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Versions: 00 01 02                                                      
Network Working Group                                         J. Iyengar
Internet-Draft                             Franklin and Marshall College
Intended status: Standards Track                             S. Cheshire
Expires: January 01, 2014                                   J. Graessley
                                                           June 30, 2013

               Minion - Service Model and Conceptual API


   Minion uses TCP-format packets on-the-wire, to provide full
   compatibility with existing NATs, Firewalls, and similar middleboxes,
   but provides a richer set of facilities to the application.  Minion's
   richer facilities include a message-oriented API rather than TCP's
   unstructured byte-stream service model, multiplexing of multiple
   messages (or message streams) on a single connection, interleaving of
   multiplexed messages (to eliminate head-of-line blocking), message
   cancellation, request/reply support, ordered and unordered messages,
   chained messages, multiple priority levels with byte-granularity
   preemption, and DTLS Security.

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 http://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 January 01, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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

1.  Conventions and Terminology Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

2.  Introduction

   Back in 1983 application developers had the choice of UDP [RFC0768]
   or TCP [RFC0793].  UDP preserves message boundaries, but provides no
   reliability, ordering, flow control, or congestion control, and only
   supports small messages (typically UDP packets larger than 1468 bytes
   result in undesirable IP fragmentation).  TCP provides these
   important facilities, and doesn't impose any message size limit --
   but only because it doesn't have any concept of messages, and doesn't
   claim to preserve message boundaries.  Consequently, whichever base
   protocol the application developer chose, they were left building
   part of the transport-layer solution themselves.

   Thirty years later, in 2013, little has changed.  Application
   developers on mainstream platforms like Android and iOS still have
   the same two choices -- UDP and TCP.

   Attempts to provide richer application facilities have failed to
   achieve widespread adoption.  Protocols like SCTP are not supported
   by mainstream NAT gateways.  Consequently, mainstream apps for
   platforms like Android and iOS don't use SCTP, because it would
   severely limit their real-world deployment.  Consequently, operating
   systems like Android and iOS don't have in-kernel native
   implementations of SCTP, because there's little developer demand for
   a protocol they can't use.  Consequently, there's little incentive
   for NAT gateway vendors to do the work to add support for a protocol
   that's neither supported in the popular operating systems nor used by
   mainstream applications.

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   Like SCTP, Minion goes beyond UDP and TCP by providing richer
   application facilities, making it possible to create applications
   that work better and more reliably (and can be brought to market
   quicker and easier) than is possible when each application has to re-
   create those facilities from scratch every time.

   However, unlike SCTP, Minion provides facilities that can be used by
   an application developer immediately, without having to wait for OS
   support or NAT gateway support.  OS support and NAT gateway support
   can come later, and provide additional incremental improvements.
   This incremental deployment path -- which begins first with the
   application developer who can choose to use Minion and immediately
   reap the benefits of that decision -- is an important property of
   Minion, and removes one of the major obstacles that hindered SCTP

   When used without kernel support, Minion acts like a typical TCP-
   based application protocol, and as such, performs as well as any
   other TCP-based application protocol.  However, unlike most
   application-specific protocols, Minion also offers the potential of
   kernel support giving better low-latency message performance and
   better prioritization.  A general application protocol is unlikely to
   receive special-case kernel support tailored to support that one
   specific application, but as a general-purpose transport protocol
   built to support a wide range of applications, special kernel support
   for Minion is feasible.

   Minion preserves the important properties of TCP, like congestion
   control, while adding a range of richer application facilities:

   Message Oriented
      Rather than an unstructured byte stream, Minion supports messages.
      TCP provides an unstructured byte stream, but virtually every
      application needs to send and receive semantic messages, which
      means that virtually every application needs to build its own
      message framing mechanism on top of TCP.  In contrast, Minion
      respects and preserves semantic boundaries.  If an application
      writes a 27-byte message followed by a 53-byte message, then a
      27-byte message and a 53-byte message are delivered to the
      receiving client, not a single combined block of 80 bytes.

   Arbitrary Size Messages
      While most Minion messages are expected to be small, Minion itself
      imposes no upper limit on message size.  For example, a 6 gigabyte
      movie download could be sent as a single Minion message.  Messages
      do not have to fit in memory.  A very large message can be
      generated incrementally by the sender, and will be delivered
      incrementally to the receiver.

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      Multiple messages can be sent, in both directions, on a single
      Minion connection.  Unlike protocols like HTTP/1.0 where each
      request used a separate connection, many Minion messages can share
      a connection.

