7.5) and framing meta-data (see Section 9.2). 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 7.3) and receive events (see Section 8.2), the application can query information about the local and remote endpoint: RemoteEndpoint := MessageContext.GetRemoteEndpoint() LocalEndpoint := MessageContext.GetLocalEndpoint() Message Contexts can also be used to send messages in reply to other messages, see Section 7.2 for details. 7.5. Message Properties Applications may need to annotate the Messages they send with extra information to control how data is scheduled and processed by the transport protocols in the Connection. Therefore a message context containing these properties can be passed to the Send Action. For other uses of the message context, see Section 7.4. Trammell, et al. Expires 14 January 2021 [Page 38] Internet-Draft TAPS Interface July 2020 Note that Message Properties are per-Message, not per-Send if partial Messages are sent (Section 7.6). 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 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 to it. 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 may be added to a MessageContext object only before the context is used for sending. Once a messageContext has been used with a Send call, modifying any of its properties is invalid. Message Properties may be inconsistent with the properties of the Protocol Stacks underlying the Connection on which a given Message is sent. For example, a Connection must provide reliability to allow setting an infinite value for the lifetime property of a Message. 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" 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 unreliably delivered. Note that changing the Reliable Data Transfer property on Messages is only possible for Connections that were established with the Selection Property "Configure Per-Message Reliability" enabled. The following Message Properties are supported: Trammell, et al. Expires 14 January 2021 [Page 39] Internet-Draft TAPS Interface July 2020 7.5.1. Lifetime Name: msgLifetime Type: Numeric Default: infinite 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 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 7.5.7). The type and units of Lifetime are implementation-specific. 7.5.2. Priority Name: msgPrio Type: Integer (non-negative) Default: 100 This property represents a hierarchy of priorities. It can specify 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. Note that this property is not a per-message override of the connection Priority - see Section 10.1.3. Both Priority properties may interact, but can be used independently and be realized by different mechanisms. 7.5.3. Ordered Name: msgOrdered Trammell, et al. Expires 14 January 2021 [Page 40] Internet-Draft TAPS Interface July 2020 Type: Boolean Default: true If true, it specifies that the receiver-side transport protocol stack may only deliver the Message to the receiving application after the previous ordered Message which was passed to the same Connection via the Send Action, when such a Message exists. If false, the Message may be delivered to the receiving application out of order. This property is used for protocols that support preservation of data ordering, see Section 5.2.4, but allow out-of-order delivery for certain messages, e.g., by multiplexing independent messages onto different streams. 7.5.4. Safely Replayable Name: safelyReplayable Type: Boolean Default: false If true, it specifies that a 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. Note that for protocols that do not protect against duplicated messages, e.g., UDP, all messages MUST be marked as "Safely Replayable". In order to enable protocol selection to choose such a protocol, "Safely Replayable" MUST 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. 7.5.5. Final Name: final Type: Boolean Default: false If true, this Message is the last one 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 Trammell, et al. Expires 14 January 2021 [Page 41] Internet-Draft TAPS Interface July 2020 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. Note that 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. 7.5.6. 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 the 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 is required, and "Full Coverage" means that the entire Message is protected by a checksum. Only "Full Coverage" is guaranteed, any other requests are advisory, meaning that "Full Coverage" is applied anyway. 7.5.7. Reliable Data Transfer (Message) Name: msgReliable Type: Boolean Default: true 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 with the Selection Property "Configure Per-Message Reliability" enabled. When this is not the case, changing it will generate an error. Disabling this property indicates that the transport system may disable retransmissions or other reliability mechanisms for this particular Message, but such disabling is not guaranteed. Trammell, et al. Expires 14 January 2021 [Page 42] Internet-Draft TAPS Interface July 2020 7.5.8. Message Capacity Profile Override Name: msgCapacityProfile Type: Enumeration 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 10.1.6). 