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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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2. Each time another duplicate ACK arrives, increment cwnd by the segment<br />

size. This inflates the congestion window for the additional segment that has<br />

left the network. Transmit a packet, if allowed by the new value of cwnd.<br />

3. When the next ACK arrives that acknowledges new data, set cwnd to<br />

ssthresh (the value set in step 1). This ACK is the acknowledgment of the<br />

retransmission from step 1, one round-trip time after the retransmission.<br />

Additionally, this ACK acknowledges all the intermediate segments sent<br />

between the lost packet <strong>and</strong> the receipt of the first duplicate ACK. This step is<br />

congestion avoidance, because <strong>TCP</strong> is down to one-half the rate it was at<br />

when the packet was lost.<br />

4.4 RFCs relevant to this chapter<br />

The following RFCs provide detailed information about the connection protocols<br />

<strong>and</strong> architectures presented throughout this chapter:<br />

► RFC 761 – DoD st<strong>and</strong>ard Transmission Control Protocol (January 1980)<br />

► RFC 768 – User Datagram Protocol (August 1980)<br />

► RFC 793 – Updated by RFC 3168 - The Addition of Explicit Congestion<br />

Notification (ECN) to <strong>IP</strong> (September 2001)<br />

170 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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