ETH Zurich

Gloriastrasse 35 8092 Zurich Switzerland ietf@trammell.ch

University of Glasgow

School of Computing Science Glasgow G12 8QQ United Kingdom csp@csperkins.org

Apple Inc.

1 Infinite Loop Cupertino, California 95014 United States of America tpauly@apple.com

ETH Zurich

Gloriastrasse 35 8092 Zurich Switzerland mirja.kuehlewind@tik.ee.ethz.ch

Apple Inc.

1 Infinite Loop Cupertino, California 95014 United States of America cawood@apple.com

Transport TAPS Working Group Internet-Draft This document describes Post Sockets, an asynchronous abstract programming interface for the atomic transmission of messages in an inherently multipath environment. Post replaces connections with long-lived associations between endpoints, with the possibility to cache cryptographic state in order to reduce amortized connection latency. We present this abstract interface as an illustration of what is possible with present developments in transport protocols when freed from the strictures of the current sockets API.

The BSD Unix Sockets API’s SOCK_STREAM abstraction, by bringing network sockets into the UNIX programming model, allowing anyone who knew how to write programs that dealt with sequential-access files to also write network applications, was a revolution in simplicity. It would not be an overstatement to say that this simple API is the reason the Internet won the protocol wars of the 1980s. SOCK_STREAM is tied to the Transmission Control Protocol (TCP), specified in 1981 . TCP has scaled remarkably well over the past three and a half decades, but its total ubiquity has hidden an uncomfortable fact: the network is not really a file, and stream abstractions are too simplistic for many modern application programming models. In the meantime, the nature of Internet access, and the variety of Internet transport protocols, is evolving. The challenges that new protocols and access paradigms present to the sockets API and to programming models based on them inspire the design elements of a new approach. Many end-user devices are connected to the Internet via multiple interfaces, which suggests it is time to promote the paths by which two endpoints are connected to each other to a first-order object. While implicit multipath communication is available for these multihomed nodes in the present Internet architecture with the Multipath TCP extension (MPTCP) , MPTCP was specifically designed to hide multipath communication from the application for purposes of compatibility. Since many multihomed nodes are connected to the Internet through access paths with widely different properties with respect to bandwidth, latency and cost, adding explicit path control to MPTCP’s API would be useful in many situations. Another trend straining the traditional layering of the transport stack associated with the SOCK_STREAM interface is the widespread interest in ubiquitous deployment of encryption to guarantee confidentiality, authenticity, and integrity, in the face of pervasive surveillance . Layering the most widely deployed encryption technology, Transport Layer Security (TLS), strictly atop TCP (i.e., via a TLS library such as OpenSSL that uses the sockets API) requires the encryption-layer handshake to happen after the transport-layer handshake, which increases connection setup latency on the order of one or two round-trip times, an unacceptable delay for many applications. Integrating cryptographic state setup and maintenance into the path abstraction naturally complements efforts in new protocols (e.g. QUIC ) to mitigate this strict layering. To meet these challenges, we present the Post-Sockets Application Programming Interface (API), described in detail in this work. Post is designed to be language, transport protocol, and architecture independent, allowing applications to be written to a common abstract interface, easily ported among different platforms, and used even in environments where transport protocol selection may be done dynamically, as proposed in the IETF’s Transport Services working group. Post replaces the traditional SOCK_STREAM abstraction with a Message abstraction, which can be seen as a generalization of the Stream Control Transmission Protocol’s SOCK_SEQPACKET service. Messages are sent and received on Carriers, which logically group Messages for transmission and reception. For backward compatibility, bidirectional byte stream protocols are represented as a pair of Messages, one in each direction, that can only be marked complete when the sending peer has finished transmitting data. Post replaces the notions of a socket address and connected socket with an Association with a remote endpoint via set of Paths. Implementation and wire format for transport protocol(s) implementing the Post API are explicitly out of scope for this work; these abstractions need not map directly to implementation-level concepts, and indeed with various amounts of shimming and glue could be implemented with varying success atop any sufficiently flexible transport protocol. The key features of Post as compared with the existing sockets API are: Explicit Message orientation, with framing and atomicity guarantees for Message transmission. Asynchronous reception, allowing all receiver-side interactions to be event-driven. Explicit support for multistreaming and multipath transport protocols and network architectures. Long-lived Associations, whose lifetimes may not be bound to underlying transport connections. This allows associations to cache state and cryptographic key material to enable fast resumption of communication, and for the implementation of the API to explicitly take care of connection establishment mechanics such as connection racing and peer-to-peer rendezvous . Transport protocol stack independence, allowing applications to be written in terms of the semantics best for the application’s own design, separate from the protocol(s) used on the wire to achieve them. This enables applications written to a single API to make use of transport protocols in terms of the features they provide, as in . This work is the synthesis of many years of Internet transport protocol research and development. It is inspired by concepts from the Stream Control Transmission Protocol (SCTP) , TCP Minion , and MinimaLT , among other transport protocol modernization efforts. We present Post as an illustration of what is possible with present developments in transport protocols when freed from the strictures of the current sockets API. While much of the work for building parts of the protocols needed to implement Post are already ongoing in other IETF working groups (e.g. MPTCP, QUIC, TLS), we argue that an abstract programming interface unifying access all these efforts is necessary to fully exploit their potential.

Post is based on a small set of abstractions, centered around a Message Carrier as the entry point for an application to the networking API. The relationships among them are shown in Figure and detailed in this section.

