Kosek et al: MUST, SHOULD, DON'T CARE: TCP Conformance in the Wild
Kosek, Mike, Leo Blöcher, Jan Rüth, Torsten Zimmermann, and Oliver Hohlfeld. “MUST, SHOULD, DON’T CARE: TCP Conformance in the Wild.” In Passive and Active Measurement, edited by Anna Sperotto, Alberto Dainotti, and Burkhard Stiller, 12048:122–38. Lecture Notes in Computer Science. Cham: Springer International Publishing, 2020. https://doi.org/10.1007/978-3-030-44081-7%5F8.
Most servers abide by TCP standards for SHOULD and MUST. A significant subset do not adhere to even basic requirements, such as urgent flags or checksums.