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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 Reliability and Flow Control Features and Protocol Modifications

Previous Topic/Section
TCP Segment Retransmission Timers and the Retransmission Queue
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3
4
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TCP Adaptive Retransmission and Retransmission Timer Calculations
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TCP Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK)
(Page 3 of 4)

There Is No Ideal Answer

Since TCP doesn't know whether these other segments showed up, it cannot know which method is better. It must simply make an “executive decision” to use one approach or the other, and hope for the best. In the example of the previous topic, as shown in Figure 223, I demonstrated the “conservative” approach—only the lost segment of the file was retransmitted. In contrast, Figure 224 shows the “aggressive” approach to retransmission.


Figure 224: TCP “Aggressive” Retransmission Example

This example is the same as that of Figure 223 except that here the server is taking an “aggressive” approach to retransmitting lost segments. When segment #3 times out, both #3 and #4 are retransmitted and their retransmission timers restarted. (In this case #4 already arrived so this extra transmission was not useful.)

 


Key Concept: There are two approaches to handling retransmission in TCP. In the more “conservative” approach, only the segments whose timers expire are retransmitted; this saves bandwidth but may cause performance degradation if many segments in a row are lost. The alternative is that when a segment’s retransmission timer expires, both it and all subsequent unacknowledged segments are retransmitted. This provides better performance if many segments are lost but may waste bandwidth on unnecessary retransmissions.



Previous Topic/Section
TCP Segment Retransmission Timers and the Retransmission Queue
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4
Next Page
TCP Adaptive Retransmission and Retransmission Timer Calculations
Next Topic/Section

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