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Specification of Internet Transmission Control Program :: RFC0675

Network Working Group                                        Vinton Cerf
Request for Comments: 675                                    Yogen Dalal
NIC: 2                                                     Carl Sunshine
INWG: 72                                                   December 1974


                         December 1974 Version


   This document describes the functions to be performed by the
   internetwork Transmission Control Program [TCP] and its interface to
   programs or users that require its services. Several basic
   assumptions are made about process to process communication and these
   are listed here without further justification. The interested reader
   is referred to [CEKA74, TOML74, BELS74, DALA74, SUNS74] for further

   The authors would like to acknowledge the contributions of R.
   Tomlinson (three way handshake and Initial Sequence Number
   Selection), D. Belsnes, J. Burchfiel, M. Galland, R. Kahn, D. Lloyd,
   W. Plummer, and J. Postel all of whose good ideas and counsel have
   had a beneficial effect (we hope) on this protocol design.  In the
   early phases of the design work, R. Metcalfe, A. McKenzie, H.
   Zimmerman, G. LeLann, and M. Elie were most helpful in explicating
   the various issues to be resolved. Of course, we remain responsible
   for the remaining errors and misstatements which no doubt lurk in the
   nooks and crannies of the text.

   Processes are viewed as the active elements of all HOST computers in
   a network. Even terminals and files or other I/O media are viewed as
   communicating through the use of processes. Thus, all network
   communication is viewed as inter-process communication.

   Since a process may need to distinguish among several communication
   streams between itself and another process [or processes], we imagine
   that each process may have a number of PORTs through which it
   communicates with the ports of other processes.

   Since port names are selected independently by each operating system,
   TCP, or user, they may not be unique. To provide for unique names at
   each TCP, we concatenate a NETWORK identifier, and a TCP identifier
   with a port name to create a SOCKET name which will be unique
   throughout all networks connected together.

Cerf, Dalal & Sunshine                                          [Page 1]
RFC 675              Specification of Internet TCP         December 1974

   A pair of sockets form a CONNECTION which can be used to carry data
   in either direction [i.e. full duplex]. The connection is uniquely
   identified by the  address pair, and
   the same local socket can participate in multiple connections to
   different foreign sockets [see Section 2.2].

   Processes exchange finite length LETTERS as a way of communicating;
   thus, letter boundaries are significant. However, the length of a
   letter may be such that it must be broken into FRAGMENTS before it
   can be transmitted to its destination. We assume that the fragments
   will normally be reassembled into a letter before being passed to the
   receiving process. Throughout this document, it is legitimate to
   assume that a fragment contains all or a part of a letter, but that a
   fragment never contains parts of more than one letter.

   We specifically assume that fragments are transmitted from Host to
   Host through means of a PACKET SWITCHING NETWORK [PSN] [ROWE70,
   POUZ73]. This assumption is probably unnecessary, since a circuit
   switched network could also be used, but for concreteness, we
   explicitly assume that the hosts are connected to one or more PACKET

   Processes make use of the TCP by handing it letters. The TCP breaks
   these into fragments, if necessary, and then embeds each fragment in
   an INTERNETWORK PACKET. Each internetwork packet is in turn embedded
   in a LOCAL PACKET suitable for transmission from the host to one of
   its serving PS. The packet switches may perform further formatting or
   other operations to achieve the delivery of the local packet to the
   destination Host.

   The term LOCAL PACKET is used generically here to mean the formatted
   bit string exchanged between a host and a packet switch. The format
   of bit strings exchanged between the packet switches in a PSN will
   generally not be of concern to us. If an internetwork packet is
   destined for a TCP in a foreign PSN, the packet is routed to a
   GATEWAY which connects the origin PSN with an intermediate or the
   destination PSN. Routing of internetwork packets to the GATEWAY may
   be the responsibility of the source TCP or the local PSN, depending
   upon the PSN Implementation.

   One model of TCP operation is to imagine that there is a basic
   GATEWAY associated with each TCP which provides an interface to the
   local network. This basic GATEWAY performs routing and packet
   reformatting or embedding, and may also implement congestion and
   error control between the TCP and GATEWAYS at or intermediate to the
   destination TCP.

Cerf, Dalal & Sunshine                                          [Page 2]
RFC 675              Specification of Internet TCP         December 1974

   At a GATEWAY between networks, the internetwork packet is unwrapped
   from its local packet format and examined to determine through which
   network the internetwork packet should travel next. The internetwork
   packet is then wrapped in a local packet format suitable to the next
   network and passed on to a new packet switch.

   A GATEWAY is permitted to break up the fragment carried by an
   internetwork packet into smaller fragments if this is necessary for
   transmission through the next network. To do this, the GATEWAY
   produces a set of internetwork packets, each carrying a new fragment.
   The packet format is designed so that the destination TCP may treat
   fragments created by the source TCP or by intermediate GATEWAYS
   nearly identically.

   The TCP is responsible for regulating the flow of internetwork
   packets to and from the processes it serves, as a way of preventing
   its host from becoming saturated or overloaded with traffic. The TCP
   is also responsible for retransmitting unacknowledged packets, and
   for detecting duplicates. A consequence of this error
   detection/retransmission scheme is that the order of letters received
   on a given connection is also maintained [CEKA74, SUNS74]. To perform
   these functions, the TCP opens and closes connections between ports
   as described in Section 4.3. The TCP performs retransmission,
   duplicate detection, sequencing, and flow control on all
   communication among the processes it serves.


2.1  The TCP as a POST OFFICE

   The TCP acts in many ways like a postal service since it provides a
   way for processes to exchange letters with each other. It sometimes
   happens that a process may offer some service, but not know in
   advance what its correspondents' addresses are. The analogy can be
   drawn with a mail order house which opens a post office box which can
   accept mail from any source. Unlike the post box, however, once a
   letter from a particular correspondent arrives, a port becomes
   specific to the correspondent until the owner of the port declares

   In addition to acting like a postal service, the TCP insures end-to-
   end acknowledgment, error correction, duplicate detection,
   sequencing, and flow control.

Cerf, Dalal & Sunshine                                          [Page 3]
RFC 675              Specification of Internet TCP         December 1974

2.2  Sockets and Addressing

   We have borrowed the term SOCKET from the ARPANET terminology
   [CACR70, MCKE73]. In general, a socket is the concatenation of a
   NETWORK identifier, TCP identifier, and PORT identifier. A CONNECTION
   is fully specified by the pair of SOCKETS at each end since the same
   local socket may participate in many connections to different foreign

   Once the connections is specified in the OPEN command [see section
   2.3.2], the TCP supplies a [short] Local Connection Name by which the
   user refers to the connection in subsequent commands. In particular
   this facilitates using connections with initially unspecified foreign

   TCP's are free to associate ports with processes however they choose.
   However, several basic concepts seem necessary in an implementation.
   There must be well known sockets [WKS] which the TCP associates only
   with the "appropriate" processes by some means. We envision that
   processes may "own" sockets, and that processes can only initiate
   connections on the sockets they own [means for implementing ownership
   is a local issue, but we envision a Request Port user call, or a
   method of uniquely allocating a group of ports to a given process,
   e.g. by associating the high order bits of a port name with a given

   Once initiated, a connection may be passed to another process that
   does not own the local socket [e.g. from logger to service process].
   Strictly speaking this is a reconnection issue which might be more
   elegantly handled by a general reconnection protocol as discussed in
   section 3.3. To simplify passing a connection within a single TCP,
   such "invisible" switches may be allowed as in TENEX systems.

   Of course, each connection is associated with exactly one process,
   and any attempt to reference that connection by another process will
   be signaled as an error by the TCP. This prevents stealing data from
   or inserting data into another process' data stream.

   A connection is initiated by the rendezvous of an arriving
   internetwork packet and a waiting Transmission Control Block [TCB]
   created by a user OPEN, SEND, INTERPUPT, or RECEIVE call [see section
   2.3]. The matching of local and foreign socket identifiers determines
   when a successful connection has been initiated. The connection
   becomes established when sequence numbers have been synchronized in
   both directions as described in section 4.3.2.

Cerf, Dalal & Sunshine                                          [Page 4]
RFC 675              Specification of Internet TCP         December 1974

   It is possible to specify a socket only partially by setting the PORT
   identifier to zero or setting both the TCP and PORT identifiers to
   zero. A socket of all zero is called UNSPECIFIED. The purpose behind
   unspecified sockets is to provide a sort of "general delivery"
   facility [useful for logger type processes with well known sockets].

   There are bounds on the degree of unspecificity of socket
   identifiers. TCB's must have fully specified local sockets, although
   the foreign socket may be fully or partly unspecified. Arriving
   packets must have fully specified sockets.

   We employ the following notation:

    x.y.z = fully specified socket with x=net, y=TCP, z=port

    x.y.u = as above, but unspecified port

    x.u.u = as above, but unspecified TCP and port

    u.u.u = completely unspecified

    with respect to implementation, u = 0 [zero]

    We illustrate the principles of matching by giving all cases of
    incoming packets which match with existing TCB's. Generally, both
    the local (foreign) socket of the TCB and the foreign (local) socket
    of the packet must match.

          TCB local   TCB foreign     Packet local    Packet foreign

    (a)     a.b.c       e.f.g           e.f.g           a.b.c

    (b)     a.b.c       e.f.u           e.f.g           a.b.c

    (c)     a.b.c       e.u.u           e.f.g           a.b.c

    (d)     a.b.c       u.u.u           e.f.g           a.b.c

    There are no other legal combinations of socket identifiers which
    match. Case (d) is typical of the ARPANET well known socket idea in
    which the well known socket (a.b.c) LISTENS for a connection from
    any (u.u.u) socket. Cases (b) and (c) can be used to restrict
    matching to a particular TCP or net.

Cerf, Dalal & Sunshine                                          [Page 5]
RFC 675              Specification of Internet TCP         December 1974


2.3.1  A Note on Style

    The following sections functionally define the USER/TCP interface.
    The notation used is similar to most procedure or function calls in
    high level languages, but this usage is not meant to rule out trap
    type service calls [e.g. SVC's, UUO's, EMT's,...].

    The user calls described below specify the basic functions the TCP
    will perform to support interprocess communication. Individual
    implementations should define their own exact format, and may
    provide combinations or subsets of the basic functions in single
    calls. In particular, some implementations may wish to automatically
    OPEN a connection on the first SEND, RECEIVE, or INTERRUPT issued by
    the user for a given connection.

    In providing interprocess communication facilities, the TCP must not
    only accept commands, but also return information to the processes
    it serves. This communication consists of:

    (a) general information about a connection [interrupts, remote
        close, binding of unspecified foreign socket].

    (b) replies to specific user commands indicating success or various
        types of failure.

   Although the means for signaling user processes and the exact format
   of replies will vary from one implementation to another, it would
   promote common understanding and testing if a common set of codes
   were adopted. Such a set of Event Codes is described in section 2.4.

   With respect to error messages, references to "local" and "foreign"
   are ambiguous unless it is known whether these refer to the world as
   seen by the sender or receiver of the error message. The authors
   attempted several different approaches and finally settled on the
   convention that these references would be as seen by the receiver of
   the message.


   Format: OPEN(local port, foreign socket [, timeout])

   We assume that the local TCP is aware of the identity of the
   processes it serves and will check the authority of the process to
   use the connection specified. Depending upon the implementation of
   the TCP, the source network and TCP identifiers will either be
   supplied by the TCP or by the processes that serve it [e.g. the

Cerf, Dalal & Sunshine                                          [Page 6]
RFC 675              Specification of Internet TCP         December 1974

   program which interfaces the TCP to its packet switch or the packet
   switch itself]. These considerations are the result of concern about
   security, to the extent that no TCP be able to masquerade as another
   one, and so on. Similarly, no process can masquerade as another
   without the collusion of the TCP.

   If no foreign socket is specified [i.e. the foreign socket parameter
   is 0 or not present], then this constitutes a LISTENING local socket
   which can accept communication from any foreign socket. Provision is
   also made for partial specification of foreign sockets as described
   in section 2.2.

   If the specified connection is already OPEN, an error is returned,
   otherwise a full-duplex transmission control block [TCB] is created
   and partially filled in with data from the OPEN command parameters.
   The TCB format is described in more detail in section 4.2.2.

   No network traffic is generated by the OPEN command. The first SEND
   or INTERRUPT by the local user or the foreign user will cause the TCP
   to synchronize the connection.

   The timeout, if present, permits the caller to set up a timeout for
   all letters transmitted on the connection. If a letter is not
   successfully transmitted within the timeout period, the user is
   notified and may ignore the condition [TCP will continue trying to
   transmit] or direct the TCP to close the connection. The present
   global default is 30 seconds, and connections which are set up
   without specifying another timeout will retransmit every letter for
   at least 30 seconds before notifying the user. The retransmission
   rate may vary, and is the responsibility of the TCP and not the user.
   Most likely, it will be related to the measured time for responses to
   return from letters sent.

   Depending on the TCP implementation, either a local connection name
   will be returned to the user by the TCP, or the user will specify
   this local connection name (in which case another parameter is needed
   in the call). The local connection name can then be used as a short
   hand term for the connection defined by the  pair.

   Responses from the TCP which may occur as a result of this call are
   detailed in section 2.4.


   Format: SEND(local connection name, buffer address, byte count, EOL
   flag [, timeout])

Cerf, Dalal & Sunshine                                          [Page 7]
RFC 675              Specification of Internet TCP         December 1974

   This call causes the data contained in the indicated user buffer to
   be sent on the indicated connection. If the connection has not been
   opened, the SEND is considered an error. Some implementations may
   allow users to SEND first, in which case an automatic OPEN would be
   done. If the calling process is not authorized to use this
   connection, an error is returned.