A Message Carrier (or simply Carrier) is a transport protocol stack-independent interface for sending and receiving messages between an application and a remote endpoint; it is roughly analogous to a socket in the present sockets API. Sending a Message over a Carrier is driven by the application, while receipt is driven by the arrival of the last packet that allows the Message to be assembled, decrypted, and passed to the application. Receipt is therefore asynchronous; given the different models for asynchronous I/O and concurrency supported by different platforms, it may be implemented in any number of ways. The abstract API provides only for a way for the application to register how it wants to handle incoming messages. All the Messages sent to a Carrier will be received on the corresponding Carrier at the remote endpoint, though not necessarily reliably or in order, depending on Message properties and the underlying transport protocol stack. A Carrier that is backed by current transport protocol stack state (such as a TCP connection; see ) is said to be “active”: messages can be sent and received over it. A Carrier can also be “dormant”: there is long-term state associated with it (via the underlying Association; see ), and it may be able to reactivated, but messages cannot be sent and received immediately. Carriers become dormant when the underlying transport protocol stack determines that an underlying connection has been lost and there is insufficient state in the Association to re-establish it (e.g., in the case of a server-side Carrier where the client’s address has changed unexpectedly). Passive close can be handled by the application via an event on the carrier. Attempting to use a carrier after passive close results in an error. If supported by the underlying transport protocol stack, a Carrier may be forked: creating a new Carrier associated with a new Carrier at the same remote endpoint. The semantics of the usage of multiple Carriers based on the same Association are application-specific. When a Carrier is forked, its corresponding Carrier at the remote endpoint receives a fork request, which it must accept in order to fully establish the new carrier. Multiple Carriers between endpoints are implemented differently by different transport protocol stacks, either using multiple separate transport-layer connections, or using multiple streams of multistreaming transport protocols. To exchange messages with a given remote endpoint, an application may initiate a Carrier given its remote (see and local (see ) identities; this is an equivalent to an active open. There are four special cases of Carriers, as well, supporting different initiation and interaction patterns, defined in the subsections below. Listener: A Listener is a special case of Message Carrier which only responds to requests to create a new Carrier from a remote endpoint, analogous to a server or listening socket in the present sockets API. Instead of being bound to a specific remote endpoint, it is bound only to a local identity; however, its interface for accepting fork requests is identical to that for fully fledged Carriers. Source: A Source is a special case of Message Carrier over which messages can only be sent, intended for unidirectional applications such as multicast transmitters. Sources cannot be forked, and need not accept forks. Sink: A Sink is a special case of Message Carrier over which messages can only be received, intended for unidirectional applications such as multicast receivers. Sinks cannot be forked, and need not accept forks. Responder: A Responder is a special case of Message Carrier which may receive messages from many remote sources, for cases in which an application will only ever send Messages in reply back to the source from which a Message was received. This is a common implementation pattern for servers in client-server applications. A Responder’s receiver gets a Message, as well as a Source to send replies to. Responders cannot be forked, and need not accept forks.