      When multiple (possibly large) messages are being sent
      concurrently on a single Minion connection, the connection
      bandwidth is shared round-robin between the messages.  This avoids
      head-of-line blocking, where messages are blocked waiting for a
      large message to complete.

      Messages do not have to be sent to completion.  If either the
      sender or the receiver determines that a message is no longer
      needed, then that single message can be cancelled without having
      to tear down the entire Minion connection.

   Request/Reply Support
      Many application protocols are request/reply-oriented.  Minion
      facilitates this by allowing an outgoing message to be explicitly
      identified as a reply to a previously-received message, which
      causes the reply message to be delivered automatically to the
      appropriate message handler at the receiving end.

      Replies do not have to be delivered in the same order that the
      requests were received.  When multiple (possibly large) replies
      are in flight at the same time, the interleaving and bandwidth-
      sharing described above applies, as it does for all Minion

      Replies can themselves generate further replies, resulting in an
      unbounded back-and-forth of ping-pong messages, each going to the
      appropriate reply handler on the receiving side.

   Unordered Messages
      One of the main arguments that is often presented to justify why a
      particular application protocol is built on UDP instead of TCP is
      that, "UDP is better for 'real time' applications."  The
      supporting reasoning for this is often that, "TCP insists on
      continuing to retransmit data long after the client doesn't need
      any more."  In truth the real problem is not retransmission; it is
      that the conventional TCP APIs don't allow received data to be
      delivered out of order.  Suppose a TCP sender has 50 packets in
      flight at any given time (e.g. the bandwidth x delay product is 75
      kB) then the loss of a single packet causes all 49 following
      packets to stall at the receiver because the API doesn't allow for

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      them to be delivered to the client until the missing packet has
      been received.  A simple kernel extension (in the form of a new
      socket option) removes this limitation, and allows out-of-order
      data to be delivered to the client.  This avoids the problem where
      a single lost TCP segment causes all the following TCP segments to
      be delayed.

      Note that this kernel extension is not *required* for a client to
      use Minion; it it an optional extension that provides better
      performance for real-time applications in situations where there
      is packet loss or reordering.  For many applications it is an
      irrelevant benefit and they can operate perfectly well without it.
      For a few applications it is a significant benefit, and it allows
      Minion to provide the low-latency performance that often drives
      developers to use UDP.

   Receiver Ordering
      Although sometimes it can be desirable to receive messages out of
      order as they arrive, often it is not.  In many cases the
      application cannot usefully use (certain) messages out of order,
      and delivering them potentially out of order would burden the
      application with the task of sorting the messages back into the
      correct order before processing them.  In such cases, it is more
      convenient for the application to have Minion deliver messages to
      it in the right order.  For this reason Minion supports ordered
      messages as well as unordered messages.  Unordered messages and
      ordered messages are supported simultaneously on a single Minion

      Other transport protocols support the notion of multiple message
      streams sharing a single connection.  Minion takes this idea and
      generalizes it to the more expressive notion of Receiver Ordering
      Message Dependencies.  Receiver Ordering Message Dependencies
      indicate that a dependent message must not be delivered before the
      message it depends upon.

      Traditional message streams can be created in Minion by using a
      sequence of Receiver Ordering Message Dependencies: If message B
      is specified to follow message A, and message C is specified to
      follow message B, and so on, then messages A,B,C... form an
      ordered "stream".  Similarly, if at the same time message Q is
      specified to follow message P, and message R is specified to
      follow message Q, and so on, then messages P,Q,R... form another
      independent ordered "stream" of their own.

      In addition to such disjoint ordered streams (A,B,C... and
      P,Q,R...), Receiver Ordering Message Dependencies also allow
      richer relationships to be expressed.  For example, in H.264

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      video, P-frames reference I-frames, but P-frames do not reference
      other P-frames.  If a single P-frame is lost or delayed, it is not
      necessary to delay all subsequent P-frames.  Each P-frame has a
      time it is due to be displayed, and when that time arrives the
      frame should be displayed if possible, even if (or especially if)
      preceding P-frames did not arrive in time.  However, there is no
      benefit in delivering a P-frame to the application before the
      I-frame it depends upon.