   If the EOL flag is set, the data is the End Of a Letter, and the EOL
   bit will be set in the last packet created from the buffer. If the
   EOL f1ag is not set, subsequent SEND's will appear as part of the
   same letter. This extended letter facility should be used sparingly
   because some TCP's may delay processing packets until an entire
   letter is received.

   If no foreign socket was specified in the OPEN, but the connection is
   established [e.g. because a listening connection has become specific
   due to a foreign letter arriving for the local port] then the
   designated letter is sent to the implied foreign socket. In general,
   users who make use of OPEN with an unspecified foreign socket can
   make use of SEND without ever explicitly knowing the foreign socket

   However, if a SEND is attempted before the foreign socket becomes
   specified, an error will be returned. Users can use the STATUS call
   to determine the status of the connection. In some implementations
   the TCP may notify the user when an unspecified socket is bound.

   If the timeout is specified, then the current default timeout for
   this connection is changed to the new one. This can affect not only
   all letters sent including and after this one, but also those which
   have not yet been sent, since the timeout is kept in the TCB and not
   associated with each letter sent. Of course, a time is maintained for
   each internetwork packet formed so as to determine how long each of
   these has been on the retransmission queue.

   In the simplest implementation, SEND would not return control to the
   sending process until either the transmission was complete or the
   timeout had been exceeded. This simple method is highly subject to
   deadlocks and is not recommended. [For example both sides of the
   connection try to do SEND's before doing any RECEIVE's.] A more
   sophisticated implementation would return immediately to allow the
   process to run concurrently with network I/O, and, furthermore, to
   allow multiple SENDs to be in progress concurrently. Multiple SENDs
   are served in first come, first served order, so the TCP will queue
   those it cannot service immediately.

Cerf, Dalal & Sunshine                                          [Page 8]
RFC 675              Specification of Internet TCP         December 1974

   NOTA BENE: In order for the process to distinguish among error or
   success indications for different letters, the buffer address should
   be returned along with the coded response to the SEND request. We
   will offer an example event code format in section 2.4, showing the
   information which should be returned to the calling process.

   The semantics of the INTERRUPT call are described later, but this
   call can have an effect on letters which have been given to the TCP
   but not yet sent. In particular, all such letters are flushed by the
   source TCP. Thus one of the responses to a SEND may be "flushed due
   to interrupt."

   Responses from the TCP which may occur as a result of this call are
   detailed in section 2.4.


   Format: RECEIVE(local connection name, buffer address, byte count)

   This command allocates a receiving buffer associated with the
   specified connection. If no OPEN precedes this command or the calling
   process is not authorized to use this connection, an error is

   In the simplest implementation, control would not return to the
   calling program until either a letter was received, or some error
   occurred, but this scheme is highly subject to deadlocks [see section
   2.3.3]. A more sophisticated implementation would permit several
   RECEIVE's to be outstanding at once, These would be filled as letters
   arrive. This strategy permits increased throughput, at the cost of a
   more elaborate scheme [possibly asynchronous] to notify the calling
   program that a letter has been received.

   If insufficient buffer space is given to reassemble a complete
   letter, an indication that the buffer holds a partial letter will be
   given; the buffer will be filled with as much data as it can hold.

   The remaining parts of a partly delivered letter will be placed in
   buffers as they are made available via successive RECEIVES. If a
   number of RECEIVES are outstanding, they may be filled with parts of
   a single long letter or with at most one letter each. The event codes
   associated with each RECEIVE will indicate what is contained in the

   To distinguish among several outstanding RECEIVES, and to take care
   of the case that a letter is smaller than the buffer supplied, the
   event code is accompanied by both a buffer pointer and a byte count
   indicating the actual length of the letter received.

Cerf, Dalal & Sunshine                                          [Page 9]
RFC 675              Specification of Internet TCP         December 1974

   The semantics of the INTERRUPT system call are discussed later, but
   this call can have an effect on outstanding RECEIVES. When the TCP
   receives an INTERRUPT, it will flush all data currently queued up
   awaiting receipt by the receiving process. If no data is waiting, but
   several buffers have been made available by anticipatory RECEIVE
   commands, these buffers are returned to the process with an error
   indicating that any data that might have been placed in those buffers
   has been flushed. This enables the receiving process to synchronize
   its RECEIVES with the interrupt. That is, the process can distinguish
   between RECEIVES issued before the receipt of the INTERRUPT and these
   issued afterwards.

   Responses from the TCP which may occur as a result of this call are
   detailed in section 2.4.


   Format: CLOSE(local connection name)

   This command causes the connection specified to be closed. If the
   connection is not open or the calling process is not authorized to
   use this connection, an error is returned. Any unfilled receive
   buffers or pending send buffers will be returned to the user with
   event codes indicating they were aborted due to the CLOSE. Users
   should wait for event codes for each SEND before closing the
   connection if they wish to be certain that all letters were
   successfully delivered.

   The user may CLOSE the connection at any time on his own initiative,
   or in response to various prompts from the TCP [remote close
   executed, transmission timeout exceeded, destination inaccessible].

   Because closing a connection requires communication with the foreign
   TCP, connections may remain in the closing state for a short time.
   Attempts to reopen the connection before the TCP replies to the CLOSE
   command will result in errors.

   Responses from the TCP which may occur as a result of this call are
   detailed in section 2.4.


   Format: INTERRUPT(local connection name)

   A special control signal is sent to the destination indicating an
   interrupt condition. This facility can be used to simulate "break"
   signals from terminals or error or completion codes from I/O devices,
   for example. The semantics of this signal to the receiving process

Cerf, Dalal & Sunshine                                         [Page 10]
RFC 675              Specification of Internet TCP         December 1974

   are unspecified. The receiving TCP will signal the interrupt to the
   receiving process immediately upon receipt, and will also flush any
   outstanding letters waiting to be delivered. Since it is possib1e to
   tell where in the letter stream this command was invoked, it is
   possible for the receiving TCP to flush only preceding data. The
   sending TCP will flush any letters pending transmission, returning a
   special error code to indicate the flush.

   If the connection is not open or the calling process is not
   authorized to use this connection, an error is returned.

   Responses from the TCP which may occur as a result of this call are
   detailed in section 2.4.

2.3.7  STATUS

   Format: STATUS(local connection name)

   This command returns a data block containing the following

    local socket, foreign socket, local connection name, receive window,
    send window, connection state, number of letters awaiting
    acknowledgment, number of letters pending receipt [including partial
    ones], default transmission timeout

    Depending on the state of the connection, some of this information
    may not be available or meaningful. If the calling process is not
    authorized to use this connection, an error is returned. This
    prevents unauthorized processes from gaining information about a

    Responses from the TCP which may occur as a result of this call are
    detailed in section 2.4.



    All messages include a type code which identifies the type of user
    call to which the message applies. Types are:

    0 - General message, does not apply to a particular user call

    1 - Applies to OPEN

    2 - Applies to CLOSE

Cerf, Dalal & Sunshine                                         [Page 11]
RFC 675              Specification of Internet TCP         December 1974

    3 - Applies to INTERRUPT

    10 - Applies to SEND

    20 - Applies to RECEIVE

    30 - Applies to STATUS

2.4.2  MESSAGE FORMAT [notional]

    All messages include the following three fields:

      Type code

      Local connection name

      Event code

   For message types 0-3 [General, Open, Close, Interrupt] only these
   three fields are necessary.

   For message type 10 [Send] one additional field is necessary:

      Buffer address

   For message type 20 [Receive] three additional fields are necessary:

      Buffer address

      Byte count

      End-of-letter flag

   For message type 30 [status] additional data might include;

      Local socket, foreign socket

      Send window [measures buffer space at foreign TCP]

      Receive window [measures buffer space at local TCP]

      Connection state [see section 4.3.6]

      Number of letters awaiting acknowledgment

      Number of letters awaiting receipt

      Retransmission timeout

Cerf, Dalal & Sunshine                                         [Page 12]
RFC 675              Specification of Internet TCP         December 1974


   The event code specifies the particular event that the TCP wishes to
   communicate to the user.

   In addition to the event code, three flags may be useful to classify
   the event into major categories and facilitate event processing by
   the user:

      E flag: set if event is an error

      L/F flag: indicates whether event was generated by Local TCP, or
      Foreign TCP or network

      P/T flag: indicates whether the event is Permanent or Temporary
      [retry may succeed]

   Events are encoded into 8 bits with the high order bits set to
   indicate the state of the E, L/F, and P/T flags, respectively.

   Events specified so far are listed below with their codes and flag
   settings. A * means a flag does not apply or can take both values for
   this event. Additional events may be defined in the course of

      0  0**  general success

      1  ELP  connection illegal for this process

      2  OF*  unspecified foreign socket has become bound

      3  ELP  connection not open

      4  ELT  no room for TCB

      5  ELT  foreign socket unspecified

      6  ELP  connection already open
         EFP  unacceptable SYN [or SYN/ACK] arrived at foreign
      TCP. Note: This is not a misprint, the local meaning is different
      from foreign.

      7  EFP  connection does not exist at foreign TCP

      8  EFT  foreign TCP inaccessible [may have subcases]

      9  ELT  retransmission timeout

Cerf, Dalal & Sunshine                                         [Page 13]
RFC 675              Specification of Internet TCP         December 1974

      10 E*P  buffer flushed due to interrupt

      11 OF*  interrupt to user

      12 **P  connection closing

      13 E**  general error

      14 E*P  connection reset

   Possible events for each message type are as follows:

      Type 0[general]: 2,11,12,14

      Type 1[open]: 0,1,4,6,13

      Type 2[close]: 0,1,3,13

      Type 3[interrupt]: 0,1,3,5,7,8,9,12,13

      Type 10[send]: 0,1,3,5,7,8,9,10,11,12,13

      Type 20[receive]: 0,1,3,10,12,13

      Type 30[status]: 0,1,13

   Note that events 6(foreign), 7, 8 are generated at the foreign TCP or
   in the network[s], and these same codes are used in the error field
   of the internet packet [see section 4.2.1].



   It is envisioned that the TCP will be able to support higher level
   protocols efficiently. It should be easy to interface existing
   ARPANET protocols like TELNET and FTP to the TCP.


   At some point, a set of well known 24 bit port numbers must be
   picked. The type of service associated with the well known ports
   might include:

      (a)  Logger

      (b)  FTP (File transfer protocol)

Cerf, Dalal & Sunshine                                         [Page 14]
RFC 675              Specification of Internet TCP         December 1974

      (c)  RJE (Remote job entry)

      (d)  Host status

      (e)  TTY Test

      (f)  HELP - descriptive, interactive system documentation

   WE RESERVE WELL KNOWN SOCKET 0 (24 bits of 0) for global messages
   destined for a particular TCP but not related to any particular
   connection. We imagine that this socket would be used for unusual TCP
   synchronization (e.g. RESET ALL) or for testing purposes (e.g.
   sending letters to TRASHCAN or ECHO). This does not conflict with the
   usage that if a socket is 0, it is unspecified, since no user can
   SEND, CLOSE, or INTERRUPT on socket 0.


   Port identifiers fall into two categories: permanent and transient.
   For example, a Logger process is generally assigned a port identifier
   that is fixed and well known. Transient processes will in general
   have ID's which are dynamically assigned.

   In the distributed processing environment of the network, two
   processes that don't have well known port identifiers may often wish
   to communicate. This can be achieved with the help of a well known
   process using a reconnection protocol. Such a protocol is briefly
   outlined using the communication facilities provided by the TCP. It
   essentially provides a mechanism by which port identifiers are
   exchanged in order to establish a connection between a pair of

   Such a protoco1 can be used to achieve the dynamic establishment of
   new connections in order to have multiple processes solving a problem
   cooperatively, or to provide a user process access to a server
   process via a logger, when the logger's end of the connection can not
   be invisibly passed to the server process.

   A paper on this subject by R. Schantz [SCHA74] discusses some of the
   issues associated with reconnection, and some of the ideas contained
   therein went into the design of the protocol outlined below.

   In the ARPANET, a protocol was implemented which would allow a
   process to connect to a well known socket, thus making an implicit
   request for service, and then be switched to another socket so that
   the well known socket could be freed for use by others. Since sockets

Cerf, Dalal & Sunshine                                         [Page 15]
RFC 675              Specification of Internet TCP         December 1974

   in our TCP are permitted to have connections with more than one
   foreign socket, this facility may not be explicitly needed (i.e.
   connections  and  are distinguishable).

   However. the well known socket may be in one network and the actual
   service socket(s) may be in another network (or at least in another
   TCP). Thus, the invisible switching of a connection from one port to
   another within a TCP may not be sufficient as an "Initial Connection
   Protocol". We imagine that a process wishes to use socket N1.T1.Q to
   access well known socket N2.T2.P. However, the process associated
   with socket N2.T2.P will actually start up a new process somewhere
   which will use N3.T3.S as its server socket. The N(i) and T(i) may be
   distinct or the same. The user will send to N2.T2.P the relevant user
   information such as user name, password, and account. The server will
   start up the server process and send to N1.T1.Q the actual service
   socket ldentif1er: N3.T3.S. The connection (N1.TI.Q,N2.T2.P) can then
   be closed, and the user can do a RECEIVE on (N1.T1.Q,N3.T3.S). The
   serving process can SEND on (N3.T3.S,N1.T1.Q). There are many
   variations on this scheme, some involving the user process doing a
   RECEIVE on a different socket (e.g. (N1.T1.X,U.U.U)) with the server
   doing SEND on (N3.T3.S,N1.T1.X).  Without showing all the detail of
   synchronization of sequence numbers and the like, we can illustrate
   the exchange as shown below.

