rfc761.txt

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  the other side to a lower level protocol such as Internet Protocol.

  The interface between an application process and the TCP is
  illustrated in reasonable detail.  This interface consists of a set of
  calls much like the calls an operating system provides to an
  application process for manipulating files.  For example, there are
  calls to open and close connections and to send and receive letters on
  established connections.  It is also expected that the TCP can
  asynchronously communicate with application programs.  Although
  considerable freedom is permitted to TCP implementors to design
  interfaces which are appropriate to a particular operating system
  environment, a minimum functionality is required at the TCP/user
  interface for any valid implementation.

  The interface between TCP and lower level protocol is essentially
  unspecified except that it is assumed there is a mechanism whereby the
  two levels can asynchronously pass information to each other.
  Typically, one expects the lower level protocol to specify this
  interface.  TCP is designed to work in a very general environment of
  interconnected networks.  The lower level protocol which is assumed
  throughout this document is the Internet Protocol [2].

1.5.  Operation

  As noted above, the primary purpose of the TCP is to provide reliable,
  securable logical circuit or connection service between pairs of
  processes.  To provide this service on top of a less reliable internet
  communication system requires facilities in the following areas:

    Basic Data Transfer
    Reliability
    Flow Control
    Multiplexing
    Connections
    Precedence and Security

  The basic operation of the TCP in each of these areas is described in
  the following paragraphs.


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  Basic Data Transfer:

    The TCP is able to transfer a continuous stream of octets in each
    direction between its users by packaging some number of octets into
    segments for transmission through the internet system.  In this
    stream mode, the TCPs decide when to block and forward data at their
    own convenience.

    For users who desire a record-oriented service, the TCP also permits
    the user to submit records, called letters, for transmission.  When
    the sending user indicates a record boundary (end-of-letter), this
    causes the TCPs to promptly forward and deliver data up to that
    point to the receiver.

  Reliability:

    The TCP must recover from data that is damaged, lost, duplicated, or
    delivered out of order by the internet communication system.  This
    is achieved by assigning a sequence number to each octet
    transmitted, and requiring a positive acknowledgment (ACK) from the
    receiving TCP.  If the ACK is not received within a timeout
    interval, the data is retransmitted.  At the receiver, the sequence
    numbers are used to correctly order segments that may be received
    out of order and to eliminate duplicates.  Damage is handled by
    adding a checksum to each segment transmitted, checking it at the
    receiver, and discarding damaged segments.

    As long as the TCPs continue to function properly and the internet
    system does not become completely partitioned, no transmission
    errors will affect the users.  TCP recovers from internet
    communication system errors.

  Flow Control:

    TCP provides a means for the receiver to govern the amount of data
    sent by the sender.  This is achieved by returning a "window" with
    every ACK indicating a range of acceptable sequence numbers beyond
    the last segment successfully received.  For stream mode, the window
    indicates an allowed number of octets that the sender may transmit
    before receiving further permission.  For record mode, the window
    indicates an allowed amount of buffer space the sender may consume,
    this may be more than the number of data octets transmitted if there
    is a mismatch between letter size and buffer size.







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  Multiplexing:

    To allow for many processes within a single Host to use TCP
    communication facilities simultaneously, the TCP provides a set of
    addresses or ports within each host.  Concatenated with the network
    and host addresses from the internet communication layer, this forms
    a socket.  A pair of sockets uniquely identifies each connection.
    That is, a socket may be simultaneously used in multiple
    connections.

    The binding of ports to processes is handled independently by each
    Host.  However, it proves useful to attach frequently used processes
    (e.g., a "logger" or timesharing service) to fixed sockets which are
    made known to the public.  These services can then be accessed
    through the known addresses.  Establishing and learning the port
    addresses of other processes may involve more dynamic mechanisms.

  Connections:

    The reliability and flow control mechanisms described above require
    that TCPs initialize and maintain certain status information for
    each data stream.  The combination of this information, including
    sockets, sequence numbers, and window sizes, is called a connection.
    Each connection is uniquely specified by a pair of sockets
    identifying its two sides.

    When two processes wish to communicate, their TCP's must first
    establish a connection (initialize the status information on each
    side).  When their communication is complete, the connection is
    terminated or closed to free the resources for other uses.

    Since connections must be established between unreliable hosts and
    over the unreliable internet communication system, a handshake
    mechanism with clock-based sequence numbers is used to avoid
    erroneous initialization of connections.

  Precedence and Security:

    The users of TCP may indicate the security and precedence of their
    communication.  Provision is made for default values to be used when
    these features are not needed.