      To give another example, a web browser client may need to retrieve
      many resources to display a page, but it cannot display *any* of
      the page until it has received the style sheet.  Consequently it
      would be beneficial if the web browser client could request all of
      the resources it needs, but for each one, indicate that it depends
      on the style sheet resource (or upon some other resource which
      depends by transitive closure on the sheet resource).  This
      dependency information tells the sender that it should not devote
      *any* bytes of available bandwidth to delivering other resources
      until after it has completed sending the all-important style

   Sender Ordering
      Even in cases where the receiver does not have a strict ordering
      requirement, it may still be useful to cause data packets to be
      sent in a favourable order.  For example, with a group of H.264
      video P-frames, the first frame of the group is likely to be
      needed for playback sooner than the last frame of the group.
      Therefore, delivering them all concurrently by sharing bandwidth
      between them may cause the first frame to be delivered too late to
      be played.  In this case the sender uses Sender Ordering to
      indicate that a particular message should follow another message
      on the wire.

      Sender Ordering is more lightweight than Receiver Ordering; it is
      used solely to control the transmission order, and is not
      communicated to the receiver.  If a message is lost or delayed in
      transit then following messages are still delivered to the
      application immediately, except when an explicit Receiver Ordering
      Message Dependency indicates that they should not be.

   Chained Messages
      Minion is intended to be used to deliver messages containing a
      single logical semantic unit.  Although Minion can "stream" a
      message of unbounded size to the receiver, Minion is not generally
      intended to be used to batch multiple logical semantic units into
      single large message, which is then "streamed" to the receiver,
      which then parses the incoming "streamed" message as it arrives
      for the logical semantic units contained within it.  Part of the

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      purpose of Minion is to take the burden of message framing off the
      application writer; treating a single unbounded "streaming" Minion
      message like a TCP connection places the burden of parsing firmly
      back in the hands of the application developer.

      In the event that a logical message contains multiple related
      parts, like a header with an associated body, Minion can
      facilitate this structuring through the use of Chained Messages.

      Chained Messages are substantially similar to Receiver Ordering
      Message Dependencies, except that in addition to controlling the
      order of data transmission on the wire, and the order of message
      delivery to the client, the chained message relationship is also
      exposed to the client application at the receiving end.  Instead
      of being delivered to the Minion connection's main message handler
      function, the way most messages are, Chained Messages are
      delivered instead to the Chained Message handler function of the
      previous message in the chain.

      The Chained Message mechanism allows an application to provide a
      main message handler function that receives and processes the
      "header" portion of each two-part message, and that main message
      handler function in turn provides a different message handler
      function that receives and processes the subsequent "body"

      As with other messages, each component in a message chain can
      optionally generate an explicit reply, which is delivered to the
      reply handler for the originating message.

      If any message in a chain is cancelled by the sender or the
      receiver, then all subsequent messages in that chain are
      implicitly cancelled.

      The sender of a chain of messages may wait at each step for a
      reply confirming that the previous message was acceptable before
      sending the next message of the chain, or it may send the entire
      chain and let the receiver cancel the message chain if an error

   Priority Levels
      While Receiver Ordering, Sender Ordering, and Message Chaining
      allow relationships between messages to be adequately expressed
      where they are known in advance, sometimes there are urgent
      messages that need to be sent at short notice that are not known
      in advance.  For example, consider a music application which is
      streaming out audio data with a generous playback buffer, and then
      the user performs a user-interface operation to change the volume

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      level.  We would like this volume change to be performed as
      promptly as possible, regardless of how much audio data is queued
      up in the transmit buffer.  For this reason Minion supports four
      priority levels.  A higher-priority message can preempt a lower-
      priority message at any arbitrary byte boundary in the lower-
      priority data stream.  (This byte-granularity preemption is made
      possible by the Minion wire protocol [minprot]).

      Minion provides strict priorities, meaning that no lower-priority
      data at all is sent as long as there is higher-priority data
      waiting.  This means that a sustained flow of higher-priority data
      can starve lower-priority data indefinitely.  For this reason,
      Minion priorities are intended to support small amounts of high-
      priority data intermixed with larger amounts of lower-priority
      data.  If the amount of high-priority data exceeds the current
      throughput of the Minion connection then all the available
      throughput will be consumed attempting to meet the high-priority
      data demand, and no lower-priority data will be sent.  If this
      outcome is undesirable to the application, it should ensure that
      it does not generate sustained high-priority data at a rate
      exceeding the network throughput for a prolonged period of time.
      Minion does not attempt to provide proportional or weighted
      bandwidth allocation between different priority levels.

      The Minion model is that if message B has a Receiver Ordering or
      Sender Ordering dependency upon message A, then Minion should not
      expend any available throughput delivering any part of message B
      until after message A has been entirely sent.  Similarly, if
      message C is higher priority than messages A and B, then Minion
      should not expend any available throughput delivering any part of
      messages A or B until after message C has been entirely sent.

