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  • Subway Synchronization Protocol

    June 17th, 2015
    kids, baby, tech, transit  [html]
    Transit Rider Protocols Working Group    J. Kaufman
    Request for Comments                      June 2015
    Category: Standards Track
    
    
         Subway Synchronization Protocol -- SSP 1.0
         ------------------------------------------
    
    Status of this Memo
    
       This document specifies a standards track
       protocol for the Transit Rider community, and
       requests discussion and suggestions for
       improvements.  Please refer to the current
       edition of the "Transit Rider Official Protocol
       Standards" for the standardization state and
       status of this protocol.  Distribution of this
       memo is unlimited.
    
    Abstract
    
       The Subway Synchronization Protocol (SSP) is a
       single-purpose stateful user-level protocol for
       distributed coordination among convergent
       parties of subway riders.
    
       SSP has been in daily use since 2014.  This
       specification defines the protocol referred to
       as "SSP 1.0".
    
    Table of Contents
    
       1   Introduction ..............................2
       1.1    Purpose ................................2
       1.2    Requirements ...........................2
       1.3    Terminology ............................2
       1.4    Notation ...............................3
       1.5    Overall Operation ......................3
       2   Structure .................................4
       2.1    Notification ...........................4
       2.2    Acknowledgement ........................4
       2.3    Resending ..............................4
       2.4    Exceptions .............................4
       3   Notifications .............................4
       3.1    Carriage Designation ...................4
       3.2    Stop Arrival ...........................5
       4   Handling Notifications ....................5
       5   Merging ...................................5
    
    
    
    
    
    
    
    J. Kaufman         Standards Track         [Page 1]
    ---------------------------------------------------
    
    RFC    Subway Synchronization Protocol    June 2015
    
    
    1 Introduction
    
    1.1 Purpose
    
       The Subway Synchronization Protocol (SSP) is a
       single-purpose stateful user-level protocol for
       distributed coordination among parties
       attempting to converge on the same subway
       carriage.
    
    1.2 Requirements
    
       The key words "MUST," "MUST NOT," "REQUIRED,"
       "SHALL," "SHALL NOT," "SHOULD," "SHOULD NOT,"
       "RECOMMENDED," "MAY," and "OPTIONAL" in this
       document are to be interpreted as described in
       RFC 2119.
    
    1.3 Terminology
    
       This specification uses a number of terms to
       refer to the roles played by participants in,
       and objects of, the SSP communication.
    
       party
          A group of one or more people traveling as a
          unit.
    
       communicator
          The member of a party responsible for
          inter-party communication.  The method of
          selection is left unspecified by SSP, but
          each party MUST have exactly one
          communicator.
    
       carriage
          A vehicle for the transportation of parties.
    
       station
          A place where parties may enter or exit a
          carriage.
    
       train
          An ordered series of carriages moving as a
          unit between stations.
    
       message
          A sequence of human-readable characters
          composed by a communicator.
    
    
    
    J. Kaufman         Standards Track         [Page 2]
    ---------------------------------------------------
    
    RFC    Subway Synchronization Protocol    June 2015
    
    
       initiator
          The communicator of the party that boards the
          train first.
    
       joiner
          The communicator of the party that attempts
          to, at a subsequent station, board the same
          train and carriage as the initiator.
    
       notification
          A message sent from the initiator to the
          joiner to assist the joiner in their task.
    
       acknowledgement
          A message sent from the joiner to the
          initiator confirming receipt.
    
    1.4 Notation
    
       When referring to a message, the message
       contents are set off in square brackets.  For
       example, [example message].  The square brackets
       are notation only, and not part of the message.
    
    1.5 Overall Operation
    
       The SSP protocol is a stateful protocol with a
       request-acknowledgement structure.  By sending a
       series of notifications an initiator can provide
       a joiner with sufficient information that the
       joiner can arrange for the parties to merge.
    
       Prior to use of SSP the communicators MUST have
       agreed through some out-of-band method on which
       stations the initiator and joiner will board the
       train.  These stations SHOULD have at least two
       other stations between them for reliable
       operation, and MUST have at least one.
    
       SSP communication usually takes place over Short
       Message Service (SMS), but other transports can
       be used.  The transport is not required to be
       reliable, but the correct operation of this
       protocol depends on low message loss and latency.
    
    
    
    
    
    
    
    
    J. Kaufman         Standards Track         [Page 3]
    ---------------------------------------------------
    
    RFC    Subway Synchronization Protocol    June 2015
    
    
    2 Structure
    
    2.1 Notification
    
       At several points described below, the initiator
       sends a message to the joiner indicating which
       point they have reached.  This message MUST be
       one of the standard statements described in
       Section 3.
    
    2.2 Acknowledgement
    
       When the joiner receives a message from the
       initiator they MUST send back the message [OK]
       to indicate that they have received the message
       and are taking appropriate action.
    
    2.3 Resending
    
       If the joiner sends a notification and does not
       receive an acknowledgement within 20 seconds,
       and has not already sent this notification three
       times, they MUST retransmit the notification.
    
    2.4 Exceptions
    
       Failure to receive an acknowledgement after
       three transmissions of a notification, or
       receipt of any message that is invalid under SSP
       indicates an abort.  After an abort the
       communicator SHOULD interpret the triggering
       message and future messages, if any, using
       whatever protocol they would otherwise use for
       the underlying transport.
    
    3 Notifications
    
    3.1 Carriage Designation
    
       When boarding the train the initiator MUST
       choose either the initial or final carriage.
       Other carriages offer a risk of confusion that
       we consider excessive.  An initiator who has
       boarded the initial carriage SHALL transmit
       [First car], while one boarding the final
       carriage SHALL transmit [Last car].
    
       Note that this notification also serves to
       indicate departure from the initial station.
    
    
    
    J. Kaufman         Standards Track         [Page 4]
    ---------------------------------------------------
    
    RFC    Subway Synchronization Protocol    June 2015
    
    
    3.2 Station Arrival
    
       Upon arriving at a station, the initiator MAY
       transmit the name of the station.  If the
       station immediately preceeds the agreed-on
       merger station they MUST send such notification.
       Sending notifications for the last two stations
       is RECOMMENDED.
    
       The station name MAY be abbreviated to a single
       letter in cases where this is not ambiguous.
       For example, upon arriving at Harvard station an
       initiator could send either [Harvard] or [H].
    
    4 Handling Notifications
    
       On receieving each notification the joiner
       knows the position of the train and SHOULD
       estimate:
    
          - the time required for their party to travel
            to the agreed-on station
    
          - the time the train will take to travel from
            the next station at which notification is
            mandatory to the agreed-on station.
    
       If the second time is less than the first, the
       joiner SHOULD begin travel to the station.
    
    4 Merging
    
       After the joiner and their party arrive at the
       agreed on station they SHOULD wait opposite
       either the initial or final carriage, as
       communicated in the carriage designation
       notification.  When each train arrives they
       SHOULD visually examine it for the initiator
       party, and board if and only if visual
       identification is successful.
    
       If the initiator arrives at the station and
       does not see the joiner, they SHOULD exit their
       carriage and wait opposite it until the joiner
       arrives.
    
       When the two parties have converged they merge
       to become a single party, the communicators lose
       their designated roles, and any future messages
       on the underlying transport SHOULD NOT be
       interpreted under SSP.
    
    
    
    
    J. Kaufman         Standards Track         [Page 5]
    
    

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