7.5.9. No Fragmentation Name: noFragmentation Type: Boolean Default: false This property specifies that a message should be sent and received as a single packet without network-layer fragmentation, if possible. Attempts to send a message with this property set with a size greater to the transport's current estimate of its maximum transmission segment size will result in a "SendError". When used with transports supporting this functionality and running over IP version 4, the Don't Fragment bit will be set. 7.6. 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. 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. Trammell, et al. Expires 14 January 2021 [Page 43] Internet-Draft TAPS Interface July 2020 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. 7.7. Batching Sends To reduce the overhead of sending multiple small Messages on a Connection, the application may want to batch several Send Actions together. This provides a hint to the system that the sending of these Messages should be coalesced when possible, and that sending any of the batched Messages may 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() 7.8. 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?) Whenever possible, a messageContext should be provided to declare the Message passed to InitiateWithSend as "Safely Replayable". This allows the transport 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(). Trammell, et al. Expires 14 January 2021 [Page 44] Internet-Draft TAPS Interface July 2020 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 InitiateError signaling the failure of Connection establishment. 8. 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, data is received in terms of 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 8.2). If Messages arrive at the transport system before Receive requests are issued, ensuing Receive requests will first operate on these Messages before awaiting any further Messages. 8.1. 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 8.2.1). 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 8.2.2 and Section 9 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. Trammell, et al. Expires 14 January 2021 [Page 45] Internet-Draft TAPS Interface July 2020 The maxLength argument indicates the maximum size of a Message in bytes 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 8.2.2). Note that maxLength does not guarantee that the application will receive that many bytes if they are available; the interface may return ReceivedPartial events with less data than maxLength according 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. 8.2. 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. 8.2.1. Received Connection -> Received 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 7.4 for details. See Section 9 for handling Message framing in situations where the Protocol Stack only provides a byte-stream transport. 8.2.2. ReceivedPartial Connection -> ReceivedPartial Trammell, et al. Expires 14 January 2021 [Page 46] Internet-Draft TAPS Interface July 2020 If a complete Message cannot be delivered in one event, one part of the Message may be delivered with a ReceivedPartial event. In order 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 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) 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. 8.2.3. ReceiveError Connection -> ReceiveError A ReceiveError occurs when data is received by the underlying Protocol Stack that cannot be fully retrieved or parsed, or when some other indication is received that reception has failed. In contrast, conditions that irrevocably lead to the termination of the Connection are signaled using ConnectionError instead (see Section 11). The ReceiveError event passes an optional associated MessageContext. This may indicate that a Message that was being partially received previously, but had not completed, encountered an error and will not be completed. Trammell, et al. Expires 14 January 2021 [Page 47] Internet-Draft TAPS Interface July 2020 8.3. 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 though additional read-only Message Properties that can be queried from the MessageContext object (see Section 7.4) passed by the receive event. The following metadata values are supported: 8.3.1. 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 purposes, and for building applications which 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. 8.3.2. Early Data In some cases it may 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 may 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. 8.3.3. 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 7.5.5. Some transport protocols and peers may not support signaling of the Final property. Applications therefore should not rely on receiving a Message marked Final to know that the other endpoint is done sending on a connection. Trammell, et al. Expires 14 January 2021 [Page 48] Internet-Draft TAPS Interface July 2020 Any calls to Receive once the Final Message has been delivered will result in errors. 9. 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 out 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 facility is designed based on the fact that many of the 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. 