A Message is the unit of communication between applications. Messages can represent relatively small structures, such as requests in a request/response protocol such as HTTP; relatively large structures, such as files of arbitrary size in a filesystem; and structures of indeterminate length, such as a stream of bytes in a protocol like TCP. In the general case, there is no mapping between a Message and packets sent by the underlying protocol stack on the wire: the transport protocol may freely segment messages and/or combine messages into packets. However, a message may be marked as immediate, which will cause it to be sent in a single packet when possible. Content may be sent and received either as Complete or Partial Messages. Dealing with Complete Messages should be preferred for simplicity whenever possible based on the underlying protocol. It is always possible to send Complete Messages, but only protocols that have a fixed maximum message length may allow clients to receive Messages using an API that guarantees Complete Messages. Sending and receiving Partial Messages (that is, a Message whose content spans multiple calls or callbacks) is always possible. To send a Message, either Complete or Partial, the Message content is passed into the Carrier, and client provides a set of callbacks to know when the Message was delivered or acknowledged. The client of the API may use the callbacks to pace the sending of Messages. To receive a Message, the client of the API schedules a completion to be called when a Complete or Partial Message is available. If the client is willing to accept Partial Messages, it can specify the minimum incomplete Message length it is willing to receive at once, and the maximum number of bytes it is willing to receive at once. If the client wants Complete Messages, there are no values to tune. The scheduling of the receive completion indicates to the Carrier that there is a desire to receive bytes, effectively creating a “pull model” in which backpressure may be applied if the client is not receiving Messages or Partial Messages quickly enough to match the peer’s sending rate. The Carrier may have some minimal buffer of incoming Messages ready for the client to read to reduce latency. When receiving a Complete Message, the entire content of the Message must be delivered at once, and the Message is not delivered at all if the full Message is not received. This implies that both the sending and receiving endpoint, whether in the application or the carrier, must guarantee storage for the full size of a Message. Partial Messages may be sent or received in several stages, with a handle representing the total Message being associated with each portion of the content. Each call to send or receive also indicates whether or not the Message is now complete. This approach is necessary whenever the size of the Message does not have a known bound, or the size is too large to process and hold in memory. Protocols that only present a concept of byte streams represent their data as single Messages with unknown bounds. In the case of TCP, the client will receive a single Message in pieces using the Partial Message API, and that Message will only be marked as complete when the peer has sent a FIN. Messages are sent over and received from Message Carriers (see ). On sending, Messages have properties that allow the application to specify its requirements with respect to reliability, ordering, priority, idempotence, and immediacy; these are described in detail below. Messages may also have arbitrary properties which provide additional information to the underlying transport protocol stack on how they should be handled, in a protocol-specific way. These stacks may also deliver or set properties on received messages, but in the general case a received messages contains only a sequence of ordered bytes. Message properties include: Lifetime and Partial Reliability: A Message may have a “lifetime” – a wall clock duration before which the Message must be available to the application layer at the remote end. If a lifetime cannot be met, the Message is discarded as soon as possible. Messages without lifetimes are sent reliably if supported by the transport protocol stack. Lifetimes are also used to prioritize Message delivery. There is no guarantee that a Message will not be delivered after the end of its lifetime; for example, a Message delivered over a strictly reliable transport will be delivered regardless of its lifetime. Depending on the transport protocol stack used to transmit the message, these lifetimes may also be signalled to path elements by the underlying transport, so that path elements that realize a lifetime cannot be met can discard frames containing the Messages instead of forwarding them. Priority: Messages have a “niceness” – a priority among other messages sent over the same Carrier in an unbounded hierarchy most naturally represented as a non-negative integer. By default, Messages are in niceness class 0, or highest priority. Niceness class 1 Messages will yield to niceness class 0 Messages sent over the same Carrier, class 2 to class 1, and so on. Niceness may be translated to a priority signal for exposure to path elements (e.g. DSCP code point) to allow prioritization along the path as well as at the sender and receiver. This inversion of normal schemes for expressing priority has a convenient property: priority increases as both niceness and lifetime decrease. A Message may have both a niceness and a lifetime – Messages with higher niceness classes will yield to lower classes if resource constraints mean only one can meet the lifetime. Dependence: A Message may have “antecedents” – other Messages on which it depends, which must be delivered before it (the “successor”) is delivered. The sending transport uses deadlines, niceness, and antecedents, along with information about the properties of the Paths available, to determine when to send which Message down which Path. Idempotence: A sending application may mark a Message as “idempotent” to signal to the underlying transport protocol stack that its application semantics make it safe to send in situations that may cause it to be received more than once (i.e., for 0-RTT session resumption as in TCP Fast Open, TLS 1.3, and QUIC). Immediacy: A sending application may mark a Message as “immediate” to signal to the underlying transport protocol stack that its application semantics require it to be placed in a single packet, on its own, instead of waiting to be combined with other messages or parts thereof (i.e., for media transports and interactive sessions with small messages). Senders may also be asynchronously notified of three events on Messages they have sent: that the Message has been transmitted, that the Message has been acknowledged by the receiver, or that the Message has expired before transmission/acknowledgement. Not all transport protocol stacks will support all of these events.

An Association contains the long-term state necessary to support communications between a Local (see ) and a Remote (see ) endpoint, such as trust model information, including pinned public keys or anchor certificates, cryptographic session resumption parameters, or rendezvous information. It uses information from the Configuration (see ) to constrain the selection of transport protocols and local interfaces to create Transients (see ) to carry Messages; and information about the paths through the network available available between them (see ). All Carriers are bound to an Association. New Carriers will reuse an Association if they can be carried from the same Local to the same Remote over the same Paths; this re-use of an Association may implies the creation of a new Transient. Associations may exist and be created without a Carrier. This may be done if peer cryptographic state such as a pre-shared key is established out-of-band. Thus, Associations may be created without the need to send application data to a peer, that is, without a Carrier. Associations are mutable. Association state may expire over time, after which it is removed from the Association, and Transients may export cryptographic state to store in an Association as needed. Moreover, this state may be exported directly into the Association or modified before insertion. This may be needed to diversify ephemeral Transient keying material from the longer-term Association keying material. A primary use of Association state is to allow new Associations and their derived Carriers to be quickly created without performing in-band cryptographic handshakes. See for more details about this separation.

A Remote represents information required to establish and maintain a connection with the far end of an Association: name(s), address(es), and transport protocol parameters that can be used to establish a Transient; transport protocols to use; trust model information, inherited from the relevant Association, used to identify the remote on connection establishment; and so on. Each Association is associated with a single Remote, either explicitly by the application (when created by the initiation of a Carrier) or a Listener (when created by forking a Carrier on passive open). A Remote may be resolved, which results in zero or more Remotes with more specific information. For example, an application may want to establish a connection to a website identified by a URL https://www.example.com. This URL would be wrapped in a Remote and passed to a call to initiate a Carrier. The first pass resolution might parse the URL, decomposing it into a name, a transport port, and a transport protocol to try connecting with. A second pass resolution would then look up network-layer addresses associated with that name through DNS, and store any certificates available from DANE. Once a Remote has been resolved to the point that a transport protocol stack can use it to create a Transient, it is considered fully resolved.

A Local represents all the information about the local endpoint necessary to establish an Association or a Listener. It encapsulates the Provisioning Domain (PvD) of a single interface in the multiple provisioning domain architecture , and adds information about the service endpoint (transport protocol port), and, per , cryptographic identities (certificates and associated private keys) bound to this endpoint.

