Engineering Metadata Placement over MQTT 5 for Multi-Robot Fleet Coordination

Abstract

MQTT-based robot fleets need metadata for routing, tracing, schema evolution, request-response correlation, and tenancy, yet inconsistent placement can defeat broker filtering or expose sensitive fields. This paper consolidates a standards-compliant placement rule: routing and authorization fields go in the topic; observability and processing hints use MQTT 5 properties; schema-bound or sensitive fields stay in a versioned payload. The contribution is an engineering guideline and reproducible evaluation, not a new protocol. We state the OASIS User Property encoding rule as a design-aid formula and check implementation conformance at packet level, including both MQTT Variable Byte Integer length fields. A Rust testbed compares zero and 16 User Properties over Mosquitto, EMQX, and HiveMQ for two payload sizes and QoS 0/1. A separate four-carrier baseline quantifies wire bytes, broker-side filtering, ACL enforcement, and latency; controlled broker-egress impairment probes adverse conditions without claiming a physical multi-host result. Direct permutation tests, effect sizes, and bootstrap intervals replace inference from overlapping confidence intervals. All 95,658 captured PUBLISH protocol data units matched the intended property count and both length ledgers. Three of 12 header strata and one of six carrier strata rejected after Holm correction, ruling out a universal latency-neutrality claim while supporting the placement rule through broker visibility and ACL results on the measured single-host Docker path.

Affiliations

St. Joseph's College, Hong Kong S.A.R. · Shun Tak Fraternal Association Seaward Woo College, Hong Kong S.A.R. · HKBU Affiliated School Wong Kam Fai Secondary and Primary School, Hong Kong S.A.R. · ESF West Island School, Hong Kong S.A.R. · Field of Dream Studio, United States of America · The Hong Kong Polytechnic University, Hong Kong S.A.R.

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