Fast Notification for tunnel-based lossless RDMA transmission in WAN

With the rapid development of Large Language Models (LLMs), many emerging AI services require lossless transmission of RDMA traffic over tunnels in Wide Area Network(WAN). Existing network mechanisms were not designed for the responsiveness and scale required by these dynamic services. WAN should support the real-time, lightweight network notification to enhance the responsiveness for traffic engineering, congestion mitigation, and failure protection. This document analyzes typical scenarios where RDMA traffic need to be tunneled across WAN, and proposes fast network notification solutions based on ICMPv6 or UDP.