Appendix A: Control Channel Slow Start and Congestion Avoidance

A.1 Overview

L2TP control channel uses reliable message transmission. To prevent network congestion, it is recommended to implement TCP-like slow start and congestion avoidance algorithms.

A.2 Slow Start Algorithm

Initial parameters:

• Initial congestion window (cwnd): 1 message
• Slow start threshold (ssthresh): Receive window size

Algorithm: For each ACK received, cwnd increases by 1. When cwnd >= ssthresh, enter congestion avoidance phase.

A.3 Congestion Avoidance

On timeout detection: ssthresh = max(cwnd/2, 2), reset cwnd = 1.

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