      USER                             SERVER

                                       1. RECEIVE(N2.T2.P,U.U.U)

      1. SEND (N1.T1.Q,N2.T2.P)==>

                                   <== 2. SEND(N2.T2.P,N1.T1.Q)

                                          With "N3.T3.S" as data

      2. RECEIVE(N1.T1.Q,N2.T2.P)

      3. CLOSE(N1.T1.Q,N2.T2.P)==>

                                   <:= 3. CLOSE(N2.T2.P,N1.T1.Q)

      4. RECEIVE(N1.T1.Q,N3.T3.S)

                                   <== 4. SEND(N3.T3.S,N1.T1.Q)

   At this point, a connection is open between N1.T1.Q and N3.T3.S. A
   variation might be to have the user do an extra RECEIVE on
   (N1.T1.X,U.U.U) and have the data "N1.T1.X" be sent in the first user
   SEND. Then, the server can start up the real serving process and do a

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RFC 675              Specification of Internet TCP         December 1974

   SEND on (N3.T3.S,N1.T1.X) without having to send the "N3.T3.S" data
   to the user. Or perhaps both server and receiver exchange this data,
   to assure security of the ultimate connection (i.e. some wild process
   might try to connect to N1.T1.X if it is merely RECEIVING on foreign
   socket U.U.U.).

   We do not propose any specific reconnection protocol here, but leave
   this to further deliberation, since it is really a user level
   protocol issue.



   Conceptually, the TCP is made up of several processes. Some of these
   deal with USER/TCP commands, and others with packets arriving from
   the network. The TCP also has an internal measurement facility which
   can be activated remotely.

   Any particular TCP could be viewed in a number of ways. It could be
   implemented as an independent process, servicing many user processes.
   It could be viewed as a set of re-entrant library routines which
   share a common interface to the local PSN, and common buffer storage.
   It could even be viewed as a set of processes, some handling the
   user, some the input of packets from the net, and some the output of
   packets to the net.



   8 bits: Internet information

      2 bits: Reserved for local PSN use

      2 bits: Header format (11 in binary)

      4 bits: Protocol version number

   8 bits: Header length in octets (32 is the current value)

   16 bits: Length of text in octets

   32 bits: Packet sequence number

   32 bits: Acknowledgment number (i.e. sequence number of next octet

Cerf, Dalal & Sunshine                                         [Page 17]
RFC 675              Specification of Internet TCP         December 1974

   16 bits: Window size (in octets)

   16 bits: Control Information

      Listed from high to low order:

      SYN: Request to synchronize sending sequence numbers

      ACK: There is a valid acknowledgment in the 32 bit ACK field

      FIN: Sender will stop SENDing and RECEIVEing on this connection

      DSN: The sender has stopped using sequence numbers and wants to
      initiate a new sequence number for sending.

      EOS: This packet is the end of a segment and therefore has a
      checksum in the 16 bit checksum field. If this bit is not set, the
      16 bit checksum field is to be ignored. The bit is usually set,
      but if fragmentation at a GATEWAY occurs, the packets preceding
      the last one will not have checksums, and the last packet will
      have the checksum for the entire original fragment (segment) as it
      was calculated by the sending TCP.

      EOL: This packet contains the last fragment of a letter. The EOS
      bit will always be set in this case.

      INT: The sender wants to INTERRUPT on this connection.

      XXX: six (6) unused control bits

      OD: three (3) bits of control dispatch:

         000: Null (the control octet contents should be ignored}

         001: Event Code is present in the control octet. These were
         defined in section 2.4.3.

         010: Special Functions

         011: Reject (codes as yet undefined)

         1XX: Unused

   8 bits: Control Data Octet

      If CD is 000 then this octet is to be ignored.

Cerf, Dalal & Sunshine                                         [Page 18]
RFC 675              Specification of Internet TCP         December 1974

      If CD is 001, this octet contains event codes defined in section

      If CD is 010, this octet contains a special function code as
      defined below:

         0: RESET all connections between Source and Destination TCPs

         l: RESET the specific connection referenced in this packet

         2: ECHO return packet to sender with the special function code
         ECHOR (Echo Reply).

         3: QUERY Query status of connection referenced in this packet

         4: STATUS Reply to QUERY with requested status.

         5: ECHOR Echo Reply

         6: TRASH Discard packet without acknowledgment

         >6: Unused

         Note: Special function packets not pertaining to a particular
         connection [RESET all, ECHO, ECHOR, and TRASH] are normally
         sent using socket zero as described in section 3.2.

      If CD is 01l, this octet contains an as yet undefined REJECT code.

      If CD is 1XX, this octet is undefined.

   4 bits: Length of destination network address in 4 bit units (current
   value is 1)

   4 bits: Destination network address

      1010-1111 are addresses of ARPANET, UCL, CYCLADES, NPL, CADC, and
      EPSS respectively.

   16 bits: Destination TCP address

   8 bits: Padding

   4 bits: length of source network address in 4 bit units (current
   value is 1)

   4 bits: source network address (as for destination address)

Cerf, Dalal & Sunshine                                         [Page 19]
RFC 675              Specification of Internet TCP         December 1974

   16 bits: Source TCP address

   24 bits: Destination port address

   24 bits: Source port address

   16 bits: Checksum (if EOS bit is set)


   It is highly likely that any implementation will include shared data
   structures among parts of the TCP and some asynchronous means of
   signaling users when letters have been delivered.

   One typical data structure is the Transmission Control Block (TCB)
   which is created and maintained during the lifetime of a given
   connection. The TCB contains the following information (field sizes
   are notional only and may vary from one implementation to another):

      16 bits: Local connection name

      48 bits: Local socket

      48 bits: Foreign socket

      16 bits: Receive window size in octets

      32 bits: Receive left window edge (next sequence number expected)

      16 bits: Receive packet buffer size of TCB (may be less than

      16 bits: Send window size in octets

      32 bits: Send left window edge (earliest unacknowledged octet)

      32 bits: Next packet sequence number

      16 bits: Send packet buffer size of TCB (may be less than window)

      8 bits: Connection state

         E/C - 1 if TCP has been synchronized at least once (i.e. has
         been established, else O, meaning it is closed; this bit is
         reset after FINS are exchanged and the user has done a CLOSE).
         The bit is not reset if the connection is only desynchronized
         on send or receive or both directions.

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RFC 675              Specification of Internet TCP         December 1974

         SS - SYNCed on send side (if set) else desynchronized

         SR - SYNCed on receive side (if set, else desynchronized)

   16 bits: Special flags

      S1 - SYN sent if set

      S2 - SYN verified if set

      R - SYN received if set

      Y - FIN sent if set

      C - CLOSE from local user received if set

      U - Foreign socket unspecified if set

      SDS - Send side DSN sent if set

      SDV - Send side DSN verified if set

      RDR - Receive side DSN received if set

   Initially, all bits are off [no pun intended] (i.e. SS, SR, E/C, S1,
   S2, R, F, C, SDS, SDV, RDR =0). When R is set, so is SR. When S1 and
   S2 are both set, so is SS. SR is reset when RDR is set. SS is reset
   when both SDS and SDV are set. These bits are used to keep track of
   connection state and to aid in arriving packet processing (e.g. Can
   sequence number be validated? Only if SR is set.).

   16 bits: Retransmission timeout (in eighths of a second#]

   16 bits: Head of Send buffer queue [buffers SENT from user to TCP,
   but not packetized]

   16 bits: Tail of Send buffer queue

   16 bits: Pointer to last octet packetized in partially packetized
   buffer (refers to the buffer at the head of the queue)

   16 bits: Head of Send packet queue

   16 bits: Tail of Send packet queue

   16 bits: Head of Packetized buffer Queue

   16 bits: Tail of Packetized buffer queue

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RFC 675              Specification of Internet TCP         December 1974

   16 bits: Head of Retransmit packet queue

   16 bits: Tail of Retransmit packet queue

   16 bits: Head of Receive buffer queue [queue of buffers given by user
   to RECEIVE letters, but unfilled]

   16 bits: Tail of Receive buffer queue

   16 bits: Head of Receive packet queue

   16 bits: Tail of receive packet queue

   16 bits: Pointer to last contiguous receive packet

   16 bits: Pointer to last octet filled in partly filled buffer

   16 bits: Pointer to next octet to read from partly emptied packet

      [Note: The above two pointers refer to the head of the receive
      buffer and receive packet queues respectively]

   16 bits: Forward TCB pointer

   16 bits: Backward TCB pointer



   The protocol places no restriction on a particular connection being
   used over and over again. New instances of a connection will be
   referred to as incarnations of the connection. The problem that
   arises owing to this is, "how does the TCP identify duplicate packets
   from previous incarnations of the connection?". This problem becomes
   harmfully apparent if the connection is being opened and closed in
   quick succession, or if the connection breaks with loss of memory and
   is then reestablished.

   The essence of the solution [TOML74] is that the initial sequence
   number [ISN] must be chosen so that a particular sequence number can
   never refer to an "o1d" octet, Once the connection is established the
   sequencing mechanism provided by the TCP filters out duplicates.

   For an association to be established or initialized, the two TCP's
   must synchronize on each other's initial sequence numbers. Hence the
   solution requires a suitable mechanism for picking an initial
   sequence number [ISN], and a slightly involved handshake to exchange

Cerf, Dalal & Sunshine                                         [Page 22]
RFC 675              Specification of Internet TCP         December 1974

   the ISN's. A "three way handshake" is necessary because sequence
   numbers are not tied to a global clock in the network, and TCP's may
   have different mechanisms for picking the ISN's. The receiver of the
   first SYN has no way of knowing whether the packet was an old delayed
   one or not, unless it remembers the last sequence number used on the
   connection which is not always possible, and so it must ask the
   sender to verify this SYN.

   The "three way handshake" and the advantages of a "clock-driven"
   scheme are discussed in [TOML74]. More on the subject, and algorithms
   for implementing the clock-driven scheme can be found in [DALA74].


   The "three way handshake" is essentially a unidirectional attempt to
   establish the connection, i.e. there is an initiator and a responder.
   The TCP's should however be able to establish the connection even if
   a simultaneous attempt is made by both TCP's to establish the
   connection. Simultaneous attempts are treated like "collisions" in
   "Aloha" systems and these conflicts are resolved into unidirectional
   attempts to establish the connection. This scheme was adopted because

      (i) Connections will normally have a passive and an active end,
      and so the mechanism should in most cases be as simple as

      (ii) It is easy to implement as special cases do not have to be
      accounted for.

   The example below indicates what a three way handshake between TCP's
   A and B looks like

         A                                                 B

         -->                                   -->

         <--                          <--

         -->                 -->

   The receiver of a "SYN" is able to determine whether the "SYN" was
   real (and not an old duplicate) when a positive "ACK" is returned for
   the receiver's "SYN,ACK" in response to the "SYN". The sender of a
   "SYN" gets verification on receipt of a "SYN,ACK" whose "ACK" part
   references the sequence number proposed in the original "SYN" [pun
   intended]. If the TCP is in the state where it is waiting for a
   response to its SYN, but gets a SYN instead, then it always thinks
   this is a collision and goes into the state prior to having sent the

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RFC 675              Specification of Internet TCP         December 1974

   SYN, i.e. it forgets that it had sent a SYN. The TCP will try to
   establish the connection again after some time, unless it has to
   respond to an arriving SYN. Even if the wait times in the two TCPs
   are the same, the varying delays in network transmission will usually
   be adequate to avoid a collision on the next cycle of attempts to
   send SYN.

   When establishing a connection, the state of the TCP is represented
   by 3 bits --

      S1 S2 R

      S1 = 1 -- SYN sent

      S2 = 1 -- My SYN verified

      R = 1 -- SYN received

   Some examples of attempts to establish the connection are now shown.
   The state of the connection is indicated when a change occurs. We
   specifically do not show the cases in which connection
   synchronization is carried out with packets containing both SYN and
   data. We do this to simplify the explanation, but we do not rule out
   an implementation which is capable of dealing with data arriving in
   the first packet (it has to be stored temporarily without
   acknowledgment or delivery to the user until the arriving SYN has
   been verified).

   The "three way handshake" now looks like --

              A                                            B
      ------------                                      ------------
      S1 S2 R                                                S1 S2 R

      0  0 0                                                 0  0 0

             -->                            -->

      1  0 0                                                 0  0 1

             <--                   <--

      1  1 1                                                 1  0 1

             --> (DATA OCTETS)        -->

      1  1 1                                                 1  1 1

Cerf, Dalal & Sunshine                                         [Page 24]
RFC 675              Specification of Internet TCP         December 1974

   The scenario for a simultaneous attempt to establish the connection
   without the arrival of any delayed duplicates is --

                    A                                     B
            ------------                               ------------
            S1 S2 R                                         S1 S2 R

             0  0 0                                          0  0 0

      (M1)   1  0 0 -->                     ...

      (M2)   0  0 0 <--   0  0 0

      (M1)   1  0 0 -->       *             -->  0  0 1

      (M3)   1  1 1 <--           <--  1  0 1

      (M4)   1  1 1 -->         -->  1  1 1

      Note: "..." means that a message does not arrive, but is delayed
      in the network. State changes are upon arrival or upon departure
      of a given message, as the case may be. Packets containing the SYN
      or INT or DSN bits implicitly contain a "dummy" data octet which
      is never delivered to the user, but which causes the packet
      sequence numbers to be incremented by 1 even if no real data is
      sent. This permits the acknowledgment of these controls without
      acknowledging receipt of any data which might also have been
      carried in the packet. A packet containing a FIN bit has a dummy
      octet following the last octet of data (if any) in the packet.

      * Once in state 000 sender selects new ISN z when attempting to
      establish the connection again.


   An established connection is said to be a "half-open" connection if
   one of the TCP's has closed the connection at its end without the
   knowledge of the other, or if the two ends of the connection have
   become desynchronized owing to a crash that resulted in loss of
   memory. Such connections will automatically become reset if an
   attempt is made to send data in either direction. However, half-open
   connections are expected to be unusual, and the recovery procedure is
   somewhat involved.

