Post Sockets, An Abstract Programming Interface for the Transport Layer
draft-trammell-taps-post-sockets-03

TAPS Working Group                                           B. Trammell
Internet-Draft                                                ETH Zurich
Intended status: Informational                                C. Perkins
Expires: March 12, 2018                            University of Glasgow
                                                                T. Pauly
                                                              Apple Inc.
                                                           M. Kuehlewind
                                                              ETH Zurich
                                                                 C. Wood
                                                              Apple Inc.
                                                      September 08, 2017


Post Sockets, An Abstract Programming Interface for the Transport Layer
                  draft-trammell-taps-post-sockets-01

Abstract

   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.

Status of This Memo

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

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

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

   This Internet-Draft will expire on March 12, 2018.








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Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Abstractions and Terminology  . . . . . . . . . . . . . . . .   5
     2.1.  Message Carrier . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Message . . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Association . . . . . . . . . . . . . . . . . . . . . . .  11
     2.4.  Remote  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     2.5.  Local . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     2.6.  Policy Context  . . . . . . . . . . . . . . . . . . . . .  12
     2.7.  Transient . . . . . . . . . . . . . . . . . . . . . . . .  13
     2.8.  Path  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   3.  Abstract Programming Interface  . . . . . . . . . . . . . . .  14
     3.1.  Example Connection Patterns . . . . . . . . . . . . . . .  15
       3.1.1.  Client-Server . . . . . . . . . . . . . . . . . . . .  16
       3.1.2.  Client-Server with Happy Eyeballs and 0-RTT
               establishment . . . . . . . . . . . . . . . . . . . .  17
       3.1.3.  Peer to Peer with Network Address Translation . . . .  17
       3.1.4.  Multicast Receiver  . . . . . . . . . . . . . . . . .  17
     3.2.  Association Bootstrapping . . . . . . . . . . . . . . . .  18
   4.  Implementation Considerations . . . . . . . . . . . . . . . .  19
     4.1.  Protocol Stack Instance (PSI) . . . . . . . . . . . . . .  19
     4.2.  Message Framing, Parsing, and Serialization . . . . . . .  20
     4.3.  Message Size Limitations  . . . . . . . . . . . . . . . .  22
     4.4.  Back-pressure . . . . . . . . . . . . . . . . . . . . . .  22
     4.5.  Associations, Transients, Racing, and Rendezvous  . . . .  22
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27




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1.  Introduction

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

   In the meantime, the nature of Internet access, and the variety of
   Internet transport protocols, is evolving.  The challenges that new
   protocols and access paradigms present to the sockets API and to
   programming models based on them inspire the design 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) [RFC6824], 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 [RFC7258].  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 [I-D.ietf-quic-transport]) to mitigate
   this strict layering.





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   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 [RFC4960] 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:

   o  Explicit Message orientation, with framing and atomicity
      guarantees for Message transmission.

   o  Asynchronous reception, allowing all receiver-side interactions to
      be event-driven.

   o  Explicit support for multistreaming and multipath transport
      protocols and network architectures.

   o  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 [RFC6555] and peer-to-peer rendezvous
      [RFC5245].





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   o  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 [I-D.ietf-taps-transports].

   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) [RFC4960], TCP Minion
   [I-D.iyengar-minion-protocol], and MinimaLT [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.

2.  Abstractions and Terminology






























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           +===============+
           |    Message    |
           +===============+
                |    ^         |               |
          send()|    |ready()  |initiate()     |listen()
                V    |         V               V
           +=====================+           +============+
           |                     |  accept() |            |
           |      Carrier        |<----------|  Listener  |
           |                     |           |            |
           +=====================+           +============+
            |1        |        n|                  |          +=========+
            |         |         |1                 |      +---|  Local  |
            |   +=========+   +=======================+   |   +=========+
            |   | Policy  |n  |                       |---+
            |   | Context |---|      Association      |       +=========+
            |   |         |  1|                       |-------|  Remote |
            |   +=========+   +=======================+       +=========+
            |         |                1| durable end-to-end
            +-------+ |                 | state via many paths,
                    | |                 | policies, and prefs
                   n| |                n|
               +===========+       +==========+
     ephemeral |           |       |          |
   transport & | Transient |-------|   Path   | properties of
  crypto state |           |n     1|          | address pair
               +===========+       +==========+


         Figure 1: Abstractions and relationships in Post Sockets

   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 Figure 1 and detailed in this section.

