Movatterモバイル変換

Network Working Group                                          S. CrockerRequest for Comments: 36                                    16 March 1970Protocol NotesI Overview  --------   The network protocol provides three facilities:         1.  Connection establishment         2.  Flow control         3.  Reconnection   Reconnection is considered separately from connection establishment   partly because of the complexity of reconnection and partly because I   don't have enough experience with the protocol to present these   concepts in an integrated fashion.   Connection Establishment   ------------------------   Connection establishment works essentially the same as in NWG/RFC   #33.  The major change is that a more general form of switching is   provided independently of establishment, so establishment is   simplified by not including switching procedures.   A rough scenario for connection establishment follows:   1.  Process PA in host A grabs socket SA and requests connection with       socket SB.  Process PA accomplishes this through a system call.   2.  Concurrently with the above, process PB in host B grabs socket SB       and requests connection with socket SA.   3.  In response to process PA's request, the network control program       in host A (referred to as NCPA) sends a Request-for-Connection       (RFC) command to host B.  NCPB in host B sends a similar command       to host A.  No ordering is implied: NCPB may send the command to       NCPA before or after receiving the command from NCPA.   4.  NCPA and NCPB are both aware the connection is established when       each has received a RFC command and each has received the RFNM       for the one it has sent.  They then notify processes PA and PB,       respectively, that the connection is established.Crocker                                                         [Page 1]

RFC 36                       Protocol Notes                   March 1970   One of the rules adhered to is that either SA is a send socket and SB   is a receive socket or vice versa.  This condition is sometimes   stated as "SA and SB must be  a send/receive pair."   5.  The sending process may now send.   Flow Control   ------------   In order to prevent a sending process from flooding a receiving   processes it is necessary for the receiving process to be able to   stop the flow(*).  Flow control is integrated into the network RFNM   handling.  When a receiving host wishes to inhibit flow on a   particular link, the host sends a special message to its IMP which   causes the next RFNM on that link to be modified.  The sending host   interprets this message as a RFNM and as a request to stop sending.   A confirming control command is returned.   When the receiving host is ready to receive again, it sends a command   (RSM) telling the sending host to resume sending.   Reconnection   ------------   For a great many reasons it is desirable to be able to switch one (or   both) ends of a connection from one socket to another.  Depending   upon the restrictions placed upon the switching process, it may be   easy or hard to implement.  To achieve maximum generality, I present   here a scheme for dynamic reconnection, which means that reconnection   can take place even after flow has started.  It may turn out that for   the majority of cases, this scheme is much more expensive than it   needs to be; however, the following virtues are claimed:      1. All various forms of switching connections are provided.      2. Reconnection introduces no overhead in the processing of         messages sent over a connection i.e., the whole cost is borne         in processing the protocol.   ---------------------------------------------------   *BB&N argues that unlimited buffering should be provided.  It is   possible that this would be a proper strategy: but it is foreign to   my way of thinking, and I have based the protocol design on the   assumption that only a small buffer is provided on the receive end of   each connection.Crocker                                                         [Page 2]

RFC 36                       Protocol Notes                   March 1970II  Data Structures    ---------------    1.  Connection Table    2.  Process Table    3.  Input Link Table    4.  Output Link Table    5.  Link Assignment Table   Connection Table   ----------------   This holds all information pertaining to local sockets, particularly   whether a socket is engaged in a connection, and if so, what state   the connection is in.  Entries are keyed by local socket, but other   tables have pointers into this table also.  (See the Process Table,   Input Link Table, and Output Link Table.)   Each entry contains the following information:         a)  local socket (key)         b)  foreign socket         c)  link         d)  connection state         e)  flow state and buffer control         f)  pointer to user's process         g)  reconnection control state         h)  queue of waiting callers   The local socket is a 32 bit number.  If no entry exists for a   particular socket, it may be created with null values.   The foreign socket is a 40 bit number.  This field will be unassigned   if no connection is established.   The link is an 8 bit number and is the link over which data is sent   from the sender to the receiver.  A socket is a receive socket iff   its low-order bit is zero.   Connection state refers to whether a connection is open or not, etc.   The following possibilities may occur.         a)  local process has requested a connection         b)  foreign process(es) has/have requested a connection         c)  connection established         d)  reconnection in progress         e)  close waiting         f)  reconnection waitingCrocker                                                         [Page 3]

RFC 36                       Protocol Notes                   March 1970   Flow state and buffer control refer to checking for RFNM's sending   and accepting cease, suspend and resume commands, and keeping track   of incoming or outgoing data.   A pointer to the user's process is necessary if the process has   requested a connection.   If reconnection is in progress, it is necessary to keep track of the   sequence of events.  A socket engaged in reconnection is either an   end or a middle.  If it's a middle, it is necessary to store the   eight bit name of the other middle attached to the same process, and   to record receipt of END and RDY commands.   Finally, if RFC's are received either when the socket is busy or when   no process has engaged it, the RFC's are stacked first-in-first-out   on a queue for the named local socket.   Process Table   -------------   This table associates a process with a socket.  It is used to process   system calls.   Input Link Table   ----------------   This table associates receive links with local sockets.  It is used   to decide for whom incoming messages are destined.   Output Link Table   -----------------   This table associates send links with local sockets.  It is used to   interpret RFNM's and RSM commands.   Link Assignment Table   ---------------------   Links are assigned by receivers.  This table shows which links are   free.Crocker                                                         [Page 4]