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]. 9.1. Adding Message Framers to 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. If multiple Framers are added, 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) Trammell, et al. Expires 14 January 2021 [Page 49] Internet-Draft TAPS Interface July 2020 9.2. Framing Meta-Data When sending Messages, applications can add specific Message values to a MessageContext (Section 7.4) that is intended for a Framer. This 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") 10. 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. 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 10.1 below. Specific Protocol Properties are defined in a transport- and implementation-specific way, and 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; however, too much reliance by an application on Specific Protocol Properties may significantly reduce the flexibility of a transport services implementation. Trammell, et al. Expires 14 January 2021 [Page 50] Internet-Draft TAPS Interface July 2020 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 5.2), 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 6.4. At any point, the application can query Connection Properties. ConnectionProperties := Connection.GetProperties() 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 Section 7.5.5. * Whether the connection can be used to receive data. A connection can not 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 8.3.3. The latter is only supported by certain transport protocols, e.g., by TCP as half-closed connection. * For Connections that are Establishing: Transport Properties that the application specified on the Preconnection, see Section 5.2. Trammell, et al. Expires 14 January 2021 [Page 51] Internet-Draft TAPS Interface July 2020 * For Connections that are Established, Closing, or Closed: Selection (Section 5.2) and Connection Properties (Section 10.1) of the actual protocols that were selected and instantiated. Selection Properties indicate whether or not the Connection has or offers a certain Selection Property. Note that the actually instantiated protocol stack may not match all Protocol Selection Properties that the application specified on the Preconnection. For example, a certain Protocol Selection Property that an application specified as Preferred may not actually be present in the chosen protocol stack because none of the currently available transport protocols had this feature. * 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. 10.1. Generic Connection Properties Generic Connection Properties are defined independent of the chosen protocol stack and therefore available on all Connections. Note that many Connection Properties have a corresponding Selection Property which enables applications to express their preference for protocols providing a supporting transport feature. 10.1.1. Retransmission Threshold Before Excessive Retransmission Notification Name: retransmitNotifyThreshold Type: Integer, with special value "Disabled" Default: Disabled This property specifies after how many retransmissions to inform the application about "Excessive Retransmissions". 10.1.2. Required Minimum Corruption Protection Coverage for Receiving Name: recvChecksumLen Type: Integer, with special value "Full Coverage" Default: Full Coverage Trammell, et al. Expires 14 January 2021 [Page 52] Internet-Draft TAPS Interface July 2020 This 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. 10.1.3. Priority (Connection) 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 6.4, 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. 10.1.4. Timeout for Aborting Connection Name: connTimeout Type: Numeric, with special value "Disabled" Default: Disabled This property specifies how long to wait before deciding that a Connection has failed when trying to reliably deliver data to the destination. Adjusting this Property will only take effect when the underlying stack supports reliability. The special value "Disabled" means that this timeout is not scheduled to happen. This can be a valid choice with unreliable data transfer (e.g., when UDP is the underlying transport protocol). 10.1.5. 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 6.4. The set of schedulers can be taken from [RFC8260]. Trammell, et al. Expires 14 January 2021 [Page 53] Internet-Draft TAPS Interface July 2020 10.1.6. 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 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 priorities on the wire for Protocol Stacks supporting prioritization. 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 system implementations that map the requested capacity profile onto per-connection DSCP signaling SHOULD assign the DSCP Default Forwarding [RFC2474] PHB. Scavenger: The application is not interactive. It expects to send 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 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 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. Trammell, et al. Expires 14 January 2021 [Page 54] Internet-Draft TAPS Interface July 2020 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 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. 