2.1.  Message Carrier

   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



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   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 Section 2.7) 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 Section 2.3), and it may be able to
   reactivated, but messages cannot be sent and received immediately.

   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 Section 2.4 and local (see
   Section 2.5) 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.

   o  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.

   o  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.





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   o  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.

   o  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.

2.2.  Message

   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,



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   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
   Section 2.1).

   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:

   o  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.




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      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.

   o  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.

   o  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.

   o  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).

   o  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




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   Message has expired before transmission/acknowledgement.  Not all
   transport protocol stacks will support all of these events.

2.3.  Association

   An Association contains the long-term state necessary to support
   communications between a Local (see Section 2.5) and a Remote (see
   Section 2.4) 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 Policy Context (see Section 2.6) to constrain
   the selection of transport protocols and local interfaces to create
   Transients (see Section 2.7) to carry Messages; and information about
   the paths through the network available available between them (see
   Section 2.8).

   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 [I-D.kuehlewind-taps-crypto-sep]
   for more details about this separation.

2.4.  Remote

   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




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   (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.

2.5.  Local

   A Local represents all the information about the local endpoint
   necessary to establish an Association or a Listener: interface, port,
   and transport protocol stack information, and, per
   [I-D.pauly-taps-transport-security], cryptographic identities
   (certificates and associated private keys) bound to this endpoint.

2.6.  Policy Context

   The Policy Context describes preferences for, and restrictions on,
   how to configure Transients to support communication between a Local
   and a Remote over one or more Paths between endpoints.  For instance,
   an application may require, or prefer to use, certain features (see
   [I-D.ietf-taps-transports]) of the transport protocol stacks used by
   the Transients underlying the Carrier.  Alternatively, it might also
   prefer Paths over one interface to those over another (e.g., WiFi
   access over LTE when roaming on a foreign LTE network, due to cost).

   These policies are expressed in the Policy Context(s) that are bound
   to the Association.  Multiple policy contexts can be active at once.
   For example, a system Policy Context can express the administrative
   preferences around network interface and protocol selection, while an
   application Policy Context expresses preferences for use of different
   transport services.  Expression of policy contexts and the resolution
   of conflicts among Policy Contexts is currently implementation-
   specific (the Policy API in the NEAT architecture [NEAT] provides an
   example of how this can be done).







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2.7.  Transient

   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 single Path 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.

2.8.  Path

   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 [RFC7556] 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:

   o  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.

   o  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.

   o  Loss Probability Expectation: expected probability of a loss of
      any given single frame along the Path.  Generally provided by




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      inline measurements performed by the transport layer, as opposed
      to signaled by path elements.

   o  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.

   o  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.

   o  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.

3.  Abstract Programming Interface

   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:

   o  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.

   o  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.




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   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.

3.1.  Example Connection Patterns

   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.





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3.1.1.  Client-Server

   Here's an example client-server application.  The server echoes
   messages.  The client sends a message and prints what it receives.

   The client in Figure 2 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.

   // connect to a server given a remote
   func sayHello() {

       carrier := Initiate(local, remote)

       carrier.Send([]byte("Hello!"))
       carrier.Ready(func (msg InMessage) {
           fmt.Println(string([]byte(msg))
           return false
       })
       carrier.Close()
   }

                         Figure 2: Example client

   The server in Figure 3 creates a Listener, which accepts Carriers and
   passes them to a server.  The server echos the content of each
   message it receives.