A Configuration encapsulates an application’s preferences around Path selection and protocol options. Each Association has exactly one Configuration, and all Carriers belonging to that Association share the same Configuration. The application cannot modify the Configuration for a Carrier or Association once it is set. If a new set of options needs to be used, then the application needs a new Carrier or Association instance. This is necessary to ensure that a single Carrier can consistently track the Paths and protocol options it uses, since it is usually not possible to modify these properties without breaking connectivity. To influence Path selection, the application can configure a set of requirements, preferences, and restrictions concerning which Paths may be selected by the Association to use for creating Transients between a Local and a Remote. For example, a Configuration can specify that the application prefers Wi-Fi access over LTE when roaming on a foreign LTE network, due to monetary cost to the user. The Association uses the Configuration’s Path preferences as a key part of determining the Paths to use for its Transients. The Configuration is provided as input when examining the complete list of available Paths on the system (to limit the list, or order the Paths by preference). The system’s policy will further restrict and modify the Path that is ultimately selected, using other aspects of the Configuration (protocol options and originating application) to select the most appropriate Path. To influence protocol selection and options, the Configuration contains one or more allowed Protocol Stack Configurations. Each of these is comprised of application- and transport-layer protocols that may be used together to communicate to the Remote, along with any protocol-specific options. For example, a Configuration could specify two alternate, but equivalent, protocol stacks: one using HTTP/2 over TLS over TCP, and the other using QUIC over UDP. Alternatively, the Configuration could specify two protocol stacks with the same protocols, but different protocol options: one using TLS with TLS 1.3 0-RTT enabled and TCP with TCP Fast-Open enabled, and one using TLS with out 0-RTT and TCP without TCP Fast-Open. Protocol-specific options within the Configuration include trust settings and acceptable cryptographic algorithms to be used by security protocols. These may be configured for specific protocols to allow different settings for each (such as between TLS over TCP and TLS for use with QUIC), or set as default security settings on the Configuration to be used by any protocol that needs to evaluate trust. Trust settings may include certificate anchors and certificate pinning options.

A Transient represents a binding between a Carrier and the instance of the transport protocol stack that implements it. As an Association contains long-term state for communications between two endpoints, a Transient contains ephemeral state for a single transport protocol over a one or more Paths at a given point in time. A Carrier may be served by multiple Transients at once, e.g. when implementing multipath communication such that the separate paths are exposed to the API by the underlying transport protocol stack. Each Transient serves only one Carrier, although multiple Transients may share the same underlying protocol stack; e.g. when multiplexing Carriers over streams in a multistreaming protocol. Transients are generally not exposed by the API to the application, though they may be accessible for debugging and logging purposes.

A Path represents information about a single path through the network used by an Association, in terms of source and destination network and transport layer addresses within an addressing context, and the provisioning domain of the local interface. This information may be learned through a resolution, discovery, or rendezvous process (e.g. DNS, ICE), by measurements taken by the transport protocol stack, or by some other path information discovery mechanism. It is used by the transport protocol stack to maintain and/or (re-)establish communications for the Association. The set of available properties is a function of the transport protocol stacks in use by an association. However, the following core properties are generally useful for applications and transport layer protocols to choose among paths for specific Messages: Maximum Transmission Unit (MTU): the maximum size of an Message’s payload (subtracting transport, network, and link layer overhead) which will likely fit into a single frame. Derived from signals sent by path elements, where available, and/or path MTU discovery processes run by the transport layer. Latency Expectation: expected one-way delay along the Path. Generally provided by inline measurements performed by the transport layer, as opposed to signaled by path elements. Loss Probability Expectation: expected probability of a loss of any given single frame along the Path. Generally provided by inline measurements performed by the transport layer, as opposed to signaled by path elements. Available Data Rate Expectation: expected maximum data rate along the Path. May be derived from passive measurements by the transport layer, or from signals from path elements. Reserved Data Rate: Committed, reserved data rate for the given Association along the Path. Requires a bandwidth reservation service in the underlying transport protocol stack. Path Element Membership: Identifiers for some or all nodes along the path, depending on the capabilities of the underlying network layer protocol to provide this. Path properties are generally read-only. MTU is a property of the underlying link-layer technology on each link in the path; latency, loss, and rate expectations are dynamic properties of the network configuration and network traffic conditions; path element membership is a function of network topology. In an explicitly multipath architecture, application and transport layer requirements can be met by having multiple paths with different properties to select from. Transport protocol stacks can also provide signaling to devices along the path, but this signaling is derived from information provided to the Message abstraction.