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 the maximum rate allowed by its congestion controller over a relatively long period of time. Transport 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 7.5.8. 10.1.7. Policy for using Multi-Path Transports Name: multipath-policy Type: Enumeration Default: Handover This property specifies the local policy of transferring data across multiple paths between the same end hosts if Parallel Use of Multiple Paths not set to Disabled (see Section 5.2.13). Possible values are: Handover: The connection should only attempt to migrate between different paths when the original path is lost or becomes unusable. The actual thresholds to declare a path unusable are implementation specific. Trammell, et al. Expires 14 January 2021 [Page 55] Internet-Draft TAPS Interface July 2020 Interactive: The connection should attempt to use multiple paths in parallel in order to minimize loss and delay. The actual strategy is implementation specific. Aggregate: The connection should attempt to use multiple paths in parallel in order to maximize bandwidth and possibly overcome bandwidth limitations of the individual paths. The actual strategy is implementation specific. Note that this is a local choice - the peer endpoint can choose a different policy. 10.1.8. Bounds on Send or Receive Rate Name: maxSendRate / maxRecvRate Type: Numeric (with special value "Unlimited") / Numeric (with special value "Unlimited") Default: 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 a data transfer useful. It is given in bits per second. The special value "Unlimited" indicates that no bound is specified. 10.1.9. Read-only Connection Properties The following generic Connection Properties are read-only, i.e. they cannot be changed by an application. 10.1.9.1. 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 7.5.4. It is given in Bytes. 10.1.9.2. Maximum Message Size Before Fragmentation or Segmentation Name: singularTransmissionMsgMaxLen Type: Integer Trammell, et al. Expires 14 January 2021 [Page 56] Internet-Draft TAPS Interface July 2020 This property, if applicable, represents the maximum Message size that can be sent without incurring network-layer fragmentation or transport layer segmentation at the sender. This property exposes the Maximum Packet Size (MPS) as described in Datagram PLPMTUD [I-D.ietf-tsvwg-datagram-plpmtud]. 10.1.9.3. Maximum Message Size on Send Name: sendMsgMaxLen Type: Integer This property represents the maximum Message size that can be sent using a send operation. 10.1.9.4. Maximum Message Size on Receive Name: recvMsgMaxLen Type: Integer This numeric property represents the maximum Message size that can be received. 10.2. TCP-specific Properties: User Timeout Option (UTO) These properties specify configurations for the User Timeout Option (UTO), in case TCP becomes the chosen transport protocol. Implementation is optional and of course only sensible if TCP is implemented in the transport 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 [I-D.ietf-taps-minset], 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). 10.2.1. Advertised User Timeout Name: tcp.userTimeoutValue Type: Integer Trammell, et al. Expires 14 January 2021 [Page 57] Internet-Draft TAPS Interface July 2020 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 10.1.4) value accordingly. 10.2.2. 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. 10.2.3. Timeout Changeable Name: tcp.userTimeoutRecv Type: Boolean Default: true This property controls whether the "Timeout for aborting Connection" (see Section 10.1.4) may be changed based on a UTO option received from the remote peer. This boolean becomes false when "Timeout for aborting Connection" (see Section 10.1.4) is used. 10.3. Connection Lifecycle Events During the lifetime of a connection there are events that can occur when configured. 10.3.1. 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<> Trammell, et al. Expires 14 January 2021 [Page 58] Internet-Draft TAPS Interface July 2020 10.3.2. Excessive retransmissions This event notifies the application of excessive retransmissions, based on a configured threshold (see Section 10.1.1). This will only happen if the underlying protocol stack supports reliability and, with it, such notifications. Connection -> ExcessiveRetransmission<> 11. 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 indeed be signaled. Connection -> Closed<> Abort terminates a Connection without delivering remaining data: Connection.Abort() A ConnectionError informs the application that data to could not be delivered after a timeout, or the other side has aborted the Connection; however, there is no guarantee that an Abort will indeed be signaled. Connection -> ConnectionError 12. Connection State and Ordering of Operations and Events As 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: Trammell, et al. Expires 14 January 2021 [Page 59] Internet-Draft TAPS Interface July 2020 * 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. * InitiateError<> 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 -----> Closed | ^ | | +-----------------------------------+ InitiateError<> (*) Ready<>, ConnectionReceived<>, RendezvousDone<> (**) Closed<>, ConnectionError<> Figure 1: Connection State Diagram The interface provides the following guarantees about the ordering of operations: * 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. Trammell, et al. Expires 14 January 2021 [Page 60] Internet-Draft TAPS Interface July 2020 * No events will occur on a Connection after it is Closed; i.e., after a Closed<> event, an InitiateError<> or ConnectionError<> 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). ConnectionError<> may occur after Closed<>, but the interface must gracefully handle all cases where application ignores these errors. 