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RFC 675              Specification of Internet TCP         December 1974

   If one end of the connection no longer exists, then any attempt by
   the other user to send any data on it will result in the sender
   receiving the event code "Connection does not exist at foreign TCP".
   Such an error message should indicate to the user process that
   something is wrong and it is expected to CLOSE the connection.

   Assume that two user processes A and B are communicating with one
   another when a crash occurs causing loss of memory to B's TCP.
   Depending on the operating system supporting B's TCP, it is likely
   that some error recovery mechanism exists. When the TCP is up again B
   is likely to start again from the beginning or from a recovery point.
   As a result B will probably try to OPEN the connection again or try
   to SEND on the connection it believes open. In the latter case 1t
   receives the error message "connection not open" from the local TCP.
   In an attempt to establish the connection B's TCP will send a packet
   containing SYN. A's TCP thinks that the connection is already
   established and so will respond with the error "unacceptable SYN (or
   SYN/ACK) arrived at foreign TCP". B's TCP knows that this refers to
   the SYN it just sent out, and so should reset the connection and
   inform the user process of this fact.

   It may happen that B is passive and only wants to receive data. In
   this case A's data will not reach B because the TCP at B thinks the
   connection is not established. As a result A'S TCP will timeout and
   send a QRY to B's TCP. B's TCP will send STATUS saying the connection
   is not synched. A's TCP will treat this as if an implicit CLOSE had
   occurred and tell the user process, A, that the connection is
   closing. A is expected to respond with a CLOSE command to his TCP.
   However, A's TCP does not send a FIN to B's TCP, since it would not
   be accepted anyway on the unsynced connection. Eventually A will try
   to reopen the connection or B will give up and CLOSE. If B CLOSES,
   B's TCP will simply delete the connection since it was not
   established as far as B's TCP is concerned. No message will be sent
   to A'S TCP as a result.


   Details of resynchronization have not yet been specified since the
   need for this should be infrequent in the initial testing stages.


   There are essentially three cases:

      a) The user initiates by telling the TCP to CLOSE the connection

      b) The remote TCP initiates by sending a FIN control signal

Cerf, Dalal & Sunshine                                         [Page 26]
RFC 675              Specification of Internet TCP         December 1974

      c) Both users CLOSE simultaneously

   Two bits are used to maintain control over the closing of a
   connection: these are called the "FIN sent" bit [F] and the "USER
   Closed" bit, [C] respectively. The control procedure uses these two
   bits to assure that the connection is properly closed.

   Case 1: Local user initiates the close

      In this case, both the F and C bits are initially zero, but the C
      bit is set immediately upon receipt of the user call "CLOSE." When
      the FIN is sent out by the TCP, the F bit is set. All pending
      RECEIVES are terminated and the user is told that they have been
      prematurely terminated ("connection closing"} without data.
      Similarly, any pending SENDS are terminated with the same
      response, "connection closing."

      Several responses may arrive as the result of sending a FIN. The
      one which is generally expected is a matching FIN. When this is
      received, the TCB CAN BE ELIMINATED. If a "connection does not
      exist at foreign TCP" message comes in response to the FIN, then
      the TCB can likewise be eliminated. If no response is forthcoming,
      or if "Foreign TCP inaccessible" arrives then the resolution is
      moot. One might simply timeout and discard the TCB. Since the
      local user wants to CLOSE anyway, this is probably satisfactory,
      although it will leave a potential "half-open" connection at the
      other side. We deal with half open connections in section 4.3.3.

      When the acknowledging FIN arrives after the connection state bits
      are set (F=1, C=1), then the TCB can be deleted.

   Case 2: TCP receives a FIN from the network

      First of all, a FIN must have a sequence number which lies in the
      valid receive window. If not, it is discarded and the left window
      edge is sent as acknowledgment. If the FIN can be processed, it is
      handled (possibly out of order, since it is taken as an imperative
      to shut down the connection). All pending RECEIVES and SENDS are
      responded to by showing that they were terminated by the other
      side's close request (i.e. "connection closing"). The user is also
      told by an unsolicited event or signal that the connection has
      been closed (in some systems, the user might have to request
      STATUS to get this information). Finally, the TCP sends FIN in

      Thus, because a FIN arrived, a FIN is sent back, so the F bit is
      set. However, the TCB stays around until the local user does a
      CLOSE in acknowledgment of the unsolicited signal that the

Cerf, Dalal & Sunshine                                         [Page 27]
RFC 675              Specification of Internet TCP         December 1974

      connection has been closed by the other side. Thus, the C bit
      remains unset until this happens. If the C and F bits go from (F=1
      C=O) to (F=l, C=1), then the connection is closed and the TCB can
      be removed.

   Case 3: both users close simultaneously

      If this happens, both connections will be in the (F=1, C=1) state.
      When the FINs arrive, the connections w11i be shut down. If one
      FIN fails to arrive, we have two choices. One is to insist on
      acknowledgments for FINs, in which case the missing one will be
      retransmitted. Another is merely to permit the half-open
      connection to remain (we prefer this solution}. It can timeout
      independently and go away after a while. If an attempt is made to
      reestablish the connection, the initiator will discover the
      existence of the open connection since an "inappropriate SYN
      received" message will be sent by the TCP which holds the "half-
      open" connection. The receiver of this message can tell the other
      TCP to reset the connection. We cannot permit the holder of the
      half-open connection to reset automatically on receipt of the SYN
      since its receipt is not necessarily prima facie evidence of a
      half open connection. (The SYN could be a delayed duplicate.)

4.3.6.  CONNECTION STATE and its relation to USER and INCOMING CONTROL

   In order to formalize the action taken by the TCP when it receives
   commands from the User, or Control information from the network, we
   define a connection to be in one of 7 states at any instant. These
   are known as the TCB Major States. Each Major State is simply a
   convenient name for a particular setting or group of settings of the
   state bits, as follows:

      S1 S2  R  U  F  C   #   name

       -  -  -  -  -  -   0   no TCB

       0  0  0 0/1 0  0   1   unsync

       1  0  0  0  0  0   2   SYN sent

       1  0  1 0/1 0  0   3   SYN received

       1  1  1  0  0  0   4   established

       1 0/1 1 0/1 1  1   5   FIN wait

       1  1  1  0  1  0   6   FIN received

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RFC 675              Specification of Internet TCP         December 1974

   The connection moves from state to state as shown below. The
   transition from one state to another will be represented as

      [X, Y]

   which means that there is a transition from state X to state Y owing
   to . The action taken by the TCP is specified as . We
   use this notation to give the important state transitions, often
   simplifying the cause and action fields to take into account a number
   of situations. Figure 1 illustrates these transitions in traditional
   state diagram form. Section 4.4.6 and section 4.4.7 fully specify the
   effect of all User commands and Control information arriving from the









      [3,1]  <(forget SYN)>







4.4.l  INTRODUCTION [See figure 2.1]

   There are many possible implementations of the TCP. We offer one
   conceptual framework in which to view the various algorithms that

Cerf, Dalal & Sunshine                                         [Page 29]
RFC 675              Specification of Internet TCP         December 1974

   make up the TCP design. In our concept, the TCP is written in two
   parts, an interrupt or signal driven part (consisting of four
   processes), and a reentrant library of subroutines or system calls
   which interface the user process to the TCP. The subroutines
   communicate with the interrupt part through shared data structures
   (TCB's, shared buffer queues etc.). The four processes are the Output
   Packet Handler which sends packets to the packet switch; the
   Packetizer which formats letters into internet packets; the Input
   Packet Handler which processes incoming packets; and the Reassembler
   which builds letters for users.

   The ultimate bottleneck is the pipe through which arriving and
   departing packets must travel. This is the Host/Packet Switch
   interface. The interrupt driven TCP shares among all TCB's its
   limited packet buffer resources for sending and receiving packets.
   From the standpoint of controlling buffer congestion, it appears
   PACKETS. That is, packet buffers which can be released by copying
   their contents into user buffers clearly help to reduce congestion.
   Neither the packetizer nor the input packet handler should be allowed
   to take up all available packet buffer space; an analogous problem
   arises in the IMP in the allocation of store and forward, and
   reassembly buffer space. One policy is to permit neither contender
   more than, say, two-thirds of the space. The buffer allocation
   routines can enforce these limits and reject buffer requests as
   needed. Conceptually, the scheduler can monitor the amounts of
   storage dedicated to the input and output routines, and can force
   either to sleep if its buffer allocation exceeds the limit.

   As an example, we can consider what happens when a user executes a
   SEND call to the TCP service routines. The buffer containing the
   letter is placed on a SEND buffer queue associated with the user's
   TCB. A 'packetizer' process is awakened to look through all the TCB's
   for 'packetizing' work. The packetizer will keep a roving pointer
   through the TCB list which enables it to pick up new buffers from the
   TCB queue and packetize them into output buffers. The packetizer
   takes no more than one letter at a time from any single TCB. The
   packetizer attempts to maintain a non-empty queue of output packets
   so that the output handler will not fall idle waiting for the
   packetizing operation. However, since arriving packets compete with
   departing packets, care must be taken to prevent either class from
   occupying all of the shared packet buffer space. Similarly since the
   TCB's all compete for space in service to their connections, neither
   input nor output packet space should be dominated by any one TCB.

   When a packet is created, it is placed on a FIFO SEND packet queue
   associated with its origin TCB. The packetizer wakes the output
   handler and then continues to packetize a few more buffers, perhaps,

Cerf, Dalal & Sunshine                                         [Page 30]
RFC 675              Specification of Internet TCP         December 1974

   before going to sleep. The output handler is awakened either by a
   'hungry' packet switch or by the packetizer; in either case, it uses
   a roving TCB pointer to select the next TCB for service. The send
   packet queue can be used as a 'work queue' for the output handler.
   After a packet has been sent, but usually before an ACK is returned,
   the output handler moves the packet to a retransmission queue
   associated with each TCB.

   Retransmission timeouts can refer to specific packets and the
   retransmission list can be searched for the specific packet. If an
   ACK is received, the retransmission entry can be removed from the
   retransmit queue. The send packet queue contains only packets waiting
   to be sent for the first time. INTERRUPT requests can remove entries
   in both the send packet queue and the retransmit packet queue.

   Since packets are never in more than one queue at a time, it appears
   possible for INT, FIN or RESET commands to remove packets from the
   receive, send, or retransmit packet queues with the assurance that an
   already issued signal to enter the reassembler, the packetizer or the
   output handler will not be confusing.

   Handling the INTERRUPT and CLOSE functions can however require some
   care to avoid confusing the scheduler, and the various processes. The
   scheduler must maintain status information for the processes. This
   information includes the current TCB being serviced. When an
   INTERRUPT is issued by a local process, the output queue of letters
   associated with the local port reference is to be deleted. The
   packetizer, for example, may however be working at that time on the
   same queue. As usual, simultaneous reading and writing of the TCB
   queue pointers must be inhibited through some sort of semaphore or
   lockout mechanism. When the packetizer wants to serve the next send
   buffer queue, it must lock out all other access to the queue, remove
   the head of the queue (assuming of course that there are enough
   buffers for packetization), advance the head of the queue, and then
   unlock access to the queue.

   If the packetizer keeps only a TCB pointer in a global place called
   CPTCB (current packetizer TCB address), and always uses the address
   in CPTCB to find the TCB in which to examine the send buffer queue,
   then removal of the output buffer queue does not require changes to
   any working storage belonging to the packetizer. Even more important,
   the arrival and processing of a RESET or CLOSE, which clears the
   system of a given TCB, can update the CPTCB pointer, as long as the
   removal does not occur while the packetizer is still working on the

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   Incoming packets are examined by the input packet handler. Here they
   are checked for valid connection sockets, and acknowledgments are
   processed, causing packets to be removed, possibly, from the SEND or
   RETRANSMIT packet queues as needed. As an example, consider the
   receipt of a valid FIN request on a particular TCB. If a FIN had not
   been sent before (i.e. F bit not set), then a FIN packet is
   constructed and sent after having cleared out the SEND buffer and
   SEND packet queues as well as the RETRANSMIT queue. Otherwise, if the
   F and C bits are both set, all queues are emptied and the TCB is
   returned to free storage.

   Packets which should be reassembled into letters and sent to users
   are queued by the input packet handler, on the receive packet queue,
   for processing by the reassembly process. The reassembler looks at
   its FIFO work queue and tries to move packets into user buffers which
   are queued up in an input buffer queue on each TCB. If a packet has
   arrived out of order, it can be queued for processing in the correct
   sequence. Each time a packet is moved into a user buffer, the left
   window edge of the receiving TCB is moved to the right so that
   outgoing packets can carry the correct ACK information. If the SEND
   buffer queue is empty, then the reassembler creates a packet to carry
   the ACK.

   As packets are moved 1nto buffers and they are filled, the buffers
   are dequeued from the RECEIVE buffer queue and passed to the user.
   The reassembler can also be awakened by the RECEIVE user call should
   it have a non-empty receive packet queue with an empty RECEIVE buffer
   queue. The awakened reassembler goes to work on each TCB, keeping a
   roving pointer, and sleeping if a cycle is made of all TCB's without
   finding any work.

4.4.2  INPUT PACKET HANDLER [See figure 2.2]

   The Input Packet Handler is awakened when a packet arrives from the
   network. It first verifies that the packet is for an existing TCB
   (i.e. the local and foreign socket numbers are matched with those of
   existing TCB's). If this fails, an error message is constructed and
   queued on the send packet queue of a dummy TCB. A signal is also sent
   to the output packet handler. Generally, things to be transmitted
   from the dummy TCB have a default retransmission timeout of zero, and
   will not be retransmitted. (We use the idea of a dummy TCB so that
   all packets containing errors, or RESET can be sent by the output
   packet handler, instead of having the originator of them interface to
   the net. These packets, it will be noticed, do not belong to any

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   The input packet handler looks out for control or error information
   and acts appropriately. Section 4.4.7 discusses this in greater
   detail, but as an example, if the incoming packet is a RESET request
   of any kind (i.e. all connections from designated TCP or given
   connection), and is believable, then the input packet handler clears
   out the related TCB(s), empties the send and receive packet queues,
   and prepares error returns for outstanding user SEND(s) and
   RECEIVE(s) on each reset TCB. The TCB's are marked unused and
   returned to storage. If the RESET refers to an unknown connection, it
   is ignored.

