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Network Working Group                                        Vinton CerfRequest for Comments: 675                                    Yogen DalalNIC: 2                                                     Carl SunshineINWG: 72                                                   December 1974SPECIFICATION OF INTERNET TRANSMISSION CONTROL PROGRAM                         December 1974 Version1.  INTRODUCTION   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   discussion.   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 <local socket, foreign socket> address pair, and   the same local socket can participate in multiple connections to   different foreign sockets [seeSection 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   SWITCHES [PS] of a PSN [HEKA7O, POUZ74, SCWI71].   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 inSection 4.3. The TCP performs retransmission,   duplicate detection, sequencing, and flow control on all   communication among the processes it serves.2.  The TCP INTERFACE to the USER2.1  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   otherwise.   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 19742.2  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   sockets.   Once the connections is specified in the OPEN command [seesection2.3.2], 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   sockets.   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   process.]   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 insection 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 [seesection2.3]. 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 insection 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 19742.3  TCP USER CALLS2.3.1  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 insection 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.2.3.2  OPEN CONNECTION   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. theCerf, 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   insection 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 insection 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 <local socket, foreign   socket> pair.   Responses from the TCP which may occur as a result of this call are   detailed insection 2.4.2.3.3 SEND LETTER   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   address.   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 insection 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 insection 2.4.2.3.4  RECEIVE LETTER   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   returned.   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 [seesection2.3.3]. 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   buffer.   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 insection 2.4.2.3.5  CLOSE CONNECTION   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 insection 2.4.2.3.6  INTERRUPT   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 processCerf, 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 insection 2.4.2.3.7  STATUS   Format: STATUS(local connection name)   This command returns a data block containing the following   information:    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    connection.    Responses from the TCP which may occur as a result of this call are    detailed insection 2.4.2.4  TCP TO USER MESSAGES2.4.1  TYPE CODES    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 CLOSECerf, 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 STATUS2.4.2  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 [seesection 4.3.6]      Number of letters awaiting acknowledgment      Number of letters awaiting receipt      Retransmission timeoutCerf, Dalal & Sunshine                                         [Page 12]

RFC 675              Specification of Internet TCP         December 19742.4.3 EVENT CODES   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   experimentation.      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 timeoutCerf, 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 [seesection 4.2.1].3.  HIGHER LEVEL PROTOCOLS3.1  INTRODUCTION   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.3.2  WELL KNOWN SOCKETS   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 0.3.3  RECONNECTION PROTOCOL (RCP)   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   sockets.   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 socketsCerf, 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 <A,B> and <A,C> 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 aCerf, Dalal & Sunshine                                         [Page 16]

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.4.  TCP IMPLEMENTATION4.1  INTRODUCTION   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.4.2  TCP DATA STRUCTURES4.2.1  INTERNETWORK PACKET FONMAT   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   expected).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 insection 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 insection2.4.3      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 insection 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)4.2.2  TRANSMISSION CONTROL BLOCK   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      window)      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.Cerf, Dalal & Sunshine                                         [Page 20]

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 queueCerf, Dalal & Sunshine                                         [Page 21]

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 pointer4.3  CONNECTION MANAGEMENT4.3.1  INITIAL SEQUENCE NUMBER SELECTION   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 exchangeCerf, 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].4.3.2 ESTABLISHING A CONNECTION   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      possible.      (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         --> <SEQ x><SYN>                                  -->         <-- <SEQ y><SYN, ACK x+l>                         <--         --> <SEQ x+1><ACK y+l><DATA BYTES>                -->   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 theCerf, Dalal & Sunshine                                         [Page 23]

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             --> <SEQ x><SYN>                           -->      1  0 0                                                 0  0 1             <-- <SEQ y><SYN, ACK x+l>                  <--      1  1 1                                                 1  0 1             --> <SEQ x+1><ACK y+1>(DATA OCTETS)        -->      1  1 1                                                 1  1 1Cerf, 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 --> <SEQ x><SYN>                    ...      (M2)   0  0 0 <-- <SEQ y><SYN)                    <--  1  0 0      (M1)              B returns no SYN sent           -->  0  0 0      (M1)   1  0 0 --> <SEQ z><SYN>      *             -->  0  0 1      (M3)   1  1 1 <-- <SEQ y+1><SYN,ACK z+1>          <--  1  0 1      (M4)   1  1 1 --> <SEQ z+1><ACK y+1><DATA>        -->  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.4.3.3 HALF-OPEN CONNECTIONS   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.Cerf, Dalal & Sunshine                                         [Page 25]