   // run a server for a specific carrier, echo all its messages
   func runMyServerOn(carrier Carrier) {
       carrier.Ready(func (msg InMessage) {
           carrier.Send(msg)
       })
   }

   // accept connections forever, spawn servers for them
   func acceptConnections() {
       listener := Listen(local)
       listener.Accept(func(carrier Carrier) bool {
           go runMyServerOn(carrier)
           return true
       })
   }

                         Figure 3: Example server





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   The Responder allows the server to be significantly simplified, as
   shown in Figure 4.

   func echo(msg InMessage, reply Sink) {
       reply.Send(msg)
   }

   Respond(local, echo)

                        Figure 4: Example responder

3.1.2.  Client-Server with Happy Eyeballs and 0-RTT establishment

   The fundamental design of a client need not change at all for happy
   eyeballs [RFC6555] (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 [RFC7413], the client
   must mark the outgoing message as idempotent.

// connect to a server given a remote and send some 0-RTT data
func sayHelloQuickly() {

    carrier := Initiate(local, remote)

    carrier.SendMsg(OutMessage{Content: []byte("Hello!"), Idempotent: true}, nil, nil, nil)
    carrier.Ready(func (msg InMessage) {
        fmt.Println(string([]byte(msg)))
        return false
    })
    carrier.Close()
}

3.1.3.  Peer to Peer with Network Address Translation

   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
   [RFC5245].  Here, each peer does its own Initiate call
   simultaneously, and the result on each side is a Carrier attached to
   an appropriate Association.

3.1.4.  Multicast Receiver

   A multicast receiver is implemented using a Sink attached to a Local
   encapsulating a multicast address on which to receive multicast




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   datagrams.  The following example prints messages received on the
   multicast address forever.

   func receiveMulticast() {
       sink = NewSink(local)
       sink.Ready(func (msg InMessage) {
           fmt.Println(string([]byte(msg)))
           return true
       })
   }

3.2.  Association Bootstrapping

   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
   [I-D.pauly-taps-transport-security], 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.

   // create security parameters with a set of trusted certificates
   func createParameters(trustedCerts []Certificate) Parameters {
       parameters := Parameters()
       parameters = parameters.SetTrustedCerts(trustedCerts)
       return parameters
   }

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

// create an Association using shared parameters
func createAssociation(local Local, remote Remote, parameters Parameters) Association {
    association := AssociationWithParameters(local, remote, parameters)
    return association
}

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









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// create an Association using a pre-shared key
func createAssociationWithPSK(local Local, remote Remote, parameters Parameters, preSharedKey []byte) Association {
    association := AssociationWithParameters(local, remote, parameters)
    association = association.SetPreSharedKey(preSharedKey)
    return association
}

   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.

// open a connection to a server using an existing Association and send some data,
// which will be sent early if possible.
func sayHelloWithAssociation(association Association) {
    carrier := InitiateWithAssociation(association)

    carrier.SendMsg(OutMessage{Content: []byte("Hello!"), Idempotent: true}, nil, nil, nil)
    carrier.Ready(func (msg InMessage) {
        fmt.Println(string([]byte(msg)))
        return false
    })
    carrier.Close()
}

4.  Implementation Considerations

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

4.1.  Protocol Stack Instance (PSI)

   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
   [I-D.ietf-taps-transports].

   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.







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   +=========+    +=========+   +==========+      +==========+
   | Carrier |    | Carrier |   | Carrier  |      | Carrier  |
   +=========+    +=========+   +==========+      +==========+
        |               |              |                 |
   +=========+    +=========+   +==========+      +==========+
   |Transient|    |Transient|   |Transient |      |Transient |
   +=========+    +=========+   +==========+      +==========+
        |                  \     /                 /        \
   +=========+           +=========+      +=========+      +=========+
   |   PSI   |           |   PSI   |      |   PSI   |      |   PSI   |
   +===+-----++          +===+-----++     +===+-----++    ++-----+===+
       |TLS   |              |SCTP  |         |TLS   |    |   TLS|
       |TCP   |              |DTLS  |         |TCP   |    |   TCP|
       |IPv6  |              |UDP   |         |IPv6  |    |  IPv4|
       |802.3 |              |IPv6  |         |802.11|    |802.11|
       +------+              |802.3 |         +------+    +------+
                             +------+
   (a) Transient  (b) Carrier multiplexing   (c) Multiple candidates
    bound to PSI   over a multi-streaming     racing during session
                   transport protocol         establishment

                Figure 5: Example Protocol Stack Instances

   For example, Figure 5(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
   Figure 5(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 Figure 5(c).
   Here, the losing PSI in a happy-eyeballs race will be terminated, and
   the carrier will continue using the winning PSI.