   Any ACK's contained in incoming packets are used to update the send
   left window edge, and to remove the ACK'ed packets from the TCB
   retransmit packet queue. If the packet being removed was the end of a
   user buffer, then the buffer must be dequeued from the packetized
   buffer queue, and the User informed. The packetizer is also signaled.
   Only one signal, or one for each packet, will have to be sent,
   depending on the scheduling scheme for the processes. See section
   4.4.7 for a detailed discussion.

   The packet sequence number, the current receive window size, and the
   receive left window edge determine whether the packet lies within the
   window or outside of it.

      Let W = window size

         S = size of sequence number space

         L = left window edge

         R = L+W-1 = right window edge

         x = sequence number to be tested

      For any sequence number, x, if

         (R-x) mod S <= W

      then x is within the window.

   A packet should be rejected only if all of it lies outside the
   window. This is easily tested by letting x be, first the packet
   sequence number, and then the sum of packet sequence number and
   packet text length, less one. If the packet lies outside the window,
   and there are no packets waiting to be sent, then the input packet
   handler should construct a dummy ACK and queue it for output on the

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   send packet queue, and signal the output packet handler. Successfully
   received packets are placed on the receive packet queue in the
   appropriate sequence order, and the reassembler signaled.

   The packet window check can not be made if the associated TCB is not
   in the 'established' state, so care must be taken to check for
   control and TCB state before doing the window check.

4.4.3  REASSEMBLER [See figure 2.3]

   The Reassembler process is activated by both the Input Packet Handler
   and the RECEIVE user call. While the reassembler is asleep, if
   multiple signals arrive, all but one can be discarded. This is
   important as the reassembler does not know the source of the signal.
   This is so in order that "dangling" signals from work in TCB's that
   have subsequently been removed don't confuse it. Each signal simply
   means that there may be work to be done. If the reassembler is awake
   when a signal arrives, it may be necessary to put 1t in a
   "hyperawake" state so that even if the reassembler tries to quit, the
   scheduler will run it one more time.

   When the reassembler is awakened it looks at the receive packet queue
   for each TCB. If there are some packets there then it sees whether
   the RECEIVE buffer queue is empty. If it is then the reassembler
   gives up on this TCB and goes on to the next one, otherwise if the
   first packet matches the left window edge, then the packet can be
   moved into the User's buffer. The reassembler keeps transferring
   packets into the User's buffer until the letter is completely
   transferred, or something causes it to stop. Note that a buffer may
   be partly filled and then a sequence 'hole' is encountered in the
   receive packet queue. The reassembler must mark progress so that the
   buffer can be filled up starting at the right place when the 'hole'
   is filled. Similarly a packet might be only partially emptied when a
   buffer is filled, so progress in the packet must be marked.

   If a letter was successfully transferred to a User buffer then the
   reassembler signals the User that a letter has arrived and dequeues
   the buffer associated with it from the TCB RECEIVE buffer queue. If
   the buffer is filled then the User is signaled and the buffer
   dequeued as before. The event code indicates whether the buffer
   contains all or part of a letter, as described in section 2.4.

   In every case when a packet is delivered to a buffer, the receive
   left window edge is updated, and the packetizer is signaled. This
   updating must take account of the extra octet included in the
   sequencing for certain control functions [SYN, INT, FIN, DSN]. If the
   send packet queue is empty then the reassembler must create a packet
   to carry the ACK, and place it on the send packet queue.

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   Note that the reassembler never works on a TCB for more than one User
   buffer's worth of time, in order to give all TCB's equal service.

   Scheduling of the reassembler is a big issue, but perhaps running to
   completion will be satisfactory, or else it can be time sliced. In
   the latter case it will continue from where it left off, but a new
   signal may have arrived producing some possible work. This work will
   be processed as part of the old incomplete signal, and so some
   wasteful processing may occur when the reassembler wakes up again.
   This is the general problem of trying to implement a protocol that is
   fundamentally asynchronous, but at least it is immune to harmful
   race-conditions. E.g. if we were to have the reassembler 'remove' the
   signal that caused it to wake up, just before it went to sleep (in
   order that new arriving ones were discarded) then a new signal may
   arrive at a critical time causing 1t not to be recognized; thus
   leaving some work pending, and this may result in a deadlock [see
   previous comments on "hyperawake" state].

4.4.4  PACKETIZER [See figure 2.4]

   The Packetizer process gets work from both the Input Packet Handler
   and the SEND user call. The signal from the SEND user call indicates
   that there is something new to send, while the one from the input
   packet handler indicates that more TCP buffers may be available from
   delivered packets. This latter signal is to prevent deadlocks in
   certain kind of scheduling schemes. We assume the same treatment of
   signals as discussed in section 4.4.3.

   When the packetizer is awakened it looks at the SEND buffer queue for
   each TCB. If there is a new or partial letter awaiting packetization,
   it tries to packetize the letter, TCB buffer and window permitting.
   It packetizes no more than one letter for a TCB before servicing
   another TCB. For every packet produced it signals the output packet
   handler (to prevent deadlock in a time sliced scheduling scheme). If
   a 'run till completion' scheme is used then one signal only need be
   produced, the first time a packet is produced since awakening. If
   packetization is not possible the packetizer goes on to the next TCB.

   If a partial buffer was transferred then the packetizer must mark
   progress in the SEND buffer queue. Completely packetized buffers are
   dequeued from the SEND buffer queue, and placed on a Packetized
   buffer queue, so that the buffer can be returned to the user when an
   ACK for the last bit is received.

   When the packetizer packetizes a letter it must see whether it is the
   first piece of data being sent on the connection, in which case it
   must include the SYN bit. Some implementations may not permit data to
   be sent with SYN and others may discard any data received with SYN.

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   The Packetizer goes to sleep if it finds no more work at any TCB.

4.4.5  OUTPUT PACKET HANDLER [see figure 2.5]

   When activated by the packetizer, or the input packet handler, or
   some of the user call routines, the Output Packet Handler attempts to
   transmit packets on the net (may involve going through some other
   network interface program). It looks at the TCB's in turn,
   transmitting some packets from the send packet queue. These are
   dequeued and put on the retransmit queue along with the time when
   they should be retransmitted.

   All data packets that are transmitted have the latest receive left
   window edge in the ACK field. Error and control messages may have no
   ACK [ACK bit off], or set the ACK field to refer to a received
   packet's sequence number.


   This process can either be viewed as a separate process, or as part
   of the output packet handler. Its implementation can vary; it could
   either perform its function, by being woken up at regular intervals,
   or when the retransmission time occurs for every packet put on the
   retransmit queue. In the first case the retransmit queue for each TCB
   is examined to see if there is anything to retransmit. If there is, a
   packet is placed on the send packet queue of the corresponding TCB.
   The output packet handler is also signaled.

   Another "demon" process monitors all user Send buffers and
   retransmittable control messages sent on each connection, but not yet
   acknowledged. If the global retransmission timeout is exceeded for
   any of these, the User is notified and he may choose to continue or
   close the connection. A QUERY packet may also be sent to ascertain
   the state of the connection [this facilitates recovery from half open
   connections as described in section 4.3.3].


   OPEN [See figure 3.1]

      1. If the process calling does not own the specified local socket,
      return with .

      2. If no foreign socket is specified, construct a new TCB and add
      it to the list of existing TCB's. Select a new local connection
      name and return it along with . If there
      is no room for the TCB, respond with .

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      3. If a foreign socket is specified, verify that there is no
      existing TCB with the same  pair
      (i.e. same connection), otherwise return . If there is no TCB space, return as in
      (2), otherwise, create the TCB and link it with the others,
      returning a local connection name with the success event code.

      Note: if a TCB is created, be sure to copy the timeout parameter
      into it, and set the "U" bit to 0 if a foreign socket is
      specified, else set U to 1 (to show unspecified foreign socket).

   SEND [see figure 3.2]

      1. Search for TCB with local connection name specified. If none
      found, return 

      2. If TCB is found, check foreign socket specification. If not set
      (i.e. U = 1 in TCB), return . If the connection is in the "closing" state (i.e.
      state 5 or 6), return  and do
      not process the buffer.

      3. Put the buffer on the Send buffer queue and signal the
      packetizer that there is work to do.

   INTERRUPT [see figure 3.3]

      1. Validate existence of the referenced connection, sending out
      error messages of the form 
      or  as appropriate. If
      the local connection refers to a connection not accessible to the
      process interrupting, send .

      2. If the connection is in the "closing" state (i.e. states 5 or
      6), return  and do not send
      an INT packet to the destination.

      3. Any pending SEND buffers should be returned with . An INT packet should be
      created and placed on the output packet queue, and the output
      packet handler should be signaled.

   RECEIVE [See figure 3.4]

      1. If the caller does not have access to the referenced local
      connection name, return . And if the connection is not open, return .

      2. Otherwise, put the buffer on the receive buffer queue and
      signal the reassembler that buffer space is available.

   CLOSE [See figure 3.5]

      1. If the connection is not accessible to the caller, return . If there is no
      such connection respond with .

      2. If the R bit is 0 (i.e. connection is in state 1 or 2), simply
      remove the TCB.

      3. If the R bit is set and the F bit is set, then remove the TCB.

      4. Otherwise, if the R bit is set, but F is 0 (i.e. states 3 or
      4), return all buffers to the User with , clear all output and input packet queues
      for this connection, create a FIN packet, and signal the output
      packet handler. Set the C and F bits to show this action.

   STATUS [See figure 3.6]

      1. If the connection is illegal for the caller to access, send

      2. If the connection does not exist, return .

      3. Otherwise set status information from the TCB and return it via


   The Input Packet Handler examines the header to see if there is any
   control information or error codes present. We do not discuss the
   action taken for various special function codes, as it is often
   implementation dependent, but we describe those that affect the state
   of the connection. After initial screening by the IPC [see section
   4.4.2 and figure 2.2], control and error packets are processed as
   shown in figures 4.l-4.7. [ACK and data processing is done within the

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   Error messages have CD=001 and do not carry user data. Depending on
   the error, zero or more octets of error information will be carried
   in the packet text field. We explicitly assume that this data is
   restricted in length so as to fall below the GATEWAY fragmentation
   threshold (probably 512 bits of data and header). Errors generally
   refer to specific connections, so the source and destination socket
   identifiers are relevant here. The ACK field of an error packet
   contains the sequence number of the packet that caused the error, and
   the ACK bit is off. [RESET and STATUS special functions may use the
   ACK field in the same way.] This allows the receiver of an error
   message to determine which packet caused the error. Error packets are
   not ACK'ed or retransmitted.



   The TCP manages buffer and window allocation on connections for two
   main purposes: equitably sharing limited TCP buffer space among all
   connections (multiplexing function), and limiting attempts to send
   packets, so that the receiver is not swamped (flow control function).
   For further details on the operation and advantages of the window
   mechanism see CEKA74.

   Good allocation schemes are one of the hardest problems of TCP
   design, and much experimentation must be done to develop efficient
   and effective algorithms. Hence the following suggestions are merely
   initial thoughts. Different implementations are encouraged with the
   hope that results can be compared and better schemes developed.

   Several of the measurements discussed in a later section are aimed at
   providing information on the performance of allocation mechanisms.
   This should aid in determining significant parameters and evaluating
   alternate schemes.

4.5.2 The SEND Side

   The window is determined by the receiver. Currently the sender has no
   control over the SEND window size, and never transmits beyond the
   right window edge. There exists the possibility of specifying two
   more special function codes so that the sender can request the
   receiver to INCREASE or DECREASE the window size, without specifying
   by how much. The receiver, of course, needn't satisfy this request.

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   Buffers must be allocated for outgoing packets from a TCP buffer
   pool. The TCP may not be willing to allocate a full window's worth of
   buffers, so buffer space for a connection may be less than what the
   window would permit. No deadlocks are possible even if there is
   insufficient buffer or window space for one letter, since the
   receiver will ACK parts of letters as they are put into the user's
   buffer, thus advancing the window and freeing buffers for the
   remainder of the letter.

   It is not mandatory that the TCP buffer outgoing packets until
   acknowledgments for them are received, since it is possible to
   reconstruct them from the actual letters sent by the user.

   However, for purposes of retransmission and processing efficiency it
   is very convenient to do.

4.5.3  The RECEIVE Side

   At the receiving side there are two requirements for buffering:

   (l) Rate Discrepancy:

      If the sender produces data much faster or much slower than the
      receiver consumes it, little buffering is needed to maintain the
      receiver at near maximum rate of operation. Simple queuing
      analysis indicates that when the production and consumption
      (arrival and service) rates are similar in magnitude, more
      buffering is needed to reduce the effect of stochastic or bursty
      arrivals and to keep the receiver busy.

   (2) Disorderly Arrivals:

      When packets arrive out of order, they must be buffered until the
      missing packets arrive so that packets (or letters) are delivered
      in sequence. We do not advocate the philosophy that they be
      discarded, unless they have to be, otherwise a poor effective
      bandwidth may be observed. Path length, packet size, traffic
      level, routing, timeouts, window size, and other factors affect
      the amount by which packets come out of order. This is expected to
      be a major area of investigation.