13. 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 4.2.1). 14. 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 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. Security consideration for these protocols should be discussed in the respective specifications. The desribed API is used to exchange information between an application and the transport system. While it is not necessarily expected that both systems are implemented by the same authority, it is expected that the transport 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 TAPS API is an internal interface that is used to change information locally between two systems. However, as the transport 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 communication peer. Most of the information provided over the TAPS API are useful to configure and select protocols and paths and are not necessarily privacy sensitive. Still, there is some information that could be privacy sensitve because this might reveal usage characteristics and habits of the user of an application. Trammell, et al. Expires 14 January 2021 [Page 61] Internet-Draft TAPS Interface July 2020 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 enties can access it. In most cases information that is provided for protocol and path selection should not directly translate to information that is can be observed by network devices on the path. But there might be specific configuration information that are intended for path exposure, such as e.g. a DiffServ codepoint setting, that is either povided directly by the application or indirectly configured over a traffic profile. Further, applications should be aware that communication attempts can lead to more than one connection establishment. This is for example the case when the transport system also excecutes name resolution; or when support mechanisms such as TURN or ICE are used to establish connectivity; or if protocols or paths are raised; or if a path fails and fallback or re-establishment is supported in the transport system. These communication activities are not different from what is used today, however, the goal of a TAPS transport 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 system but at the same time may reduce 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. 15. 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. Trammell, et al. Expires 14 January 2021 [Page 62] Internet-Draft TAPS Interface July 2020 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. 16. References 16.1. Normative References [I-D.ietf-taps-arch] Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G., Perkins, C., Tiesel, P., and C. Wood, "An Architecture for Transport Services", Work in Progress, Internet-Draft, draft-ietf-taps-arch-07, 9 March 2020, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . 16.2. Informative References Trammell, et al. Expires 14 January 2021 [Page 63] Internet-Draft TAPS Interface July 2020 [I-D.ietf-taps-impl] Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K., Jones, T., Tiesel, P., Perkins, C., and M. Welzl, "Implementing Interfaces to Transport Services", Work in Progress, Internet-Draft, draft-ietf-taps-impl-06, 9 March 2020, . [I-D.ietf-taps-minset] Welzl, M. and S. Gjessing, "A Minimal Set of Transport Services for End Systems", Work in Progress, Internet- Draft, draft-ietf-taps-minset-11, 27 September 2018, . [I-D.ietf-taps-transport-security] Enghardt, T., Pauly, T., Perkins, C., Rose, K., and C. Wood, "A Survey of the Interaction Between Security Protocols and Transport Services", Work in Progress, Internet-Draft, draft-ietf-taps-transport-security-12, 23 April 2020, . [I-D.ietf-tsvwg-datagram-plpmtud] Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and T. Voelker, "Packetization Layer Path MTU Discovery for Datagram Transports", Work in Progress, Internet-Draft, draft-ietf-tsvwg-datagram-plpmtud-22, 10 June 2020, . [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, . [RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, DOI 10.17487/RFC2597, June 1999, . [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914, DOI 10.17487/RFC2914, September 2000, . Trammell, et al. Expires 14 January 2021 [Page 64] Internet-Draft TAPS Interface July 2020 [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, . [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, . [RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, DOI 10.17487/RFC4594, August 2006, . [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, . [RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option", RFC 