We now turn to the design of an abstract programming interface to provide a simple interface to Post’s abstractions, constrained by the following design principles: Flexibility is paramount. So is simplicity. Applications must be given as many controls and as much information as they may need, but they must be able to ignore controls and information irrelevant to their operation. This implies that the “default” interface must be no more complicated than BSD sockets, and must do something reasonable. Reception is an inherently asynchronous activity. While the API is designed to be as platform-independent as possible, one key insight it is based on is that an Message receiver’s behavior in a packet-switched network is inherently asynchronous, driven by the receipt of packets, and that this asynchronicity must be reflected in the API. The actual implementation of receive and event handling will need to be aligned to the method a given platform provides for asynchronous I/O. A new API cannot be bound to a single transport protocol and expect wide deployment. As the API is transport-independent and may support runtime transport selection, it must impose the minimum possible set of constraints on its underlying transports, though some API features may require underlying transport features to work optimally. It must be possible to implement Post over vanilla TCP in the present Internet architecture. The API we design from these principles is centered around a Carrier, which can be created actively via initiate() or passively via a listen(); the latter creates a Listener from which new Carriers can be accept()ed. Messages may be created explicitly and passed to this Carrier, or implicitly through a simplified interface which uses default message properties (reliable transport without priority or deadline, which guarantees ordered delivery over a single Carrier when the underlying transport protocol stack supports it). For each connection between a Local and a Remote a new Carrier is created and destroyed when the connection is closed. However, a new Carrier may use an existing Association if present for the requested Local-Remote pair and permitted by the PolicyContext that can be provided at Carrier initiation. Further the system-wide PolicyContext can contain more information that determine when to create or destroy Associations other than at Carrier initiation. E.g. an Association can be created at system start, based on the configured PolicyContext or also by a manual action of an single application, for Local-Remote pairs that are known to be likely used soon, and to pre-establish, e.g., cryptographic context as well as potentially collect current information about path capabilities. Every time an actual connection with a specific PSI is established between the Local and Remote, the Association learns new Path information and stores them. This information can be used when a new transient is created, e.g. to decide which PSI to use (to provide the highest probably for a successful connection attempt) or which PSIs to probe for (first). A Transient is created when an application actually sends a Message over a Carrier. As further explained below this step can actually create multiple transients for probing or assign a new transient to an already active PSI, e.g. if multi-streaming is provided and supported for these kind of use on both sides.

[EDITOR’S NOTE: ensure these are in line with the below.] Here, we illustrate the usage of the API for common connection patterns. Note that error handling is ignored in these illustrations for ease of reading.

Here’s an example client-server application. The server echoes messages. The client sends a message and prints what it receives. The client in connects, sends a message, and sets up a receiver to print messages received in response. The carrier is inactive after the Initiate() call; the Send() call blocks until the carrier can be activated.

The server in creates a Listener, which accepts Carriers and passes them to a server. The server echos the content of each message it receives.

The Responder allows the server to be significantly simplified, as shown in .

The fundamental design of a client need not change at all for happy eyeballs (selection of multiple potential protocol stacks through connection racing); this is handled by the Post Sockets implementation automatically. If this connection racing is to use 0-RTT data (i.e., as provided by TCP Fast Open , the client must mark the outgoing message as idempotent.

In the client-server examples shown above, the Remote given to the Initiate call refers to the name and port of the server to connect to. This need not be the case, however; a Remote may also refer to an identity and a rendezvous point for rendezvous as in ICE . Here, each peer does its own Initiate call simultaneously, and the result on each side is a Carrier attached to an appropriate Association.

A multicast receiver is implemented using a Sink attached to a Local encapsulating a multicast address on which to receive multicast datagrams. The following example prints messages received on the multicast address forever.

Here, we show how Association state may be initialized without a carrier. The goal is to create a long-term Association from which Carriers may be derived and, if possible, used immediately. Per , a first step is to specify trust model constraints, such as pinned public keys and anchor certificates, which are needed to create Remote connections. We begin by creating shared security parameters that will be used later for creating a remote connection.

Using these statically configured parameters, we now show how to create an Association between a Local and Remote using these parameters.

We may also create an Association with a pre-shared key configured out-of-band.

We now show how to create a Carrier from an existing, pre-configured Association. This Association may or may not contain shared cryptographic static between the Local and Remote, depending on how it was configured.

As Carriers provide the central entry point to Post, they are key to API dynamics. The lifecycle of a carrier is shown in . Carriers are created by active openers by calling Initiate() given a Local and a Remote, and by passive openers by calling Listen() given a Local; the .Accept() method on the listener Carrier can then be used to create active carriers. By default, the underlying Association is automatically created and managed by the underlying API. This underlying Association can be accessed by the Carrier’s .Association() method. Alternately, an association can be explicitly created using NewAssociation(), and a Carrier on the association may be accessed or initiated by calling the association’s .Initiate() method. Once a Carrier has been created (via Initiate(), Association.Initiate(), NewSource(), NewSink(), or Listen()/Accept()), it may be used to send and receive Messages. The existence of a Carrier does not imply the existence of an active Transient or associated transport-layer connection; these may be created when the carrier is, or may be deferred, depending on the network environment, configuration, and protocol stacks available.

[Carrier] -+----------> .Close() | ^ | close [ Carrier ] | | +- event -> [ (closed) ] | | .Association() .Carriers() | .Initiate() V | [Association] ^ | NewAssociation(local,remote) ]]>

Access to more detailed information is possible through accessors on Carriers and Associations, as shown in . The set of currently active Transients can be accessed through the Carrier’s .Transients() methods. The active path(s) used by a Transient can be accessed through the Transient’s .Paths() method, and the set of all paths for which properties are cached by an Association can be accessed through the Association’s .Paths() method. The set of active carriers on an association can be accessed through the Association’s .Carriers() method. Access to transients and paths is not necessary in normal operation; these accessors are provided primarily for logging and debugging purposes.

[Transient] | ^ | | | | .Association() .Carriers() .Paths() | .Initiate() | V | V [Association]---.Paths()------>[Path] ]]>

Each Carrier has a .Send() method, by which Messages can be sent with given properties, and a .Ready() method, which supplies a callback for reading Messages from the remote side. .Send() is not available on Sinks, and .Ready() is not available on Sources. Carriers also provide .OnSent(), .OnAcked(), and .OnExpired() calls for binding default send event handlers to the Carrier, and .OnClosed() for handling passive close notifications.