   The considerations for choosing an appropriate window are as follows:

   Suppose that the receiver knows the sender's retransmission timeout,
   also, that the receiver's acceptance rate is 'U' bits/sec, and the
   window size is 'W' bits. Ignoring line errors and other traffic, the
   sender transmits at a rate between W/K and the maximum line rate (the
   sender can send a window's worth of data each timeout period).

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   If W/K is greater than U, the difference must be retransmissions
   which is undesirable, so the window should be reduced to W', such
   that W'/K is approximately equal to U. This may mean that the entire
   bandwidth of the transmission channel is not being used, but it is
   the fastest rate at which the receiver is accepting data, and the
   line capacity is free for other users. This is exactly the same case
   where the rates of the sender and receiver were almost equal, and so
   more buffering is needed. Thus we see that line utilization and
   retransmissions can be traded off against buffering.

   If the receiver does not accept data fast enough (by not performing
   sufficient RECEIVES) the sender may continue retransmitting since
   unaccepted data will not be ACK'ed. In this case the receiver should
   reduce the window size to "throttle" the sender and inhibit useless

   Receiver window control:

      If the user at the receiving side is not accepting data, the
      window should be reduced to zero. In particular, if all TCP
      incoming packet buffers for a connection are filled with received
      packets, the window must go to zero to prevent retransmissions
      until the user accepts some packets.

      Short term flow control:

      Let F = the number of user receive buffers filled

         B = the total user receive buffers

         W = the long-term or nominal window size

         W' = the window size returned to the sender

      then a possible value for W' is

         W' = W*[1-F/B]**a

      The value of 'a' should be greater than one, in order to shut the
      window faster as buffers run out. The values of W' and F actually
      used could be averages of recent values, in order to get smooth
      control. Note that W' is constantly being recomputed, while the
      value of W, which sets the upper limit of W', only changes slowly
      in response to other factors.

      The value of W can be large (up to half the sequence number space)
      to allow for good throughput on high delay channels. The sender
      needn't allocate W worth of buffer space anyway. The long-term

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      variation of W to match flow requirements may be a separate

   This short-term mechanism for flow control allows some buffering in
   the two TCP's at either end, (as much as they are willing), and the
   rest in the user process at the send side where the data is being
   created. Hence the cost of buffering to smooth out bursty traffic is
   borne partly by the TCP's, and partly by the user at the send side.
   None of it is borne by the communication subnet.



   We have in mind a program which will exercise a given TCP, causing it
   to cycle through a number of states; opening, closing, and
   transmitting on a variety of connections. This program will collect
   statistics and will generally try to detect deviation from TCP
   functional specifications. Clearly there will have to be a copy of
   this program both at the local site being tested and some site which
   has a certified TCP. So we will have to produce a specification for
   this user level diagnostic program also.

   There needs to be a master and a slave side to all this so the master
   can tell the slave what's going wrong with the test.


   Round trip delay times

      Time from moment the packet is sent by the TCP to the time that
      the ACK is received by the TCP.

      Time from the moment the USER issues the SEND to the time that the
      USER gets the successful return code.

         Note: packet size should be used to distinguish from one set of
         round trip times and another.

         Network destination, and current configuration and traffic load
         may also be issues of importance that must be taken into

         What if the destination TCP decides to queue up ACKs and send a
         single ACK after a while? How does this affect round trip

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         What about out of order arrivals and the bunched ACK for all of

         The histogram of round trip times include retransmission times
         and these must be taken into account in the analysis and
         evaluation of the collected data.

         Packet size statistics

      Histogram of packet length in both directions on the full duplex

      Histogram of letter size in both directions.

   Measure of disorderly arrival

      Distance from the first octet of arriving packet to the left
      window edge. A histogram of this measure gives an idea of the out
      of order nature of packet arrivals. It will be 0 for packets
      arriving in order.

   Retransmission Histogram

   Effective throughput

      This is the effective rate at which the left edge of the window
      advances. The time interval over which the measure is made is a
      parameter of the measurement experiment. The shorter the interval,
      the more bursty we would expect the measure to be.

      It is possible to measure effective data throughput in both
      directions from one TCP by observing the rate at which the left
      window edge is moving on ACK sent and received for the two

      Since throughput is largely dependent upon buffer allocation and
      window size, we must record these values also. Varying window for
      a fixed file transmission might be a good way to discover the
      sensitivity of throughput to window size.

   Output measurement

      The throughput measurement is for data only, but includes
      retransmission. The output rate should include all octets
      transmitted and will give a measure of retransmission overhead.
      Output rate also includes packet format overhead octets as well as

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      The effective throughput divided by the output rate gives a
      measure of utilization of the communication connection.

   Window and buffer allocation measurements

      Histogram of letters outstanding, measured at the instant of SEND
      receipt by TCP from user or at instant of arrival of a letter for
      a receiving user.

      Buffers in use on the SEND side upon packet departure into the
      net; buffers in use on the RECEIVE side upon delivery of packet
      into a USER Buffer.


   Statistics on User Commands sent to the local TCP

   Statistics of error or success codes returned [histogram of each type
   of error or return response]

   Statistics of control bit use

      Counter for each control bit over all packets emitted by the TCP
      and another for packets accepted

   Count data carrying packets

   Count ACK packets with no data

   Error packets distribution by error type code received from the net
   and sent out into the net


   We view the measurement process as something which occurs internal to
   the TCP but which is controllable from outside. A well known socket
   owned by the TCP can be used to accept control which will select one
   or more measurement classes to be collected. The data would be
   periodically sent to a designated foreign socket which would absorb
   the data for later processing, in the manner currently used in the
   ARPANET IMPs. Each measurement class has its own data packet format
   to make the job of parsing and analyzing the data easier.

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   We would restrict access to TCP measurement control to a few
   designated sites [e.g. NMC, SU-DSL, BBN]. This is easily done by
   setting up listening control connections on partially specified
   foreign sockets.



   1. CEKA74

      V. Cerf and R. Kahn, "A Protocol For Packet Network
      Intercommunication," IEEE Transactions on Communication, vol. C-
      2O, No. 5. May 1974, pp. 637-648.

   2. CERF74

      V. Cerf, "An Assessment of ARPANET Protocols," in Proceedings of
      the Jerusalem Conference on Information Technology, July l974
      [RFC#635, INWG Note # ***].


      V. Cerf and C. Sunshine, "Protocols and Gateways for the
      Interconnection of Packet Switching Networks," Proc. of the
      Subconference on Computer Nets, Seventh Hawaii International
      Conference on Systems Science, January 1974.

   4. HEKA70

      F. Heart, R.E. Kahn, et al, "The Interface Message Processor for
      the ARPA Computer Network," AFIPS 1970 SJCC Proceedings, vol. 36,
      Atlantic City, AFIPS Press, New Jersey, pp. 551-567.

   5. POUZ74

      L. Pouzin, "CIGALE, the packet switching machine of the CYCLADES
      computer network," Proceedings of the IFIP74 Congress, Stockholm,

   6. ROWE74

      L. Roberts and B. Wessler, "Computer Network Development to
      achieve resource sharing," AFIPS 1970, SJCC Proceedings, vol. 36,
      Atlantic City, AFIPS Press, New Jersey, pp. 543-549.

Cerf, Dalal & Sunshine                                         [Page 45]
RFC 675              Specification of Internet TCP         December 1974

   7. POUZ73

      L. Pouzin, "Presentation and major design aspects of the CYCLADES
      Computer Network," Data Networks: Analysis and Design, Third Data
      Communications Symposium, St. Petersburg, Florida, November 1973,
      pp. 80-87.

   8. SCWI71

      R. Scantlebury and P.T. Wilkinson, "The Design of a Switching
      System to allow remote Access to Computer Services by other
      computers and Terminal Devices," Second Symposium on Problems in
      the Optimization of Data Communication Systems Proceedings, Palo
      Alto, California, 0ctober 1971, pp. 160-167.

   9. POST72

      J. Postel, "Official Initial Connection Protocol," Current Network
      Protocols, Network Information Center, Stanford Research
      Institute, Menlo Park, California. January 1972 (NIC 7101).

   10. CACR70

      C.S. Carr, S.D. Crocker, and V.G. Cerf, "Host-Host Communication
      Protocol in the ARPA Network," AFIPS Conference Proceedings, vol.
      36, 1970 SJCC, AFIPS Press, Montvale, N.J.

   11. ZIEL74

      H. Zimmerman and M. Elie, "Transport Protocol. Standard Host-Host
      Protocol for heterogeneous computer networks," INWG#61, April

   12. CRHE72

      S. D. Crocker, J. F. Heafner, R. M. Metcalfe and J. B. Postel,
      "Function-oriented protocols for the ARPA Computer Network," AFIPS
      Conference Proceedings, vol. 41, 1972 FJCC, AFIPS Press, Montvale,

   13. DALA74

      Y. Dalal, "More on selecting sequence numbers," INWG Protocol Note
      #4, October 1974.

Cerf, Dalal & Sunshine                                         [Page 46]
RFC 675              Specification of Internet TCP         December 1974

   14. SUNS74

      C. Sunshine, "Issues in communication protocol design -- formal
      correctness." INWG Protocol Note #5, October 1974


      D. Belsnes, "Note on single message communication," INWG Protocol
      Note #3. September 1974.

   16. TOML74

      R. Tomlinson, "Selecting sequence numbers," INWG Protocol Note #2,
      September 1974.

   17. SCHA74

      R. Schantz, "Reconnection Protocol", private communication;
      available from Schantz at BBN.

   18. POUZ74A

      L. Pouzin, "A proposal for interconnecting packet switching
      networks, INWG Note #60, March 1974 [also submitted to EUROCOMP

   19. DLMG74

      D. Lloyd, M. Galland, and P. T. Kirstein, "Aims and objectives of
      internetwork experiments," to be published as an INWG Experiments

   20. MCKE73

      A. McKenzie, "Host-Host Protocol for the ARPANET," NIC # 8246,
      Stanford Research Institute [also in ARPANET Protocols Notebook
      NIC 7104].

   21. BELS74A

      D. Belsnes, "Flow control in packet switching networks," INWG Note
      #63, October 1974.

Cerf, Dalal & Sunshine                                         [Page 47]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 1: TCB Major States

                              0-no TCB
                       OPEN    |    A   CLOSE           CLOSE    A
                    ---------- |    | ----------      ---------- |
                    set up TCB |    | remove TCB      remove TCB |
                               |    |                            |
                               |    |       collision retry,     |
        SYN arrives          __V____|__       SEND, INTER        |
       -------------        / S1=0     \    ----------------     |
       send SYN, ACK       |  S2=0 F=0  |       send SYN         |
     ______________________|  R=0  C=0  |_____________________   |
    |                      |  U=0/1     |                     |  |
    |                      |            |   SYN arrives       |  |
    |      error,timeout   |   1-OPEN   |   -----------       |  |
    |      -------------    \__________/    collision;        |  |
    |        clear TCB         A    A       set timeout       |  |
    |     _____________________|    |_____________________    |  |
  __V____|__                                             _|___V__|_
 / S1=1     \                                           / S1=1     \
|  S2=0 F=0  |                                         |  S2=0 F=0  |
|  R=1  C=0  |                       SYN, ACK arrives  |  R=0  C=0  |
|  U=0/1     |  ACK arrives          ----------------  |  U=0       |
|            |  -----------              send ACK      |            |
| 3-SYN rcvd |_________________       _________________| 2-SYN sent |
 \__________/                  |     |                  \__________/
    |                        __V_____V__
    |                       / S1=1      \
    |  CLOSE               |  S2=1 F=0   |
    | --------             |  R=1  C=0   |     FIN arrives
    | send FIN             |  U=0        | -------------------
    |                      |             | tell user, send FIN
    |      ________________|4-established|______________________
    |     |    CLOSE        \___________/                       |
    |     |   -------                                           |
  __V_____V_  send FIN                                   _______V__
 / S1=1     \                                           / S1=1     \
| S2=0/1 F=1 |     timeout or                          |  S2=1 F=1  |
|  R=1   C=1 | FIN, error, arrives            CLOSE    |  R=1  C=0  |
|  U=0/1     | -------------------          ---------- |  U=0       |
|            |     remove TCB               remove TCB |            |
| 5-FIN wait |_____________________       _____________| 6-FIN rcvd |
 \__________/                      |     |              \__________/
                                   |     |
      /                                                          \
                                  0-no TCB

Cerf, Dalal & Sunshine                                         [Page 48]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.1: Structure of the TCP

      |       _____________            _______________       |
      |      |             |          |               |      |
      |      |             |          | INPUT PACKET  |<---->|
      |      | REASSEMBLER |          |    HANDLER    |      |
      |      |_____________|          |_______________|      |
      |             |_______________          |              |
      |                             |         |              |
      |       _________             |         |              |
      |      |         |          __V_________V____          |  NETWORK
      |<=====| SYSTEM  |         |                 |         |    or
      |      |  CALLS  |<========|       TCB's     |<========|   some
USERS |=====>|   or    |         |        and      |         |  NETWORK
      |      |  USER   |========>|ASSOCIATED QUEUES|========>| INTERFACE
      |<---->|INTERFACE|         |_________________|         |  PROGRAM
      |      |_________|            A         A              |
      |                             |         |              |
      |               ______________|         |              |
      |       _______|_____            _______|_______       |
      |      |             |          |               |      |
      |      | PACKETIZER  |          | OUTPUT PACKET |      |
      |      |             |          |    HANDLER    |<---->|
      |      |_____________|          |_______________|      |
      |                                                      |

     =======> Logical or physical flow of data (packets/letters)

     -------> "Interaction"