5482, DOI 10.17487/RFC5482, March 2009, . [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, . [RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real- Time Communication Use Cases and Requirements", RFC 7478, DOI 10.17487/RFC7478, March 2015, . [RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015, . [RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services (Diffserv) and Real-Time Communication", RFC 7657, DOI 10.17487/RFC7657, November 2015, . Trammell, et al. Expires 14 January 2021 [Page 65] Internet-Draft TAPS Interface July 2020 [RFC8084] Fairhurst, G., "Network Transport Circuit Breakers", BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017, . [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, . [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, . [RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April 2019, . [RFC8622] Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for Differentiated Services", RFC 8622, DOI 10.17487/RFC8622, June 2019, . [RFC8699] Islam, S., Welzl, M., and S. Gjessing, "Coupled Congestion Control for RTP Media", RFC 8699, DOI 10.17487/RFC8699, January 2020, . [TCP-COUPLING] "ctrlTCP: Reducing Latency through Coupled, Heterogeneous Multi-Flow TCP Congestion Control", IEEE INFOCOM Global Internet Symposium (GI) workshop (GI 2018) , 15 April 2018. Appendix A. Convenience Functions A.1. Adding Preference Properties As Selection Properties of type "Preference" will be added to a TransportProperties object quite frequently, implementations should provide special actions for adding each preference level i.e, "TransportProperties.Add(some_property, avoid)" is equivalent to "TransportProperties.Avoid(some_property)": Trammell, et al. Expires 14 January 2021 [Page 66] Internet-Draft TAPS Interface July 2020 TransportProperties.Require(property) TransportProperties.Prefer(property) TransportProperties.Ignore(property) TransportProperties.Avoid(property) TransportProperties.Prohibit(property) TransportProperties.Default(property) A.2. Transport Property Profiles To ease the use of the interface specified by this document, implementations should provide a mechanism to create Transport Property objects (see Section 5.2) that are pre-configured with frequently used sets of properties. Implementations should at least offer short-hands to specify the following property profiles: A.2.1. reliable-inorder-stream This profile provides reliable, in-order transport service with congestion control. An example of a protocol that provides this service is TCP. It should consist of the following properties: +=======================+=========+ | Property | Value | +=======================+=========+ | reliability | require | +-----------------------+---------+ | preserveOrder | require | +-----------------------+---------+ | congestionControl | require | +-----------------------+---------+ | preserveMsgBoundaries | ignore | +-----------------------+---------+ Table 2 A.2.2. reliable-message This profile provides message-preserving, reliable, in-order transport service with congestion control. An example of a protocol that provides this service is SCTP. It should consist of the following properties: Trammell, et al. Expires 14 January 2021 [Page 67] Internet-Draft TAPS Interface July 2020 +=======================+=========+ | Property | Value | +=======================+=========+ | reliability | require | +-----------------------+---------+ | preserveOrder | require | +-----------------------+---------+ | congestionControl | require | +-----------------------+---------+ | preserveMsgBoundaries | require | +-----------------------+---------+ Table 3 A.2.3. unreliable-datagram This profile provides unreliable datagram transport service. An example of a protocol that provides this service is UDP. It should consist of the following properties: +=======================+=========+ | Property | Value | +=======================+=========+ | reliability | ignore | +-----------------------+---------+ | preserveOrder | ignore | +-----------------------+---------+ | congestionControl | ignore | +-----------------------+---------+ | preserveMsgBoundaries | require | +-----------------------+---------+ | safely replayable | true | +-----------------------+---------+ Table 4 Applications that choose this Transport Property Profile for latency reasons should also consider setting the Capacity Profile Property, see Section 10.1.6 accordingly and my benefit from controlling checksum coverage, see Section 5.2.7 and Section 5.2.8. Trammell, et al. Expires 14 January 2021 [Page 68] Internet-Draft TAPS Interface July 2020 Appendix B. Relationship to the Minimal Set of Transport Services for End Systems [I-D.ietf-taps-minset] 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 [I-D.ietf-taps-minset], updated according to the discussion in Section 5 of [I-D.ietf-taps-minset]. This list is a subset of the transport features in Appendix A of [I-D.ietf-taps-minset], 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 6.1). * Listen: "Listen" Action (Section 6.2). * Specify number of attempts and/or timeout for the first establishment message: "timeout" parameter of "Initiate" (Section 6.1) or "InitiateWithSend" Action (Section 7.8). * Disable MPTCP: "Parallel Use of Multiple Paths" Property (Section 5.2.13). * Hand over a message to reliably transfer (possibly multiple times) before connection establishment: "InitiateWithSend" Action (Section 7.8). * Change timeout for aborting connection (using retransmit limit or time value): "Timeout for Aborting Connection" property, using a time value (Section 10.1.4). * Timeout event when data could not be delivered for too long: "ConnectionError" Event (Section 11). * Suggest timeout to the peer: "TCP-specific Property: User Timeout" (Section 10.2). * Notification of Excessive Retransmissions (early warning below abortion threshold): "Notification of excessive retransmissions" property (Section 5.2.16). * Notification of ICMP error message arrival: "Notification of ICMP soft error message arrival" property (Section 5.2.17). Trammell, et al. Expires 14 January 2021 [Page 69] Internet-Draft TAPS Interface July 2020 * Choose a scheduler to operate between streams of an association: "Connection Group Transmission Scheduler" property (Section 10.1.5). * Configure priority or weight for a scheduler: "Priority (Connection)" property (Section 10.1.3). * "Specify checksum coverage used by the sender" and "Disable checksum when sending": "Corruption Protection Length" property (Section 7.5.6) and "Full Checksum Coverage on Sending" property (Section 5.2.7). * "Specify minimum checksum coverage required by receiver" and "Disable checksum requirement when receiving": "Required Minimum Corruption Protection Coverage for Receiving" property (Section 10.1.2) and "Full Checksum Coverage on Receiving" property (Section 5.2.8). * "Specify DF" field and "Request not to bundle messages": the "No Fragmentation" Message Property combines both of these requests, i.e. if a request not to bundle messages is made, this also turns off fragmentation (i.e., sets DF=1) in the case of a protocol that allows this (only UDP and UDP-Lite, which cannot bundle messages anyway) (Section 7.5.9). * 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 10.1.9.2). * Get max. transport-message size that may be received from the configured interface: "Maximum Message Size on Receive" property (Section 10.1.9.4). * Obtain ECN field: "ECN" is a defined UDP(-Lite)-specific read-only Message Property of the MessageContext object (Section 8.3.1). * "Specify DSCP field", "Disable Nagle algorithm", "Enable and configure a "Low Extra Delay Background Transfer"": as suggested in Section 5.5 of [I-D.ietf-taps-minset], these transport features are collectively offered via the "Capacity Profile" property (Section 10.1.6). Per-Message control is offered via the "Message Capacity Profile Override" property (Section 7.5.8). Trammell, et al. Expires 14 January 2021 [Page 70] Internet-Draft TAPS Interface July 2020 * 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 [I-D.ietf-taps-minset] (Section 11). * "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 11). * "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 7). Reliability is controlled via the "Reliable Data Transfer (Connection)" (Section 5.2.1) property and the "Reliable Data Transfer (Message)" Message Property (Section 7.5.7). Transmitting data as a message or without delimiters is controlled via Message Framers (Section 9). The choice of congestion control is provided via the "Congestion control" property (Section 5.2.9). * Configurable Message Reliability: the "Lifetime" Message Property implements a time-based way to configure message reliability (Section 7.5.1). * "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 7.5.3). * Request not to delay the acknowledgement (SACK) of a message: should the protocol support it, this is one of the transport features the transport system can apply when an application uses the "Capacity Profile" Property (Section 10.1.6) or the "Message Capacity Profile Override" Message Property (Section 7.5.8) with value "Low Latency/Interactive". * Receive data (with no message delimiting): "Received" Event (Section 8.2.1). See Section 9 for handling Message framing in situations where the Protocol Stack only provides a byte-stream transport. * Receive a message: "Received" Event (Section 8.2.1), using Message Framers (Section 9). Trammell, et al. Expires 14 January 2021 [Page 71] Internet-Draft TAPS Interface July 2020 * Information about partial message arrival: "ReceivedPartial" Event (Section 8.2.2). * Notification of send failures: "Expired" Event (Section 7.3.2) and "SendError" Event (Section 7.3.3). * Notification that the stack has no more user data to send: applications can obtain this information via the "Sent" Event (Section 7.3.1). * Notification to a receiver that a partial message delivery has been aborted: "ReceiveError" Event (Section 8.2.3). 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 Trammell, et al. Expires 14 January 2021 [Page 72] Internet-Draft TAPS Interface July 2020 URI: http://www.erg.abdn.ac.uk/ Mirja Kuehlewind Ericsson Ericsson-Allee 1 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 TU Berlin Einsteinufer 25 10587 Berlin 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 Trammell, et al. Expires 14 January 2021 [Page 73]