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.4.3.4  RESYNCHRONIZING A CONNECTION   Details of resynchronization have not yet been specified since the   need for this should be infrequent in the initial testing stages.4.3.5 CLOSING A CONNECTION   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 signalCerf, 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 insection 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      response.      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 theCerf, 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   REQUESTS   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 receivedCerf, Dalal & Sunshine                                         [Page 28]

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]<cause><action>   which means that there is a transition from state X to state Y owing   to <cause>. The action taken by the TCP is specified as <action>. 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 andsection 4.4.7 fully specify the   effect of all User commands and Control information arriving from the   network.      [0,l] <OPEN> <create TCB>      [1,2] <SEND,INTERRUPT, or collision timeout> <send SYN>      [1,3] <SYN arrives> <send SYN,ACK>      [1,0] <CLOSE> <remove TCB>      [2,1] <SYN arrives (collision)> <set timeout, forget SYNs>      [2,0] <CLOSE> <remove TCB>      [2,4] <appropriate SYN,ACK arrives> <send ACK>      [3,4] <appropriate ACK arrives> <none>      [3,1] <error arrives or timeout> <(forget SYN)>      [3,5] <CLOSE> <send FIN>      [4,5] <CLOSE> <send FIN>      [4,6] <appropriate FIN arrives> <send FIN, inform user>      [5,0] <FIN or error arrives, or timeout> <remove TCB>      [6,0] <CLOSE> <remove TCB>4.4  STRUCTURE 0F THE TCP4.4.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 thatCerf, 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   better to TREAT INCOMING PACKETS WITH HIGHER PRIORITY THAN OUTGOING   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   TCB.Cerf, Dalal & Sunshine                                         [Page 31]

RFC 675              Specification of Internet TCP         December 1974   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   TCB).Cerf, Dalal & Sunshine                                         [Page 32]

RFC 675              Specification of Internet TCP         December 1974   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. Seesection4.4.7 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 theCerf, Dalal & Sunshine                                         [Page 33]

RFC 675              Specification of Internet TCP         December 1974   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 insection 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.Cerf, Dalal & Sunshine                                         [Page 34]

RFC 675              Specification of Internet TCP         December 1974   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 insection 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.Cerf, Dalal & Sunshine                                         [Page 35]

RFC 675              Specification of Internet TCP         December 1974   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.   The RETRANSMIT PROCESS:   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 insection 4.3.3].4.4.6  USER CALL PROCESSING   OPEN [See figure 3.1]      1. If the process calling does not own the specified local socket,      return with <type 1><ELP 1 "connection illegal for this process">.      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 <type 1><OLP 0 "success">. If there      is no room for the TCB, respond with <type 1><ELT 4 "No room for      TCB">.Cerf, Dalal & Sunshine                                         [Page 36]

RFC 675              Specification of Internet TCP         December 1974      3. If a foreign socket is specified, verify that there is no      existing TCB with the same <local socket, foreign socket> pair      (i.e. same connection), otherwise return <type l><ELP 6      "connection already open">. 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 <type 10><ELP 3 "connection not open">      2. If TCB is found, check foreign socket specification. If not set      (i.e. U = 1 in TCB), return <type 10><ELT 5 "foreign socket      unspecified">. If the connection is in the "closing" state (i.e.      state 5 or 6), return <type 3><ELP 12 "connection closing"> 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 <type 3><ELP 3 "connection not open">      or <type 3><ELT 5 "foreign socket unspecified"> as appropriate. If      the local connection refers to a connection not accessible to the      process interrupting, send <type 3><ELP 1 "connection illegal for      this process">.      2. If the connection is in the "closing" state (i.e. states 5 or      6), return <type 3><ELT 12 "connection closing"> and do not send      an INT packet to the destination.      3. Any pending SEND buffers should be returned with <type 10><ELP      10 "buffer flushed due to interrupt">. 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 <type 20><ELP 1 "connection illegal for      this process">. And if the connection is not open, return <typeCerf, Dalal & Sunshine                                         [Page 37]

RFC 675              Specification of Internet TCP         December 1974      20><ELP 3 "connection not open"). If the connection is in the      closing state (e.g. a FIN has been received or a user CLOSE is      being processed), return <type 20><ELP 12 "connection closing">.      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 <type      2><ELP 1 "connection illegal for this process">. If there is no      such connection respond with <type 2><ELP 3 "connection not      open">.      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 <type x><ELP 12      "connection closing">, 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      <type 30><ELP 1 "connection illegal for this process">.      2. If the connection does not exist, return <type 30><ELP 3      "connection not open">.      3. Otherwise set status information from the TCB and return it via      <type 30><O-T 0 "status data...">.4.4.7  NETWORK CONTROL PROCESSING   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 [seesection4.4.2 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   IPC.]Cerf, Dalal & Sunshine                                         [Page 38]