    







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                             2.  PHILOSOPHY

2.1.  Elements of the Internetwork System

  The internetwork environment consists of hosts connected to networks
  which are in turn interconnected via gateways.  It is assumed here
  that the networks may be either local networks (e.g., the ETHERNET) or
  large networks (e.g., the ARPANET), but in any case are based on
  packet switching technology.  The active agents that produce and
  consume messages are processes.  Various levels of protocols in the
  networks, the gateways, and the hosts support an interprocess
  communication system that provides two-way data flow on logical
  connections between process ports.

  We specifically assume that data is transmitted from host to host
  through means of a set of  networks.  When we say network, we have in
  mind a packet switched network (PSN).  This assumption is probably
  unnecessary, since a circuit switched network or a hybrid combination
  of the two could also be used; but for concreteness, we explicitly
  assume that the hosts are connected to one or more packet switches of
  a PSN.

  The term packet is used generically here to mean the data of one
  transaction between a host and a packet switch.  The format of data
  blocks exchanged between the packet switches in a network will
  generally not be of concern to us.

  Hosts are computers attached to a network, and from the communication
  network's point of view, are the sources and destinations of packets.
  Processes are viewed as the active elements in host computers (in
  accordance with the fairly common definition of a process as a program
  in execution).  Even terminals and files or other I/O devices are
  viewed as communicating with each other through the use of processes.
  Thus, all 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.

2.2.  Model of Operation

  Processes transmit data by calling on the TCP and passing buffers of
  data as arguments.  The TCP packages the data from these buffers into
  segments and calls on the internet module to transmit each segment to
  the destination TCP.  The receiving TCP places the data from a segment
  into the receiving user's buffer and notifies the receiving user.  The
  TCPs include control information in the segments which they use to
  ensure reliable ordered data transmission.


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  The model of internet communication is that there is an internet
  protocol module associated with each TCP which provides an interface
  to the local network.  This internet module packages TCP segments
  inside internet datagrams and routes these datagrams to a destination
  internet module or intermediate gateway.  To transmit the datagram
  through the local network, it is embedded in a local network packet.

  The packet switches may perform further packaging, fragmentation, or
  other operations to achieve the delivery of the local packet to the
  destination internet module.

  At a gateway between networks, the internet datagram is "unwrapped"
  from its local packet and examined to determine through which network
  the internet datagram should travel next.  The internet datagram is
  then "wrapped" in a local packet suitable to the next network and
  routed to the next gateway, or to the final destination.

  A gateway is permitted to break up an internet datagram into smaller
  internet datagram fragments if this is necessary for transmission
  through the next network.  To do this, the gateway produces a set of
  internet datagrams; each carrying a fragment.  Fragments may be broken
  into smaller ones at intermediate gateways.  The internet datagram
  fragment format is designed so that the destination internet module
  can reassemble fragments into internet datagrams.

  A destination internet module unwraps the segment from the datagram
  (after reassembling the datagram, if necessary) and passes it to the
  destination TCP.

  This simple model of the operation glosses over many details.  One
  important feature is the type of service.  This provides information
  to the gateway (or internet module) to guide it in selecting the
  service parameters to be used in traversing the next network.
  Included in the type of service information is the precedence of the
  datagram.  Datagrams may also carry security information to permit
  host and gateways that operate in multilevel secure environments to
  properly segregate datagrams for security considerations.

2.3.  The Host Environment

  The TCP is assumed to be a module in a time sharing operating system.
  The users access the TCP much like they would access the file system.
  The TCP may call on other operating system functions, for example, to
  manage data structures.  The actual interface to the network is
  assumed to be controlled by a device driver module.  The TCP does not
  call on the network device driver directly, but rather calls on the
  internet datagram protocol module which may in turn call on the device
  driver.


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  Though it is assumed here that processes are supported by the host
  operating system, the mechanisms of TCP do not preclude implementation
  of the TCP in a front-end processor.  However, in such an
  implementation, a host-to-front-end protocol must provide the
  functionality to support the type of TCP-user interface described
  above.

2.4.  Interfaces

  The TCP/user interface provides for calls made by the user on the TCP
  to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain
  STATUS about a connection.  These calls are like other calls from user
  programs on the operating system, for example, the calls to open, read
  from, and close a file.

  The TCP/internet interface provides calls to send and receive
  datagrams addressed to TCP modules in hosts anywhere in the internet
  system.  These calls have parameters for passing the address, type of
  service, precedence, security, and other control information.