Deterministic LAN State Replication and Passive SIP/RTP Packet Telemetry for Offline-First Commercial Terminals

Cloud-dependent commercial point-of-sale (POS) and telephony workstations suffer from wide-area network (WAN) outages, queueing jitter, and split-brain synchronization anomalies. When retail branches and service desks encounter WAN disconnects, traditional synchronous REST-to-cloud architectures either abort in-flight customer transactions or risk inventory counter divergence upon reconnection. Concurrently, active Session Initiation Protocol (SIP) back-to-back user agents (B2BUAs) introduce single-point-of-failure vulnerabilities and audio latency into enterprise private branch exchange (PBX) audio trunks. This technical note presents an offline-first architecture that unifies raw TCP/IP deterministic state replication across local area network (LAN) peer nodes with non-intrusive, zero-copy passive SIP/RTP packet capture via kernel-level packet filter drivers (libpcap / Npcap). Local transactional state consistency is maintained through monotonically increasing Lamport timestamps, idempotent command logging, and SQLite Write-Ahead Logging (WAL) snapshot streaming. Passive voice telemetry operates with strictly 0.00 ms of added audio transmission delay by monitoring network interfaces in promiscuous mode and reconstructing G.711 RTP voice streams via memory-mapped ring buffers. Empirical benchmarks conducted across a 72-hour testbed simulating WAN disruptions with 10,000 transactions and concurrent call streams demonstrate P99 LAN replication latency of 4.2 ms, zero transaction loss during network partitions, and packet drop rates below 0.02% under peak load.