RFC 675              Specification of Internet TCP         December 19744.4.8  TCP ERROR HANDLING   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.4.5.  BUFFER AND WINDOW ALLOCATION4.5.1  INTRODUCTION   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.Cerf, Dalal & Sunshine                                         [Page 39]

RFC 675              Specification of Internet TCP         December 1974   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).Cerf, Dalal & Sunshine                                         [Page 40]

RFC 675              Specification of Internet TCP         December 1974   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   retransmissions.   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-termCerf, Dalal & Sunshine                                         [Page 41]

RFC 675              Specification of Internet TCP         December 1974      variation of W to match flow requirements may be a separate      question   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.5.  NETWORK MEASUREMENT PLANS FOR TCP5.1  USERLEVEL DIAGNOSTICS   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.5.2  SINGLE CONNECTION MEASUREMENTS   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         account.         What if the destination TCP decides to queue up ACKs and send a         single ACK after a while? How does this affect round trip         statistics?Cerf, Dalal & Sunshine                                         [Page 42]

RFC 675              Specification of Internet TCP         December 1974         What about out of order arrivals and the bunched ACK for all of         them?         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      connection.      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      windows.      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      data.Cerf, Dalal & Sunshine                                         [Page 43]

RFC 675              Specification of Internet TCP         December 1974   Utilization      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.5.3  MULTICONNECTION MEASUREMENTS   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 net5.4  MEASUREMENT IMPLEMENTATION PHILOSOPHY   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.Cerf, Dalal & Sunshine                                         [Page 44]

RFC 675              Specification of Internet TCP         December 1974   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.6.  SCHEDULE OF IMPLEMENTATION7.  REFERENCES   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 # ***].   3.CESU74      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,      Sweden.   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      1974.   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,      N.J.   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   BELS74      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      74].   19. DLMG74      D. Lloyd, M. Galland, and P. T. Kirstein, "Aims and objectives of      internetwork experiments," to be published as an INWG Experiments      Note.   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 1974FIGURE 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 | \__________/                      |     |              \__________/                                   |     |       ____________________________V_____V_______________________      /                                                          \                                  0-no TCBCerf, Dalal & Sunshine                                         [Page 48]

RFC 675              Specification of Internet TCP         December 1974FIGURE 2.1: Structure of the TCP      |       _____________            _______________       |      |      |             |          |               |      |      |      |             |          | INPUT PACKET  |<---->|      |      | REASSEMBLER |          |    HANDLER    |      |      |      |_____________|          |_______________|      |      |             |_______________          |              |      |                             |         |              |      |       _________             |         |              |      |      |         |          __V_________V____          |  NETWORK      |<=====| SYSTEM  |         |                 |         |    or      |      |  CALLS  |<========|       TCB's     |<========|   someUSERS |=====>|   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 shownCerf, Dalal & Sunshine                                         [Page 49]

RFC 675              Specification of Internet TCP         December 1974FIGURE 2.2a:                               ________Address Check                             / Begin  \                                          \________/                                               |                                              _V_                                           .'     '.                                         .' 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 1974FIGURE 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                             |                              '                             V                           to "Y"                       in Fig 2.2b-2Cerf, Dalal & Sunshine                                         [Page 51]

RFC 675              Specification of Internet TCP         December 1974FIGURE 2.2b-2: Input Packet Handler (continued)                            "Y"                             |           .'.              _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. |              |          |               |______________________|              |          |                                                     |          |_____________________________________________________|                                  |                                  V                      to "Begin" in Fig 2.2b-1Cerf, Dalal & Sunshine                                         [Page 52]

RFC 675              Specification of Internet TCP         December 1974FIGURE 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                                      Vto "T"                                 to "U"in Fig 2.3-2                        in Fig 2.3-2Cerf, Dalal & Sunshine                                         [Page 53]

RFC 675              Specification of Internet TCP         December 1974FIGURE 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 2.3-1Cerf, Dalal & Sunshine                                         [Page 54]

RFC 675              Specification of Internet TCP         December 1974FIGURE 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 1974FIGURE 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 1974FIGURE 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 1974FIGURE 3.1:OPEN                                 _______                                / Begin \                                \_______/                                    |                                  __V__                                .'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|              ||              |________________________________________|              ||                                    |                                 | ----------------------------------->|<--------------------------------'                                 ____V___                                / Return \                                \________/Cerf, Dalal & Sunshine                                         [Page 58]

