1. Introduction

RFC 9959 specifies Careful Resume, a cautious method for Internet transports to speed up congestion-control startup by reusing saved Congestion Control (CC) parameters from a previously observed path between the same transport endpoints.

The goal is to reduce transfer completion time when the congestion window (CWND) limits startup, especially on paths with high or varying Bandwidth-Delay Product (BDP). The method avoids simply trusting old state: it first confirms the path, uses a bounded unvalidated increase, and retreats safely if congestion or path change is detected.

Saved state is associated with a Remote Endpoint, source and destination IP addresses, possibly a DSCP, and a Lifetime. Careful Resume does not define multicast or broadcast support.