Answer: Karn/Partridge Algorithm
{"Image DocumentName": "Answer: Karn/Partridge Algorithm", "Image ImageDescription": "Surprisingly simple solution! Refines RTT Calculation: Do not sample RTT when retransmitting. Only measures SampleRTT for segments that have been sent only once. Algorithm also included a second small change: Double timeout after each retransmission. Do NOT base timeout on the last EstimatedRTT. Use Exponential backoff (similar to Ethernets) Why? Why? The motivation for using exponential backoff is simple: Congestion is the most likely cause of lost segments, meaning that the TCP source should not react too aggressively to a timeout. In fact, the more times the connection times out, the more cautious the source should become. 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.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}