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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Transport Layer Protocols
           9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
                9  TCP/IP Transmission Control Protocol (TCP)
                     9  TCP Basic Operation: Connection Establishment, Management and Termination

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TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange
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TCP Connection Termination
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TCP Connection Management and Problem Handling, the Connection Reset Function, and TCP "Keepalives"
(Page 2 of 3)

Handling Reset Segments

When a device receives a segment with the RST bit sent, it tells the device to reset the connection so it can be re-established. Like all segments, the reset itself must be checked to ensure that it is valid (by looking at the value of its Sequence Number field). This prevents a spurious reset from shutting down a connection. Assuming the reset is valid, the handling of the message depends on the state of the device that receives it:

  • If the device is in the LISTEN state, the reset is ignored and it remains in that state.

  • If the device is in the SYN-RECEIVED state but was previously in the LISTEN state (which is the normal course of events for a server setting up a new connection), it returns to the LISTEN state.

  • In any other situation, the reset causes the connection to be aborted and the device returns to the CLOSED state for that connection. The device will advise the higher-layer process using TCP that the connection has been closed.

Key Concept: TCP includes a special connection reset feature to allow devices to deal with problem situations, such as half-open connections or the receipt of unexpected message types. To use the feature, the device detecting the problem sends a TCP segment with the RST (reset) flag set to 1. The receiving device either returns to the LISTEN state, if it was in the process of connection establishment, or closes the connection and returns to the CLOSED state pending a new session negotiation.



Previous Topic/Section
TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange
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Next Page
TCP Connection Termination
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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