RFC 2140 (rfc2140) - Page 2 of 11

TCP Control Block Interdependence

RFC 2140           TCP Control Block Interdependence          April 1997


The TCP Control Block (TCB)

   A TCB is associated with each connection, i.e., with each association
   of a pair of applications across the network. The TCB can be
   summarized as containing [9]:


        Local process state

            pointers to send and receive buffers
            pointers to retransmission queue and current segment
            pointers to Internet Protocol (IP) PCB

        Per-connection shared state

            macro-state

                connection state
                timers
                flags
                local and remote host numbers and ports

            micro-state

                send and receive window state (size*, current number)
                round-trip time and variance
                cong. window size*
                cong. window size threshold*
                max windows seen*
                MSS#
                round-trip time and variance#


   The per-connection information is shown as split into macro-state and
   micro-state, terminology borrowed from [5]. Macro-state describes the
   finite state machine; we include the endpoint numbers and components
   (timers, flags) used to help maintain that state. This includes the
   protocol for establishing and maintaining shared state about the
   connection. Micro-state describes the protocol after a connection has
   been established, to maintain the reliability and congestion control
   of the data transferred in the connection.

   We further distinguish two other classes of shared micro-state that
   are associated more with host-pairs than with application pairs. One
   class is clearly host-pair dependent (#, e.g., MSS, RTT), and the
   other is host-pair dependent in its aggregate (*, e.g., cong. window
   info., curr. window sizes).




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