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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 Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK)
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Pages in Current Topic/Section
12
3
Next Page
TCP Window Size Adjustment and Flow Control
Next Topic/Section

TCP Adaptive Retransmission and Retransmission Timer Calculations
(Page 3 of 3)

Refinements to RTT Calculation and Karn's Algorithm

TCP's solution to round-trip time calculation is based on the use of a technique called Karn's algorithm, after its inventor, Phil Karn. The main change this algorithm makes is the separation of the calculation of average round-trip time from the calculation of the value to use for timers on retransmitted segments.

The first change made under Karn's algorithm is to not use measured round-trip times for any segments that are retransmitted in the calculation of the overall average round-trip time for the connection. This completely eliminates the problem of acknowledgment ambiguity.

However, this by itself would not allow increased delays due to retransmissions to affect the average round-trip time. For this, we need the second change: incorporation of a timer backoff scheme for retransmitted segments. We start by setting the retransmission timer for each newly-transmitted segment based on the current average round-trip time. When a segment is retransmitted, the timer is not reset to the same value it was set for the initial transmission. It is “backed off” (increased) using a multiplier (typically 2) to give the retransmission more time to be received. The timer continues to be increased until a retransmission is successful, up to a certain maximum value. This prevents retransmissions from being sent too quickly and further adding to network congestion.

Once the retransmission succeeds, the round-trip timer is kept at the longer (backed-off) value until a valid round-trip time can be measured on a segment that is sent and acknowledged without retransmission. This permits a device to respond with longer timers to occasional circumstances that cause delays to persist for a period of time on a connection, while eventually having the round-trip time settle back to a long-term average when normal conditions resume.

Key Concept: TCP uses an adaptive retransmission scheme that automatically adjusts the amount of time to which retransmission timers are set, based on the average amount of time it takes to send segments between devices. This helps avoid retransmitting potentially lost segments too quickly or too slowly.



Previous Topic/Section
TCP Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK)
Previous Page
Pages in Current Topic/Section
12
3
Next Page
TCP Window Size Adjustment and Flow Control
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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