     NOTE:    The signalling of processes by others is not shown

Cerf, Dalal & Sunshine                                         [Page 49]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.2a:                               ________
Address Check                             / Begin  \
                                           .'     '.
                                         .' packet  '.
                                       .'   foreign   '.
                  ___________________.'  socket matches '.
                 |                no  '.  a TCB local  .'
                 |                      '.   socket  .'
                 |                        '.   ?   .'
                 |                          '.___.'
                 |                             | yes
                 |                            _V_
                 |                         .'     '.
                 |                       .' packet  '.           ___
                 |                     .'local socket '.        /   \
                 |                   .'  matches fully  '.____\| YES |
                 |                    '. specified TCB .'     / \___/
                 |                      '.fgn socket .'
                 |                        '.   ?   .'
                _V_                         '.___.'
             .'     '.                         | no
           .'   SYN,  '.                      _V_
         .'FIN,INT,DSN, '.                 .'     '.
  _____.'or text length>0 './_____       .' matches '.
 |  no  '.   or QUERY    .' \     |    .'partly spec. '.
 |        '.           .'         |___.'  or unspec. TCB '.
 |          '.   ?   .'            no  '.     foreign   .'
 |            '.___.'                  '.   socket  .'
 |               | yes                   '.   ?   .'
 |     __________V_________                 '.___.'
 |    |                    |                   | yes
 |    |   Create error 7   |                  _V_
 |    | packet. Signal OPH |               .'     '.
 |    |____________________|             .' packet  '.
 |               |               ______.' has SYN set '.
 |           ____V____          |   no  '.           .'
 |          |         |         |         '.   ?   .'
 |_________\| discard |/________|          '.___.'
           /|_________|\                      |
                 |                           _V_
                _V_                         /   \
               /   \                       | YES |
              | NO  |                       \___/

Cerf, Dalal & Sunshine                                         [Page 50]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.2b-1:                         _______
Input Packet Handler                  / Begin \
|                              A         /|\                         |
|                              |          |                          |
|                              |         _V_                         |
|                              |       .'   '.          _______      |
|                              |     .' input '.       | go to |     |
|                              |   .'  packet   '.____\| sleep |     |
|                              |    '.available.'  no /|_______|     |
|                              |      '.__?__.'                      |
|                              |          | yes                      |
|                              |         _V_                         |
|                              |       .'   '.                       |
|   .->SPECIAL FUNCT. Fig 4.7  |     .'address'.                     |
|   | .->ERR Fig 4.5,4.6       |___.' check OK  '.                   |
|   | | .->SYN Fig 4.1,4.2      no  '.    ?    .'                    |
|   | | | .->INT Fig 4.3              '._____.'                      |
|   | | | | .->FIN Fig 4.4                | yes              ________|_
|   | | | | |                            _V_                | discard  |
|  _|_|_|_|_|___________               .'   '.              |(or queue)|
|  |                    |            .' error '.            |__________|
|<-| Control Processing |/_________.'or control '.                   A
   |____________________|\     yes  '.    ?    .'                    |
        |                             '._____.'                      |
        | (INT with data)                 | no                       |
        |                                 |                          |
        V                                _V_                         |
      to "X"                           .'   '.              .        |
  in Fig 2.2b-2                      .'(estab)'.          .' '.      |
                              _____.' R=S1=S2=1 '.----->.'seq.#'.--->|
                             | yes  '.    ?    .'  no    '.OK .'  no |
                             |        '._____.'            '.'       |
                             |                              | yes    |
                             |    _______________           |        |
                             |   | Set S2=1, U=0 |          V        |
                             |   | Notify user   |         .'.       |
                             |<--| with event 2  |       .'ACK'.     |
                             |   | if U was 1    |<-----'.  OK .'--->'
                             |   |_______________|  yes   '. .'   no
                             |                              '
                           to "Y"
                       in Fig 2.2b-2

Cerf, Dalal & Sunshine                                         [Page 51]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.2b-2: Input Packet Handler (continued)

           .'.              _V_
         .'txt'.          .'   '.        ______________________________
       .'lgth>0 '.      .'within '.     |Use ACK to advance send window|
,<----'. or DSN  .'<---'. window  .'--->|Release ACK'ed packets from   |
|  no   '.  ?  .'   no   '.  ?  .'  yes |retransmit or send queues. If |
|         '._.'            '._.'        |any packet had EB bit set     |
|           | yes                       |remove buffer from Packetized |
|   ________V____________________       |buffer queue and inform user  |
|  |Create ACK packet. Put on    |      |(success). Signal Packetizer. |
|<-|Send packet queue. Signal OPH|      |______________________________|
|  |_____________________________|                      |
|                                                       |
|          _____________________________________________|
|         |
|         |
|         |                      "X"
|         |                       |
|        _V_                     _V_              _____________________
|      .'   '.                 .'TCB'.           |Put packet on        |
|    .' text  '.   yes       .'Receive'.   yes   |Receive packet queue |
|  .' length>0  '.-------->.'  buffer   '.------>|in the right order.  |
|   '. or DSN  .'   A       '.available.'        |Signal Reassembler.  |
|     '.  ?  .'     |         '.  ?  .'          |_____________________|
|       '._.'       |           '._.'                           |
|         | no      |             | no                          |
|         |         |            _V_                            |
|________\|         |          .'   '.                          |
         /|         |        .' seq # '.         ________       |
          |         |      .' of packet '.  yes |Discard |      |
          |         |     '.  highest so .'---->|packet  |----->|
          |         |       '.   far   .'       |________|      |
          |         |         '.  ?  .'                         |
          |         |           '._.'                           |
          |         |             | no                          |
          |         |      _______V______________               |
          |         |     |Discard packet with   |              |
          |         |_____|highest seq. no from  |              |
          |               |Receive packet queue. |              |
          |               |______________________|              |
          |                                                     |
                      to "Begin" in Fig 2.2b-1

Cerf, Dalal & Sunshine                                         [Page 52]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.3-1: Reassembler

      / Begin \
          |                      _____                    | yes
    ______V_____               .'     '.                 _|_
   |Get ready   |            .' Receive '.   yes       .'any'.
   |for next TCB|--------->.'Packet Queue '.-------->.' more  '.
   |____________|     A     '.  empty ?  .'     A     '.work?.'
                      |       '._______.'       |       '._.'
                      |            | no         |         | no
   "R"------>---------'          __V__          |     ____V____
                               .' is  '.        |    |  Go to  |
                             .' packet  '.      |    |  Sleep  |
  .--<----------------------'.DSN with no.'     |    |_________|
  |                     yes   '. data? .'       |
  |                             '.___.'         |
  |                                | no         |
  |                              __V__          |
  |                            .'     '.        |
  |                          .' Receive '.  yes |
  |                        .'Buffer Queue '.--->|
  |                         '.  empty ?  .'     |
  |  ________________         '._______.'       |
  | |Copy from packet|             | no         |<-------------"S"
  | |to buffer until |           __V__          |
  | |one is exhausted|         .'First'.        |
  | |Update receive  | yes   .' packet  '.   no |
  | |window.         |<----.'matches Recv '.--->'
  | |________________|      '.left window.'
  |         |                 '. edge ?.'
  |       __V__                 '.___.'
  |     .'Send '.
  |   .' Packet  '.   yes  _____________________________
  | .' Queue empty '.---->|Create ACK packet containing |
  |  '.     ?     .'      |new window. Signal OPH.      |
  |    '._______.'        |_____________________________|
  |      no |                            |
  |         |                            |
  |         '--------------------------->|
  |                                      |
  V                                      V
to "T"                                 to "U"
in Fig 2.3-2                        in Fig 2.3-2

Cerf, Dalal & Sunshine                                         [Page 53]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.3-2:  Reassembler (continued)

     "T"                                "U"
      |                                  |
      |                                  |           _____________
   ___V____           ___              __V__        |Mark progress|
  |process |  yes   .'   '.    yes   .'whole'.  no  |in packet.   |
  |  DSN   |<-----.'  DSN  '.<-----.' packet  '.--->|Return buffer|--->.
  |________|       '. set?.'        '.copied?.'     |to user.     |    |
      |              '._.'            '.___.'       |_____________|    |
      |                | no                                            |
      '--------------->|                                               |
                       |                                               |
                     __V__              __________________________     |
                   .' EOL '.  yes      |Return buffer to user.    |    |
                  '.  set? .'--------->|Return packet to free     |--->|
                    '.___.'            |storage. Signal Packetizer|    |
                    no |               |__________________________|    |
                       |                   A                           |
                     __V__                 |                           |
                   .' full'.               |                           |
                  '. buffer.'--------------'                           |
                    '.___.'   yes                                      |
                       | no                                            |
                       |                                               |
    ___________________V__________________                             |
   |Mark progress in buffer. Return packet|                            |
   |to free storage. Signal Packetizer.   |                   ,--------'
   |______________________________________|                   |
                       |                                      |
                       |                                      |
                       V                                      V
              to "R" in Fig 2.3-1                    to "S" in Fig 2.3-1

Cerf, Dalal & Sunshine                                         [Page 54]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.4: Packetizer

    _______               ________________________
   / Begin \____________\| Get ready for next TCB |/___________________
   \_______/            /|________________________|\                   |
                                      |                                |
                                    __V__               _____          |
                                  .'Send '.           .' any '.        |
                            no  .' Buffer  '.  yes  .'  more   '.  yes |
                 .-------------'.   Queue   .'---->'.   work    .'-----'
                 |               '.empty? .'   A     '.   ?   .'
     ____________V____________     '.___.'     |       '.___.'
    |Pick packet size depend- |                |          | no
,-->|ing on send buffer, TCB  |                |    ______V______
|   |buffer space, window, etc|                |   | go to sleep |
|   |_________________________|                |   |_____________|
|                |                             |
|              __V__                           |
|            .'Send '.                         |
|          .' window  '.  no                   |
|         '.has room ? .'--------------------->|
|           '._______.'                        |
|                | yes                         |
|              __V__                           |
|            .' TCB '.                         |
|          .' buffer  '.   no                  |
|        .'space avail- '.---------------------'
|         '.  able ?   .'                   A
|           '._______.'                     |
|                | yes                      |
|   _____________V____________     _________|_______     ____________
|  |Copy from Send buffer to  |   |Move buffer from |   |Set EOL bit |
|  |packet until packet full. |   |Send queue to    |<--|in packet   |
|  |Put packet on Send packet |   |packetized queue |   |header      |
|  |queue. Signal OPH.        |   |_________________|   |____________|
|  |__________________________|             A                  A
|                |                          | no               |
|              __V__                      __|__                |
|            .'whole'.                  .' EOL '.              |
|          .'  Send   '.  yes         .' set in  '.  yes       |
|         '.  buffer   .'----------->'.   Send    .'-----------'
|           '.copied?.'                '.buffer?.'
|             '.___.'                    '.___.'
|                | no
|   _____________V__________
|  |Note in TCB where in    |
 --|Send buffer we stopped. |

Cerf, Dalal & Sunshine                                         [Page 55]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.5a:
Output Packet Handler
                                       / Begin \
                               ____________V___________                |
                              | Get ready for next TCB |               |
                              |________________________|               |
                                      |                                |
,------------------------------------>|                                |
|                                   __V__               _____          |
|               _____             .'Send '.           .' any '.        |
|        yes  .' ACK '.     no  .' Buffer  '.  yes  .'  more   '.  yes |
|      .-----'.bit set.'<------'.   Queue   .'---->'.   work    .'-----'
|      |       '.___.'           '.empty? .'    A    '.   ?   .'
|      |       no |________        '.___.'      |      '.___.'
|      |                   |__________          |         | no
|  ____V__________________            |         |         |
| |Put latest receive left|   ________v______   |   ______V______
| |window edge in ACK.    |->|Transmit packet|  |  | go to sleep |
| |_______________________|  |_______________|  |  |_____________|
|                                     |         |
|     ________________              __V__       |
|    |Return packet to|           .'pckt '.     |_________________
|    |buffer pool as  |    no   .'seq # to '.                     |
|    |it has been     |<------.'rgt of Send  '.                   |
|    |ACKed           |        '.left window.'                    |
|    |________________|          '.  edge .'                      |
|             |                    '.___.'                        |
|             |                       | yes                       |
|             |        _______________V________________           |
|             |       |Move packet to retransmit queue;|          |
|             |       |set new retrans. time for it.   |          |
|             |       |________________________________|          |
|             |                       |                           |
|             '---------------------->|                           |
|                                   __V__                         |
|                            no   .'Time '.   yes                 |
 -------------------------------.'to switch'.---------------------'
                                 '.TCB's? .'