RFC 36                       Protocol Notes                   March 1970III   Control Commands      ----------------                          Command Summary   0         <NOP>   1         <RFC> <me> <you>   or   <RFC> <me> <you> <link>   2         <CLS> <me> <you>   3         <RSM> <link>   4         <SPD> <link>   5         <FND> <me> <you> <asker>   6         <END> <link> <end>   7         <RDY> <link>   8         <ASG> <me> <you> <link>                               CommandsNo Operation   Form:    NOP            NOP  is X'00'   Purpose:  This command is included for completeness and             convenience.Request for connection   Form:    <RFC> <my socket>  <your socket>     or     <RFC> <my socket>  <your socket>  <link>            <RFC> is X'01'            <my socket> is a 32 bit socket number local to the            sender            <your socket> is a 32 bit socket number local to the            receiver            <link> is an eight bit link number.            <my socket> and your socket must be a send/receive pair.            <link> is included if and only if <my socket> is a            receive socket   Purpose:  This command is used to initiate a connection.  When             two hosts have exchanged  RFC  commands with the same             arguments (reversed), the connection is established.             Links are assigned by the receiver.Crocker                                                         [Page 5]

RFC 36                       Protocol Notes                   March 1970Close   Form:    <CLS> <my socket> <your socket>            <CLS> is X'02'            <my socket> and <your socket> are the same as for <RFC>   Purpose:  This command is used to block a connection.  It may             also be used to abort the establishment of a connection             or to refuse a request.  It may happen that no             connection between the named sockets was established,             or was in the process of being established.  In this             event, the <CLS> should be discarded.Resume   Form:    <RSM> <link>            <RSM> is X'03'   Purpose:  This command is sent by a receiving host to cause the             sending host to resume transmission on the named link.             A sending host suspends sending if it receives a             special RFNM for some message.  (Special RFNM's are             generated by the receiving IMP upon request by its             host.)Suspended   Form:    <SPD> <link>            <SPD> is X'04'   Purpose:  This command is sent by a sending host to acknowledge             that it has stopped sending over the named link.             Transmission will resume if a <RSM> command is             received.Final End   Form:    <FND> <my socket> <your socket> <asker>            <FND> is X'05'            <my socket> is a 32 bit socket number of a socket local            to the sender            <your socket> is a 32 bit socket number of a socket            local to the receiver            <my socket> and <your socket> form a send/receive pair.            A connection should be established between them.            <asker> is a 40 bit socket number of the same type as            <my socket>Crocker                                                         [Page 6]

RFC 36                       Protocol Notes                   March 1970   Purpose:  If a process decides to short-circuit itself by connecting             one of its receive sockets to one of its send sockets, the             NCP sends out two <FND> commands -- one in each direction.             Each one has <asker> initialized to <my socket>.             Upon receiving an <FND> command, the NCP checks its <your             socket>.  If <your socket> is already engaged in a             reconnection, the command is passed on with a new <my             socket> and <your socket>.  However, before it is passed             on, the <asker> is compared with the new <my socket>.  If             they are equal, a loop has been detected and both sockets             are closed.             If <your socket> is not engaged in a reconnection, it is             marked as the end of a chain of reconnections and an <END>             is sent back.             If the connection named is not in progress, a <CLS> is sent             back and the <FND> is discarded.End Found   Form:    <END> <link> <end socket>            <END> is X'06'            <link> is an 8 bit link            <end socket> is a 40 bit socket   Purpose:  This command indicates which socket is at the end of a             chain of reconnections.  It is generated at <end             socket> and passed back to the other terminal socket             via all the intermediate sockets.  If <end socket> is a             send socket, <link> refers to a connection with the             send socket in the sending host and the receive socket             in the receiving host.  If <end socket> is a receive             socket, <link> refers to a connection with the send             socket in the receiving host and the receive socket in             the sending hose.  ("sending" end "receiving" refer to             the transmission of this control command.)Crocker                                                         [Page 7]

RFC 36                       Protocol Notes                   March 1970Ready   Form:    <RDY> <link>            <RDY> is X'07'            <link> is an 8 bit link number   Purpose:  This command is sent from a send socket to a receive             socket to indicate that all messages have been             forwarded and that reconnection may occur.Assign New Link   Form:    <ASG> <my socket> <your socket> <link>            <ASG> is X'08'   Purpose:  This command completes a reconnection.  It is sent from             a receive socket to a send socket after the receive             socket has received a <RDY>.  A new link is assigned             and transmission commences.          [ This RFC was put into machine readable form for entry ]           [ into the online RFC archives by Marc Blanchett 3/00 ]Crocker                                                         [Page 8]