TCP IP Illustrated

The cost is that the server must remember the last cc

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Unformatted text preview: ved in an acceptable SYN segment from that host. 4. When a CC option is received on an initial SYN, the receiver compares the value with the cached value for the sender. If the received CC is greater than the cached CC, the SYN is new and any data in the segment is passed to the receiving application (the server). The connection is called half-synchronized. If the received CC is not greater than the cached CC, or if the receiving host doesn't have a cached CC for this client, the normal TCP three-way handshake is performed. 5. The SYN, ACK segment in response to an initial SYN echoes the received CC value in another new option named CCECHO. 6. The CC value in a non-SYN segment detects and rejects any duplicate segments from previous incarnations of the same connection. The accelerated open avoids the need for a three-way handshake unless either the client or server has crashed and rebooted. The cost is that the server must remember the last CC received from each client. The TIME_WAIT state is shortened by calculating the TIME_WAIT delay dynamically, based on the measured RTT between the two hosts. The TIME_WAIT delay is set to 8 times RTO, the retransmission timeout value (Section 21.3). Using these features the minimal transaction sequence is an exchange of three segments: 1. Client to server, caused by an active open: client-SYN, client-data (the request), client-FIN, and client-CC. When the server TCP with the passive open receives this segment, if the client-CC file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (17 of 20) [12/09/2001 14.47.34] Chapter 24. TCP Futures and Performance is greater than the cached CC for this client host, the client-data is passed to the server application, which processes the request. 2. Server to client: server-SYN, server-data (reply), server-FIN, ACK of client-FIN, server-CC, and CCECHO of client-CC. Since TCP acknowledgments are cumulative, this ACK of the client FIN acknowledges the client's SYN, data, and FIN. When the client TCP receives this segment it passes the reply to the client application. 3. Client to server: ACK of server-FIN, which acknowledges the server's SYN, data, and FIN. The client's response time to its request is RTT plus SPT. There are many fine points to...
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