.Send() ---> [Carrier] <---- .Ready() <---- [Receiver] [Message ] | +--- .OnSent() +--- .OnAcked() +--- .OnExpired() +--- .OnClosed() ]]>

An application may have a global Configuation, as well as more specific Configurations to apply to the establishment of a given Association or Carrier. These Configurations are optional arguments to the Association and Carrier creation calls. [EDITOR’S NOTE (bht): the text below does not belong here, figure out what to do with it when https://github.com/mami-project/draft-trammell-post-sockets/pull/23 lands: Each Configuration is conceptually a key-value store, where information in more specific scopes overrides information in less specific scopes: application defaults can be overriden by specific Configurations bound to Carriers or Associations, all of which may be overriden by system or user-scoped configuration parameters. Configurations are also made directly available to protocol stack instances (PSIs, see ) for fine-grained control of implementation-specific configuration parameters.] In order to initiate a connection with a remote endpoint, a user of Post Sockets must start from a Remote (see ). A Remote encapsulates identifying information about a remote endpoint at a specific level of resolution. A new Remote can be wrapped around some identifying information by via the NewRemote() call. A Remote has a .Resolve() method, which can be iteratively revoked to increase the level of resolution; a call to Resolve on a given Remote may result in one to many Remotes, as shown in . Remotes at any level of resolution may be passed to Post Sockets calls; each call will continue resolution to the point necessary to establish or resume a Carrier.

[Remote] --.Resolve()---+ ]]>

Information about the local endpoint is also necessary to establish an Association, whether explicitly or implicitly through the creation of a Carrier or Listener. This is passed in the form of a Local (see ). A Local is created with a NewLocal() call, which takes a Configuration (including certificates to present and secret keys associated with them) and identifying information (interface(s) and port(s) to use).

Here we discuss an incomplete list of API implementation considerations that have arisen with experimentation with prototype implementations of Post.

A PSI encapsulates an arbitrary stack of protocols (e.g., TCP over IPv6, SCTP over DTLS over UDP over IPv4). PSIs provide the bridge between the interface (Carrier) plus the current state (Transients) and the implementation of a given set of transport services . A given implementation makes one or more possible protocol stacks available to its applications. Selection and configuration among multiple PSIs is based on system-level or application policies, as well as on network conditions in the provisioning domain in which a connection is made.

For example, (a) shows a TLS over TCP stack, usable on most network connections. Protocols are layered to ensure that the PSI provides all the transport services required by the application. A single PSI may be bound to multiple Carriers, as shown in (b): a multi-streaming transport protocol like QUIC or SCTP can support one carrier per stream. Where multi-streaming transport is not available, these carriers could be serviced by different PSIs on different flows. On the other hand, multiple PSIs are bound to a single transient during establishment, as shown in (c). Here, the losing PSI in a happy-eyeballs race will be terminated, and the carrier will continue using the winning PSI.

While some transports expose a byte stream abstraction, most higher level protocols impose some structure onto that byte stream. That is, the higher level protocol operates in terms of messages, protocol data units (PDUs), rather than using unstructured sequences of bytes, with each message being processed in turn. Protocols are specified in terms of state machines acting on semantic messages, with parsing the byte stream into messages being a necessary annoyance, rather than a semantic concern. Accordingly, Post Sockets exposes a message-based API to applications as the primary abstraction. Protocols that deal only in byte streams, such as TCP, represent their data in each direction as a single, long message. When framing protocols are placed on top of byte streams, the messages used in the API represent the framed messages within the stream. There are other benefits of providing a message-oriented API beyond framing PDUs that Post Sockets should provide when supported by the underlying transport. These include: the ability to associate deadlines with messages, for transports that care about timing; the ability to provide control of reliability, choosing what messages to retransmit in the event of packet loss, and how best to make use of the data that arrived; the ability to manage dependencies between messages, when some messages may not be delivered due to either packet loss or missing a deadline, in particular the ability to avoid (re-)sending data that relies on a previous transmission that was never received. All require explicit message boundaries, and application-level framing of messages, to be effective. Once a message is passed to Post Sockets, it can not be cancelled or paused, but prioritization as well as lifetime and retransmission management will provide the protocol stack with all needed information to send the messages as quickly as possible without blocking transmission unnecessarily. Post Sockets provides this by handling message, with known identity (sequence numbers, in the simple case), lifetimes, niceness, and antecedents. Transport protocols such as SCTP provide a message-oriented API that has similar features to those we describe. Other transports, such as TCP, do not. To support a message oriented API, while still being compatible with stream-based transport protocols, Post Sockets must provide APIs for parsing and serialising messages that understand the protocol data. That is, we push message parsing and serialisation down into the Post Sockets stack, allowing applications to send and receive strongly typed data objects (e.g., a receive call on an HTTP Message Carrier should return an object representing the HTTP response, with pre-parsed status code, headers, and any message body, rather than returning a byte array that the application has to parse itself). This is backwards compatible with existing protocols and APIs, since the wire format of messages does not change, but gives a Post Sockets stack additional information to allow it to make better use of modern transport services. The Post Sockets approach is therefore to raise the semantic level of the transport API: applications should send and receive messages in the form of meaningful, strongly typed, protocol data. Parsing and serialising such messages should be a re-usable function of the protocol stack instance not the application. This is well-suited to implementation in modern systems languages, such as Swift, Go, Rust, or C++, but can also be implemented with some loss of type safety in C.

