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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

TCP Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK) 1 2 3 TCP Window Size Adjustment and Flow Control

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.

TCP Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK) 1 2 3 TCP Window Size Adjustment and Flow Control