  Deployment of billions of Commercial off-the-shelf (COTS) RFID tags has drawn
much of the attention of the research community because of the performance gaps
of current systems. In particular, hash-enabled protocol (HEP) is one of the
most thoroughly studied topics in the past decade. HEPs are designed for a wide
spectrum of notable applications (e.g., missing detection) without need to
collect all tags. HEPs assume that each tag contains a hash function, such that
a tag can select a random but predicable time slot to reply with a one-bit
presence signal that shows its existence. However, the hash function has never
been implemented in COTS tags in reality, which makes HEPs a 10-year
untouchable mirage. This work designs and implements a group of analog on-tag
hash primitives (called Tash) for COTS Gen2-compatible RFID systems, which
moves prior HEPs forward from theory to practice. In particular, we design
three types of hash primitives, namely, tash function, tash table function and
tash operator. All of these hash primitives are implemented through selective
reading, which is a fundamental and mandatory functionality specified in Gen2
protocol, without any hardware modification and fabrication. We further apply
our hash primitives in two typical HEP applications (i.e., cardinality
estimation and missing detection) to show the feasibility and effectiveness of
Tash. Results from our prototype, which is composed of one ImpinJ reader and 3,
000 Alien tags, demonstrate that the new design lowers 60% of the communication
overhead in the air. The tash operator can additionally introduce an overhead
drop of 29.7%.