Ideally, Messages can be of infinite size. However, protocol stacks and protocol stack implementations may impose their own limits on message sizing; For example, SCTP and TLS impose record size limitations of 64kB and 16kB, respectively. Message sizes may also be limited by the available buffer at the receiver, since a Message must be fully assembled by the transport layer before it can be passed on to the application layer. Since not every transport protocol stack implements the signaling necessary to negotiate or expose message size limitations, these may need to be defined out of band, and are probably best exposed through the Configuration. A truly infinite message service – e.g. large file transfer where both endpoints have committed persistent storage to the message – is probably best realized as a layer above Post Sockets, and may be added as a new type of Message Carrier to a future revision of this document.

Regardless of how asynchronous reception is implemented, it is important for an application to be able to apply receiver back-pressure, to allow the protocol stack to perform receiver flow control. Depending on how asynchronous I/O works in the platform, this could be implemented by having a maximum number of concurrent receive callbacks, or by bounding the maximum number of outstanding, unread bytes at any given time, for example.

As the network has evolved, even the simple act of establishing a connection has become increasingly complex. Clients now regularly race multiple connections, for example over IPv4 and IPv6, to determine which protocol to use. The choice of outgoing interface has also become more important, with differential reachability and performance from multiple interfaces. Name resolution can also give different outcomes depending on the interface the query was issued from. Finally, but often most significantly, NAT traversal, relay discovery, and path state maintenance messages are an essential part of connection establishment, especially for peer-to-peer applications. Post Sockets accordingly breaks communication establishment down into multiple phases: Gathering Locals The set of possible Locals is gathered. In the simple case, this merely enumerates the local interfaces and protocols, and allocates ephemeral source ports for transients. For example, a system that has WiFi and Ethernet and supports IPv4 and IPv6 might gather four candidate locals (IPv4 on Ethernet, IPv6 on Ethernet, IPv4 on WiFi, and IPv6 on WiFi) that can form the source for a transient. If NAT traversal is required, the process of gathering locals becomes broadly equivalent to the ICE candidate gathering phase . The endpoint determines its server reflexive locals (i.e., the translated address of a local, on the other side of a NAT) and relayed locals (e.g., via a TURN server or other relay), for each interface and network protocol. These are added to the set of candidate locals for this association. Gathering locals is primarily an endpoint local operation, although it might involve exchanges with a STUN server to derive server reflexive locals, or with a TURN server or other relay to derive relayed locals. It does not involve communication with the remote. Resolving the Remote The remote is typically a name that needs to be resolved into a set of possible addresses that can be used for communication. Resolving the remote is the process of recursively performing such name lookups, until fully resolved, to return the set of candidates for the remote of this association. How this is done will depend on the type of the Remote, and can also be specific to each local. A common case is when the Remote is a DNS name, in which case it is resolved to give a set of IPv4 and IPv6 addresses representing that name. Some types of remote might require more complex resolution. Resolving the remote for a peer-to-peer connection might involve communication with a rendezvous server, which in turn contacts the peer to gain consent to communicate and retrieve its set of candidate locals, which are returned and form the candidate remote addresses for contacting that peer. Resolving the remote is not a local operation. It will involve a directory service, and can require communication with the remote to rendezvous and exchange peer addresses. This can expose some or all of the candidate locals to the remote. Establishing Transients The set of candidate locals and the set of candidate remotes are paired, to derive a priority ordered set of Candidate Paths that can potentially be used to establish a connection. Then, communication is attempted over each candidate path, in priority order. If there are multiple candidates with the same priority, then transient establishment proceeds simultaneously and uses the transient that wins the race to be established. Otherwise, transients establishment is sequential, paced at a rate that should not congest the network. Depending on the chosen transport, this phase might involve racing TCP connections to a server over IPv4 and IPv6 , or it could involve a STUN exchange to establish peer-to-peer UDP connectivity , or some other means. Confirming and Maintaining Transients Once connectivity has been established, unused resources can be released and the chosen path can be confirmed. This is primarily required when establishing peer-to-peer connectivity, where connections supporting relayed locals that were not required can be closed, and where an associated signalling operation might be needed to inform middleboxes and proxies of the chosen path. Keep-alive messages may also be sent, as appropriate, to ensure NAT and firewall state is maintained, so the transient remains operational. By encapsulating these four phases of communication establishment into the PSI, Post Sockets aims to simplify application development. It can provide reusable implementations of connection racing for TCP, to enable happy eyeballs, that will be automatically used by all TCP clients, for example. With appropriate callbacks to drive the rendezvous signalling as part of resolving the remote, we believe a generic ICE implementation ought also to be possible. This procedure can even be repeated fully or partially during a connection to enable seamless hand-over and mobility within the network stack.

Many thanks to Laurent Chuat and Jason Lee at the Network Security Group at ETH Zurich for contributions to the initial design of Post Sockets. Thanks to Joe Hildebrand, Martin Thomson, and Michael Welzl for their feedback, as well as the attendees of the Post Sockets workshop in February 2017 in Zurich for the discussions, which have improved the design described herein. This work is partially supported by the European Commission under Horizon 2020 grant agreement no. 688421 Measurement and Architecture for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat for Education, Research, and Innovation under contract no. 15.0268. This support does not imply endorsement.