RFC 675              Specification of Internet TCP         December 1974FIGURE 3.2:SEND                   _______                  / Begin \                  \_______/                      |                    __V__                  .'conn-'.                .' 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|         | |_____________________________________________________|         |                      |                                          |                      |<-----------------------------------------'                  ____V___                 / Return \                 \________/Cerf, Dalal & Sunshine                                         [Page 59]

RFC 675              Specification of Internet TCP         December 1974FIGURE 3.3:INTERRUPT                   _______                  / Begin \                  \_______/                      |                      |                      V                Same as SEND                      |                                          |                      |                                          |  ____________________V_________________________                 | |Return any pending Send buffers with code 10. |                | |Create INT packet on outgoing packet queue.   |                | |Signal Output Packet Handler.                 |                | |______________________________________________|                |                      |                                          |                      |<-----------------------------------------'                  ____V___                 / Return \                 \________/Cerf, Dalal & Sunshine                                         [Page 60]

RFC 675              Specification of Internet TCP         December 1974FIGURE 3.4:RECEIVE                   _______                  / Begin \                  \_______/                      |                    __V__                  .'conn-'.                .' 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|-->|    |_____________________|                        |_________|   |               |                                                 |               |<------------------------------------------------'           ____V___          / Return \          \________/Cerf, Dalal & Sunshine                                         [Page 61]

RFC 675              Specification of Internet TCP         December 1974FIGURE 3.5:CLOSE                   _______                  / Begin \                  \_______/                      |                    __V__                  .'conn-'.                .' 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    |    || |_____________________________________|       |___________|    ||                      |                                         | --------------------->|<----------------------------------------'                   ____V___                  / Return \                  \________/Cerf, Dalal & Sunshine                                         [Page 62]

RFC 675              Specification of Internet TCP         December 1974FIGURE 3.6:STATUS                   _______                  / Begin \                  \_______/                      |                    __V__                  .'conn-'.                .' 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.|                              |          |_______________________|                              |                      |                                          |                      |<-----------------------------------------'                  ____V___                 / Return \                 \________/Cerf, Dalal & Sunshine                                         [Page 63]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.1:SYN (no ACK)                              _______                             / Begin \                             \_______/                                 |                                _V_                              .'   '.                            .'       '.                          .' 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 #. |                       |     |  |______________________________________|                       |     |                        |                                        |     |                        |                                        |     '----------------------->|<---------------------------------------'                           ___V__                          / Done \                          \______/Cerf, Dalal & Sunshine                                         [Page 64]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.2:SYN,ACK                        _______                       / Begin \                       \_______/                           |                         __V__                       .'     '.                     .' State 2 '.  no                    '.S1=1;S2=R=0.'----------------.                      '.   ?   .'                  |                        '.___.'                    |                           | yes                   |                         __V__              _______V______                       .' ACK '.   no      |              |                     .' correct '.-------->| send error 6 |                      '.   ?   .'          |______________|                        '.___.'                    |                           | yes                   |                  _________V_________              |                 |Set S2=R=1. Process|             |                 |ACK. Send ACK.     |             |                 |___________________|             |                           |                       |                           |<----------------------'                        ___V__                       / Done \                       \______/Cerf, Dalal & Sunshine                                         [Page 65]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.3:INT (from net)                   _______       ____________                  / Begin \____\|Process ACK |                  \_______/    /|(may set S2)|------.                                |____________|      |                                                    |                                                  __V__                        ____________            .' 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              '------------------------------------>|                                                 ___V__                                                / Done \                                                \______/Cerf, Dalal & Sunshine                                         [Page 66]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.4:FIN                 _______       ____________                / Begin \____\|Process ACK |                \_______/    /|(may set S2)|------.                              |____________|      |                                                  |                                                __V__                                              .'     '.                                        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)   |    |________________________|   |  |__________________________________|                 |   |                  |                                    |   '----------------->|<-----------------------------------'                   ___V__                  / Done \                  \______/Cerf, Dalal & Sunshine                                         [Page 67]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.5:Error 6 (bad SYN)                _______               / Begin \               \_______/                   |                   |                 __V__               .'     '.             .'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             |                   |<--------------------------------------------------'                ___V__               / Done \               \______/Cerf, Dalal & Sunshine                                         [Page 68]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.6:Error 7,8                   _______                  / Begin \                  \_______/                      |                    __V__                  .'     '.                .'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             |    '------------------------------>|<---------------------------------'                                 ___V__                                / Done \                                \______/Cerf, Dalal & Sunshine                                         [Page 69]

RFC 675              Specification of Internet TCP         December 1974FIGURE 4.7:RESET                                _______                               / Begin \                               \_______/                                   |                                 __V__                           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.       |                        ||_________________________________|                        |                 |                                         |                 |<----------------------------------------'              ___V__             / 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]