Case I: Streamed Distribution of Traffic Data

 

Assume that a municipality has traffic sensors distributed over streets. It broadcasts this data over the Internet so citizens (and robot driven vehicles) can improve their trip planning. The system requirements are as follows:

• The data rate of the stream is about 8 Kbps, about $20$ packets of $64$ bytes each are sent every second.
• The packet drop rate is at most $5\%$, where the average length of burst drops is $5$ packets.
• The verification delay should be less than $10$ seconds.

Many different instantiations of EMSSresult in efficient schemes which satisfy these requirements. The following scheme offers low overhead with high verification probability. Each packet has two hashes, and the length of each hash chain element is chosen uniformly distributed over the interval . Each hash is $80$ bits long, hence, only one hash is necessary for verification. A signature packet is sent every $100$ packets, or every five seconds, which is not necessary to achieve robustness in this case, but to ensure that the verification delay is less than ten seconds, with high probability. Each signature packet carries the hash of five data packets. The simulation predicts an average verification probability per packet of $98.7\%$.

The computation overhead is minimal. The sender only needs to compute one signature every five seconds, and only $20$ hash functions per second. The communication overhead is low also. Each data packet carries $20$ bytes containing the hash of two previous packets. The signature packet contains five hashes and a signature, and its length is hence $50$ bytes plus the signature length. Assuming a $1024$ bit RSA signature, the signature packet is $178$ bytes long. The average per-packet overhead is therefore about $22$ bytes, which is much lower than previous schemes, which we review in section 4.