Cerf, Dalal & Sunshine                                         [Page 56]
RFC 675              Specification of Internet TCP         December 1974

FIGURE 2.5b:
Retransmit Process

                               / Begin \
                       ____________V___________                        |
                      | Get ready for next TCB |                       |
                      |________________________|                       |
                                   |                                   |
 .-------------------------------->|                                   |
 |                               __V__                                 |
 |                             .' Any '.                _____          |
 |                           .'packet's '.            .' any '.        |
 |                         .'retrans. time'.  no    .'  more   '.  yes |
 |                        '. has occurred  .'----->'.   work    .'-----'
 |                          '.  for this .'          '.   ?   .'
 |                            '. TCB ? .'              '.___.'
 |                              '.___.'                   |
 |                                 | yes                  | no
 |                                 |                ______V______
 |                         ________V________       | go to sleep |
 |                        |Move packet to   |      |_____________|
 '------------------------|Send Packet      |
                          |queue. Signal OPH|

Cerf, Dalal & Sunshine                                         [Page 57]
RFC 675              Specification of Internet TCP         December 1974

                                / Begin \
                                .'User '.          _______
                              .'permitted'.   no  |       |
                            .'  access to  '.---->|error 1|------------.
                             '.this local .'      |_______|            |
                               '.socket?.'                             |
                                 '.___.'                               |
                                    | yes                              |
                                  __V__                                |
                                .' fgn '.                              |
                         yes  .' socket  '.  no                        |
                       .-----'. specified .'----.                      |
                       |       '.   ?   .'      |                      |
                     __V__       '.___.'      __V__         _______    |
   _______         .'conn-'.                .'space'.  no  |       |   |
  |       |  yes .' ection  '.             '.for TCB.'---->|error 4|-->|
,-|error 6|<----'.  already  .'              '.___.'       |_______|   |
| |_______|       '.exists?.'                   | yes                  |
|                   '.___.'                     |                      |
|                      | no                 ____V__________            |
|   _______          __V__                 |Create TCB. Set|           |
|  |       |   no  .'space'.               |S1=S2=R=F=C=1  |           |
|<-|error 4|<-----'.for TCB.'              |Set U=1        |           |
|  |_______|        '.___.'                |_______________|           |
|                      | yes                       |                   |
|                      |                           |                   |
|             _________V__________                 |                   |
|            |Create TCB. Set U=0 |                |                   |
|            |Set S1=S2=R=F=C=1   |                |                   |
|            |____________________|                |                   |
|                      |                           |                   |
|                      '-------------.-------------'                   |
|                                    |                                 |
|               _____________________V__________________               |
|              |Return local connection name and Success|              |
|              |________________________________________|              |
|                                    |                                 |
                                / Return \

Cerf, Dalal & Sunshine                                         [Page 58]
RFC 675              Specification of Internet TCP         December 1974

                  / Begin \
                .' ection  '.               _________
              .'  legal for  '.  no        |         |
             '. this process  .'---------->| error 1 |-----------.
               '.     ?     .'             |_________|           |
                 '._______.'                                     |
                      | yes                                      |
                    __V__                                        |
                  .'conn-'.                 _________            |
                .' ection  '.   no         |         |           |
              .'    open     '.----------->| error 3 |---------->|
               '.     ?     .'             |_________|           |
                 '._______.'                                     |
                      | yes                                      |
                    __V__                                        |
                  .' fgn '.                 _________            |
                .' socket  '.  no          |         |           |
               '. specified .'------------>| error 5 |---------->|
                 '.(U=0)? .'               |_________|           |
                   '.___.'                                       |
                      | yes                                      |
                    __V__                                        |
                  .'conn-'.                 _________            |
                .' ection  '.  yes         |         |           |
               '. closing ? .'------------>| error 12|---------->|
                 '.(F,C=1).'               |_________|           |
                   '.___.'                                       |
                      | no                                       |
  ____________________V________________________________          |
 |Put buffer on Send Buffer queue and signal Packetizer|         |
 |_____________________________________________________|         |
                      |                                          |
                 / Return \

Cerf, Dalal & Sunshine                                         [Page 59]
RFC 675              Specification of Internet TCP         December 1974


                  / Begin \

                Same as SEND

                      |                                          |
                      |                                          |
  ____________________V_________________________                 |
 |Return any pending Send buffers with code 10. |                |
 |Create INT packet on outgoing packet queue.   |                |
 |Signal Output Packet Handler.                 |                |
 |______________________________________________|                |
                      |                                          |
                 / Return \

Cerf, Dalal & Sunshine                                         [Page 60]
RFC 675              Specification of Internet TCP         December 1974

                  / Begin \
                .' ection  '.               _________
              .'  legal for  '.  no        |         |
             '. this process  .'---------->| error 1 |-----------.
               '.     ?     .'             |_________|           |
                 '._______.'                                     |
                      | yes                                      |
                     _V_                                         |
                   .'   '.                                       |
                 .'       '.                                     |
               .'connection '.                                   |
             .'     state     '.                                 |
            :___________________:                   _________    |
               |      |      |                     |         |   |
           1-4 |  5,6 |    0 '-------------------->| error 3 |-->|
               |      '---------------------.      |_________|   |
     __________V__________                  |                    |
    |Put buffer on Receive|                 |       _________    |
    |Buffer queue. Signal |                 |      |         |   |
    |Reassembler          |                 '----->| error 12|-->|
    |_____________________|                        |_________|   |
               |                                                 |
          / Return \

Cerf, Dalal & Sunshine                                         [Page 61]
RFC 675              Specification of Internet TCP         December 1974

                  / Begin \
                .' ection  '.               _________
              .'  legal for  '.  no        |         |
             '. this process  .'---------->| error 1 |-----------.
               '.     ?     .'             |_________|           |
                 '._______.'                                     |
                      | yes                                      |
                     _V_                                         |
                   .'   '.                                       |
                 .'       '.                                     |
               .'connection '.                                   |
             .'     state     '.                                 |
            :___________________:                   _________    |
            5|   |3,4  |1,2,6  |0                  |         |   |
             |   |     |       '------------------>| error 3 |-->|
,------------'   |     '-------------------.       |_________|   |
|  ______________V______________________   |                     |
| |Return all buffers to user with error|  |     ___________     |
| |12; clear all packet queues, create  |  |    |Remove TCB |    |
| |FIN packet, signal Output Packet     |  '--->|Return     |--->|
| |Handler, set C=F=1                   |       |Success    |    |
| |_____________________________________|       |___________|    |
|                      |                                         |
                  / Return \

Cerf, Dalal & Sunshine                                         [Page 62]
RFC 675              Specification of Internet TCP         December 1974

                  / Begin \
                .' ection  '.               _________
              .'  legal for  '.  no        |         |
             '. this process  .'---------->| error 1 |-----------.
               '.     ?     .'             |_________|           |
                 '._______.'                                     |
                      | yes                                      |
                    __V__                   __________           |
                  .'conn-'.                |Return    |          |
                .' ection  '.  no          |state=0 or|          |
               '.   open ?  .'------------>|error 3   |--------->|
                 '._______.'               |__________|          |
                      | yes                                      |
           ___________V___________                               |
          |Fill in reply from TCB.|                              |
          |Return Success to user.|                              |
          |_______________________|                              |
                      |                                          |
                 / Return \

Cerf, Dalal & Sunshine                                         [Page 63]
RFC 675              Specification of Internet TCP         December 1974

SYN (no ACK)
                             / Begin \
                              .'   '.
                            .'       '.
                          .' S1, S2, R '.
                        .'       ?       '.
                       :___________________: 1,1,1        _________
 __________             |     |     |     | (states 4-6) |         |
|Treat as a|      1,0,1 |     |     |     '------------->| error 6 |-->.
|duplicate.|<-----------'     |     |                    |_________|   |
|Retransmit|                  |     | 1.0,0                            |
|SYN, ACK  |            0,0,0 |     | (Syn sent)   ________________    |
|__________|      (listening) |     '------------>|Collision: Clear|   |
     |                        |                   |S1, set timeout,|   |
     |   _____________________V________________   |remove SYN from |-->|
     |  |Set R=S1=1. If U=1 set foreign socket |  |retransmit queue|   |
     |  |in TCB to match packet local socket.  |  |________________|   |
     |  |Send SYN, ACK. Signal OPH. Fill in TCB|                       |
     |  |with send window, receive sequence #. |                       |
     |  |______________________________________|                       |
     |                        |                                        |
     |                        |                                        |
                          / Done \

Cerf, Dalal & Sunshine                                         [Page 64]
RFC 675              Specification of Internet TCP         December 1974


                       / Begin \
                       .'     '.
                     .' State 2 '.  no
                      '.   ?   .'                  |
                        '.___.'                    |
                           | yes                   |
                         __V__              _______V______
                       .' ACK '.   no      |              |
                     .' correct '.-------->| send error 6 |
                      '.   ?   .'          |______________|
                        '.___.'                    |
                           | yes                   |
                  _________V_________              |
                 |Set S2=R=1. Process|             |
                 |ACK. Send ACK.     |             |
                 |___________________|             |
                           |                       |
                       / Done \

Cerf, Dalal & Sunshine                                         [Page 65]
RFC 675              Specification of Internet TCP         December 1974

INT (from net)
                   _______       ____________
                  / Begin \____\|Process ACK |
                  \_______/    /|(may set S2)|------.
                                |____________|      |
                        ____________            .' in  '.
                       | Discard    |     no  .' state 4 '.
              .<-------| (or queue) |<-------'. S1=S2=R=1 .'
              |        |____________|          '. F=0 ? .'
              |                                  '.___.'
              |                                     | yes
              |                                   __V__
              |         ____________            .'     '.
              |        | ACK and    |     no  .' within  '.
              |<-------| discard    |<-------'.  window   .'
              |        |____________|          '.   ?   .'
              |                                  '.___.'
              |                                     | yes
              |         ____________________________V_______________
              |        |Move Receive Left window edge to sequence   |
              |        |number of INT. Return event 10 with any     |
              |        |pending Receive buffers. Ruturn event 11 to |
              |        |user. Send ACK for INT.                     |
              |        |____________________________________________|
              |                                     |
              |                                   __V__
              |                 see       yes   .'data '.
              |              Figure<----------.' in this '.
              |                 2.2            '.packet?.'
              |                                  '.___.'
              |                                     | no
                                                / Done \

Cerf, Dalal & Sunshine                                         [Page 66]
RFC 675              Specification of Internet TCP         December 1974

                 _______       ____________
                / Begin \____\|Process ACK |
                \_______/    /|(may set S2)|------.
                              |____________|      |
                                              .'     '.
                                        no  .'S1=S2=R=1'.
                            .--------------'.  (estab-  .'
                            |                '.lished).'
                            |                  '.___.'
                            |                     | yes
                            |                   __V__
                      ______V_____            .'     '.
                     |            |     no  .' within  '.
   .-----------------| discard    |<-------'.  window   .'
   |                 |____________|          '.   ?   .'
   |                                           '.___.'
   |                                              | yes
   |                                            __V__
   |                             (state 4) 0  .'F bit'.  1 (state 5)
   |                            .------------'. value .'------------.
   |                            |              '.___.'              |
   |   _________________________V________                           |
   |  |Return all user buffers (event 12)|     _____________________V__
   |  |Clear all packet queues. Send FIN |    |Return success to User's|
   |  |packet. Set F=1. Inform user      |    |CLOSE.  Remove TCB.     |
   |  |"connection closing" (event 12)   |    |________________________|
   |  |__________________________________|                 |
   |                  |                                    |
                  / Done \

Cerf, Dalal & Sunshine                                         [Page 67]
RFC 675              Specification of Internet TCP         December 1974

Error 6 (bad SYN)

               / Begin \
               .'     '.
             .'refers to'.
           .'current pckt?'.                      _________
         .'(ACK matches seq '.  no               |         |
        '.  # of packet on   .'----------------->| discard |-----------.
          '.retrans or send.'                    |_________|           |
            '.  queues?) .'                                            |
              '._______.'                                              |
                   | yes                                               |
                   |                                                   |
                  _V_                                                  |
                .'   '.   1 (state 3)                                  |
              .' value '.--------------------------------.             |
               '. of R.'  bad SYN,ACK                    |             |
                 '._.'                                   |             |
                   |                                     |             |
                   | 0 (state 2)                         |             |
                   | bad SYN                             |             |
 __________________V__________________            _______V______       |
|Other side is established. Send RESET|          |Clear S1, R   |      |
|(put error packet's seq. # in ACK    |          |Remove SYN,ACK|      |
|field. Return all user buffers with  |          |from retrans  |      |
|code 14. Inform user with event 14   |          |queue.        |      |
|_____________________________________|          |______________|      |
                   |                                     |             |
                   |                                     V             |
               / Done \

Cerf, Dalal & Sunshine                                         [Page 68]
RFC 675              Specification of Internet TCP         December 1974

Error 7,8

                  / Begin \
                  .'     '.
                .'refers to'.                     _________
              .'   current   '.  no              |         |
             '. packet (check .'---------------->| discard |-----------.
               '.   ACK)?   .'         A         |_________|           |
                 '._______.'           |                               |
                      | yes            |                               |
                     _V_               |                               |
                   .'   '.             |                               |
                 .'       '.           |                               |
               .'connection '.         |                               |
             .'     state     '.       |                               |
            :___________________:      |                               |
           4|   5|   3|   2|   6|      |                               |
    .-------'    |    |    |    '------'                               |
    |            |    |    '-----------------------------.             |
    |            |    '-------------.                    |             |
    |            |                  |                    |             |
 ___V___     ____V_______     ______V_______     ________V_________    |
|Pass to|   |Remove TCB. |   |Clear S1, R.  |   |Discard. SYN will |   |
|user   |   |Return      |   |Remove SYN,ACK|   |be retrans to     |   |
|_______|   |success to  |   |from transmit |   |avoid receiver    |   |
    |       |user's CLOSE|   |queue (go to  |   |having to queue it|   |
    |       |____________|   |state 1).     |   |__________________|   |
    |            |           |______________|            |             |
    |            V                  |                    V             |
                                / Done \

Cerf, Dalal & Sunshine                                         [Page 69]
RFC 675              Specification of Internet TCP         December 1974

                               / Begin \
                           no  .'Reset'.  yes
                 .------------'. All ? .'------------------.
                 |              '.___.'                    |
                 |                                _________V_________
                 |                               |Clear all TCB's for|
                 |                               |foreign TCP. Inform|
                 |                               |users with event 14|
                 |                               |___________________|
               __V__                                       |
             .' Is  '.             _________               |
           .'  RESET  '.   no     |         |              |
         .'believable ? '.------->| discard |------------->|
          '.(check ACK .'         |_________|              |
            '.field) .'                                    |
              '.___.'                                      |
                 | yes                                     |
 ________________V________________                         |
|Clear all queues for this TCB.   |                        |
|Return event 14 for user buffers.|                        |
|Inform User with event 14.       |                        |
|_________________________________|                        |
                 |                                         |
             / Done \

       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by Alex McKenzie with    ]
       [ support from GTE, formerly BBN Corp.           2/2000 ]

Cerf, Dalal & Sunshine                                         [Page 70]