This document describes, surveys, and classifies the protocol mechanisms provided by existing IETF protocols, as background for determining a common set of transport services. It examines the Transmission Control Protocol (TCP), Multipath TCP, the Stream Control Transmission Protocol (SCTP), the User Datagram Protocol (UDP), UDP-Lite, the Datagram Congestion Control Protocol (DCCP), the Internet Control Message Protocol (ICMP), the Realtime Transport Protocol (RTP), File Delivery over Unidirectional Transport/ Asynchronous Layered Coding Reliable Multicast (FLUTE/ALC), and NACK- Oriented Reliable Multicast (NORM), Transport Layer Security (TLS), Datagram TLS (DTLS), and the Hypertext Transport Protocol (HTTP), when HTTP is used as a pseudotransport. This survey provides background for the definition of transport services within the TAPS working group. This document obsoletes RFC 2960 and RFC 3309. It describes the Stream Control Transmission Protocol (SCTP). SCTP is designed to transport Public Switched Telephone Network (PSTN) signaling messages over IP networks, but is capable of broader applications.SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. It offers the following services to its users:-- acknowledged error-free non-duplicated transfer of user data,-- data fragmentation to conform to discovered path MTU size,-- sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages,-- optional bundling of multiple user messages into a single SCTP packet, and-- network-level fault tolerance through supporting of multi-homing at either or both ends of an association. The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks. [STANDARDS-TRACK] This document describes a protocol for Network Address Translator (NAT) traversal for UDP-based multimedia sessions established with the offer/answer model. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN). ICE can be used by any protocol utilizing the offer/answer model, such as the Session Initiation Protocol (SIP). [STANDARDS-TRACK] When a server's IPv4 path and protocol are working, but the server's IPv6 path and protocol are not working, a dual-stack client application experiences significant connection delay compared to an IPv4-only client. This is undesirable because it causes the dual- stack client to have a worse user experience. This document specifies requirements for algorithms that reduce this user-visible delay and provides an algorithm. [STANDARDS-TRACK] Encrypted communication on the Internet often uses Transport Layer Security (TLS), which depends on third parties to certify the keys used. This document improves on that situation by enabling the administrators of domain names to specify the keys used in that domain's TLS servers. This requires matching improvements in TLS client software, but no change in TLS server software. [STANDARDS-TRACK] TCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failure.Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document defines an Experimental Protocol for the Internet community. Pervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible. This document describes an experimental TCP mechanism called TCP Fast Open (TFO). TFO allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, and saves up to one full round-trip time (RTT) compared to the standard TCP, which requires a three-way handshake (3WHS) to complete before data can be exchanged. However, TFO deviates from the standard TCP semantics, since the data in the SYN could be replayed to an application in some rare circumstances. Applications should not use TFO unless they can tolerate this issue, as detailed in the Applicability section. This document is a product of the work of the Multiple Interfaces Architecture Design team. It outlines a solution framework for some of the issues experienced by nodes that can be attached to multiple networks simultaneously. The framework defines the concept of a Provisioning Domain (PvD), which is a consistent set of network configuration information. PvD-aware nodes learn PvD-specific information from the networks they are attached to and/or other sources. PvDs are used to enable separation and configuration consistency in the presence of multiple concurrent connections. This document defines the core of the QUIC transport protocol. This document describes connection establishment, packet format, multiplexing and reliability. Accompanying documents describe the cryptographic handshake and loss detection. Due to the latency involved in connection setup and security handshakes, there is an increasing deployment of cryptographic session resumption mechanisms. While cryptographic context and endpoint capabilities need to be be known before encrypted application data can be sent, there is otherwise no technical constraint that the crypto handshake must be performed on the same transport connection. This document recommends a logical separation between the mechanism(s) used to negotiate capabilities and set up encryption context (handshake protocol), the application of encryption and authentication state to data (record protocol), and the associated transport connection(s). This document provides a survey of commonly used or notable network security protocols, with a focus on how they interact and integrate with applications and transport protocols. Its goal is to supplement efforts to define and catalog transport services [RFC8095] by describing the interfaces required to add security protocols. It examines Transport Layer Security (TLS), Datagram Transport Layer Security (DTLS), Quick UDP Internet Connections with TLS (QUIC + TLS), MinimalT, CurveCP, tcpcrypt, and Internet Key Exchange with Encapsulating Security Protocol (IKEv2 + ESP). This survey is not limited to protocols developed within the scope or context of the IETF. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. Minion uses TCP-format packets on-the-wire, for compatibility with existing NATs, Firewalls, and similar middleboxes, but provides a richer set of facilities to the application, as described in the Minion Service Model document. This document specifies the details of the on-the-wire protocol used to provide those services. draft-trammell-plus-abstract-mech-00 Abstract This document describes the operation of three abstract mechanisms for supporting an explicitly cooperative path layer in the Internet architecture. Three mechanisms are described: sender to path signaling with receiver integrity verification; path to receiver signaling with confidential feedback to sender; and direct path to sender signaling. This document describes a simple state machine for stateful network devices on a path between two endpoints to associate state with traffic traversing them on a per-flow basis, as well as abstract signaling mechanisms for driving the state machine. This state machine is intended to replace the de-facto use of the TCP state machine or incomplete forms thereof by stateful network devices in a transport-independent way, while still allowing for fast state timeout of non-established or undesirable flows.

This document is under active development; a list of current open issues is available at https://github.com/mami-project/draft-trammell-post-sockets/issues