4.2.  Message Framing, Parsing, and Serialization

   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



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   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:

   o  the ability to associate deadlines with messages, for transports
      that care about timing;

   o  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;

   o  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



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   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.

4.3.  Message Size Limitations

   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 [RFC4960] and TLS
   [I-D.ietf-tls-tls13] 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 are currently configured out of band, and are probably best
   exposed through the policy context.

   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.

4.4.  Back-pressure

   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.

4.5.  Associations, Transients, Racing, and Rendezvous

   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




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   of connection establishment, especially for peer-to-peer
   applications.

   Post Sockets accordingly breaks communication establishment down into
   multiple phases:

   o  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
      [RFC5245].  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.

   o  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.



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      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.

   o  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 [RFC6555], or it could involve a STUN
      exchange to establish peer-to-peer UDP connectivity [RFC5245], or
      some other means.

   o  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.








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5.  Acknowledgments

   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.

6.  References

6.1.  Normative References

   [I-D.ietf-taps-transports]
              Fairhurst, G., Trammell, B., and M. Kuehlewind, "Services
              provided by IETF transport protocols and congestion
              control mechanisms", draft-ietf-taps-transports-14 (work
              in progress), December 2016.

6.2.  Informative References

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-05 (work
              in progress), August 2017.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

   [I-D.iyengar-minion-protocol]
              Jana, J., Cheshire, S., and J. Graessley, "Minion - Wire
              Protocol", draft-iyengar-minion-protocol-02 (work in
              progress), October 2013.

   [I-D.kuehlewind-taps-crypto-sep]
              Kuehlewind, M., Pauly, T., and C. Wood, "Separating Crypto
              Negotiation and Communication", draft-kuehlewind-taps-
              crypto-sep-00 (work in progress), July 2017.





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   [I-D.pauly-taps-transport-security]
              Pauly, T. and C. Wood, "A Survey of Transport Security
              Protocols", draft-pauly-taps-transport-security-00 (work
              in progress), July 2017.

   [I-D.trammell-plus-abstract-mech]
              Trammell, B., "Abstract Mechanisms for a Cooperative Path
              Layer under Endpoint Control", draft-trammell-plus-
              abstract-mech-00 (work in progress), September 2016.

   [I-D.trammell-plus-statefulness]
              Kuehlewind, M., Trammell, B., and J. Hildebrand,
              "Transport-Independent Path Layer State Management",
              draft-trammell-plus-statefulness-03 (work in progress),
              March 2017.

   [MinimaLT]
              Petullo, W., Zhang, X., Solworth, J., Bernstein, D., and
              T. Lange, "MinimaLT, Minimal-latency Networking Through
              Better Security", May 2013.

   [NEAT]     Grinnemo, K-J., Tom Jones, ., Gorry Fairhurst, ., David
              Ros, ., Anna Brunstrom, ., and . Per Hurtig, "Towards a
              Flexible Internet Transport Layer Architecture", June
              2016.

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

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <https://www.rfc-editor.org/info/rfc5245>.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <https://www.rfc-editor.org/info/rfc6555>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
              <https://www.rfc-editor.org/info/rfc6824>.



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   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
              <https://www.rfc-editor.org/info/rfc7556>.

Appendix A.  Open Issues

   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

Authors' Addresses

   Brian Trammell
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: ietf@trammell.ch


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

   Email: csp@csperkins.org


   Tommy Pauly
   Apple Inc.
   1 Infinite Loop
   Cupertino, California 95014
   United States of America

   Email: tpauly@apple.com






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   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

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


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

   Email: cawood@apple.com



































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