   DTLS Security
      Minion includes security support.  Because of the potential for
      out-of-order message reception, Minion uses DTLS (which includes
      an explicit record number) instead of TLS (which assumes strictly-
      ordered delivery over TCP).

3.  Conceptual API

   While different implementations in different languages may provide
   APIs that differ in details and programming model, the common
   conceptual framework of Minion APIs is as follows:

   o  Outbound Connections

      *  Create new Minion Connection to remote peer, with handler
         function or object to receive incoming messages.

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      *  Close Minion Connection when it is no longer needed.

   o  Inbound Connections

      *  Listen on a port for incoming connections.

      *  Upon receipt of incoming connection request, a new Minion
         Connection object (substantially similar to the Outbound Minion
         Connection object above) is generated, and delivered to the

      *  Set handler function or object to handle incoming messages
         received on an inbound connection.

      *  Stop listening for incoming connections.

   o  Sending Messages

      *  Create new Outbound Minion Message, associated with an existing
         connection (either outbound or inbound), specifying the
         priority level for the message.

      *  Optionally, indicate Sender Ordering for this message by
         reference to some previously-created Minion message.

      *  Optionally, indicate Receiver Ordering for this message, or
         Chaining for this message, or that this message is a reply to
         some previously received Minion message.  Note that these three
         options are mutually exclusive.  An outgoing message can be
         identified as a response to a received message, or a subsequent
         member of a multi-part message chain, or a message with a
         Receiver Ordering Message Dependency, but not more than one of
         these three things.

      *  Optionally, provide a reply handler function or object to
         receive replies to this message.

      *  Provide (possibly incomplete) data for the message.

      *  Optionally, add further units of data to the message.

      *  Indicate when message is complete.  This tells the Minion
         implementation layer that it should now send the message.
         Alternatively, a message can also be cancelled if it is no
         longer needed.

      *  Dispose of the message when it is no longer needed.

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   o  Receiving Messages

      *  Upon receipt of a message, a handler function or object is
         handed a new inbound message:

      *  If the message is a chained continuation message, and a
         specific handler exists for that chain, then that specific
         handler is invoked.

      *  Else, if the message is a reply, and a specific handler exists
         for the originating message, then that specific handler is

      *  Else, the Minion Connection's generic message handler is

      *  Read data from the message.  For large messages, this may not
         be the entire message.  After one or more reads, a return code
         (or similar) indicates to the application when the message is
         complete (or alternatively, that is is incomplete, and will not
         be completed, because it has been cancelled by the sender).

      *  The application may decide to reject a message before it has
         been entirely received, by canceling it.

      *  The message handler may generate outbound messages in response
         to the received message, including outbound explicit reply
         messages, outbound chained messages, and simple outbound
         standalone messages.

      *  Dispose of the received message when it is no longer needed.

4.  Client Isolation

   Minion allows multiple messages to share the available throughput of
   a single connection.  The sources of those multiple messages (if not
   the same application) are assumed to be mutually trusting.  Minion
   does not attempt to prevent one message source on a connection from
   consuming an unfair share of the bandwidth, nor does Minion attempt
   to guard against a client that fails to read its messages, causing
   the receive window to close, thereby preventing any messages from
   being received.

   In the event that some proxy or similar technology allows multiple
   mutually untrusting clients to share a single Minion connection, that
   application-layer code that is allowing the single Minion connection
   to be shared is responsible for policing the traffic so that the
   single Minion connection is shared reasonably.

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5.  IANA Considerations

   No IANA actions are required by this document.

6.  Security Considerations

   No new security risks occur as a result of using this protocol.

7.  Acknowledgements

   Many thanks to Bryan Ford, Padma Bhooma and Anumita Biswas for their
   contributions to the development of the Minion API.

8.  References

8.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

   [minprot]  Iyengar, J., "Minion - Wire Protocol", draft-iyengar-
              minion-protocol-00 (work in progress), June 2013.

Authors' Addresses

   Janardhan Iyengar
   Franklin and Marshall College
   Mathematics and Computer Science
   PO Box 3003
   Lancaster, Pennsylvania  17604-3003

   Phone: +1 717 358 4774
   Email: janardhan.iyengar@fandm.edu

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   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014

   Phone: +1 408 974 3207
   Email: cheshire@apple.com

   Josh Graessley
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014

   Phone: +1 408 974 5710
   Email: jgraessley@apple.com

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