Chapter 3 Time-to-live Covert Channels - CAIA

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Block corruption rate 1e−01 1e−03 1e−05 ● ● ● 0.90 0.92 0.94 0.96 0.98 Code rate ● ● MED0 MED CHAPTER 3. TIME-TO-LIVE COVERT CHANNELS ● ● AMI DED ● Block corruption rate 1e−01 1e−03 1e−05 ● ● ● ● ● Code rate ● ● ● MED0 MED ● 0.75 0.80 0.85 0.90 Figure 3.19: Block corruption rate depending on code rate for CAIA (left) and Leipzig (right) for 0% packet loss and reordering We used packet loss rates of 0%, 0.1%, 0.5% and 1% with uniform distribution. For packet reordering we used rates of 0%, 0.1% and 0.5% using a uniform distribution, and each reordered packet was swapped with the previous packet (d = 1 and b = 1, see Section 3.3). We limited our analysis to a small set of loss and reordering rates to keep the effort reasonable. Also, for high rates the capacity is expected to be low. For the RS code we always used one-byte symbols, limiting the maximum block size to 255 bytes. We used the largest possible block size for the non-deletion technique and varied sizes for the marker-based technique depending on the packet loss and reordering rate (see Appendix B.11). Longer RS codes could be used with larger symbols, but then the decoding time and the time for transmitting blocks increase. Since the rate of the covert channel is low it takes a while to transmit a data block. For example, for the CAIA trace on average it takes around 30 seconds to transmit one 255 byte block. The following graphs show the average rate of corrupted blocks over increasing code rate. We only show some of the graphs for the CAIA and Leipzig traces (graphs for other traces are in Appendix B.7). Figures 3.19 and 3.20 show the block corruption rates for 0% and 0.5% packet loss with 0% packet reordering. Figure 3.19 shows that for the CAIA trace AMI and DED outperform both MED schemes, but for Leipzig the performance of most schemes is similar. MED0 performs by far the worst across all traces because of the higher bit error rates, as discussed in Section 3.5.2. In the rest of the experiments we omitted MED0 since the high error rates result in extremely long analysis times. Figure 3.20 shows that AMI performs clearly worse than MED and DED. Packet loss causes additional substitution errors for differential schemes, such as AMI, as discussed in Section 3.3. MED and DED perform equally good, with MED being better at high code 65 AMI DED ●
Block corruption rate Block corruption rate 1e−01 1e−03 1e−05 ● ● ● ● 0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80 ● Code rate ● CHAPTER 3. TIME-TO-LIVE COVERT CHANNELS MED AMI ● DED ● ● ● ● ● 0.60 0.65 0.70 0.75 Code rate Figure 3.20: Block corruption rate depending on code rate for CAIA (left) and Leipzig (right) for 0.5% packet loss and 0% packet reordering 1e−01 1e−03 1e−05 ● ● ● ● ● Code rate ● ● MED AMI ● DED 0.70 0.72 0.74 0.76 0.78 0.80 0.82 ● Block corruption rate 1e−01 1e−03 1e−05 ● MED AMI ● ● ● ● ● ● ● ● ● DED 0.55 0.60 0.65 0.70 0.75 Code rate Figure 3.21: Block corruption rate depending on code rate for CAIA (left) and Leipzig (right) for 0.1% packet loss and 0.5% packet reordering rates for Leipzig. In the experiments with 1% packet loss we omitted AMI because of the long analysis times. Figure 3.21 shows the block corruption rate for 0.1% packet loss and 0.5% packet reordering. The block corruption rate is worse than for 0.5% packet loss without reordering. In general reordering is worse than loss for our reliable transport technique. For each reordering there is a 50% chance of two wrong consecutive bits. Even if these fall into one data symbol two RS symbols are needed for correction compared to only one RS symbol needed per erasure. In the worst case the bits are in two different symbols and four RS symbols are required for correction. In all the figures we see that at some point the block corruption rate does not decrease quickly anymore with increasing amount of redundancy. This is because of the burstiness of the errors. Few large bursts require a much larger amount of redundancy than needed 66 Block corruption rate 1e−01 1e−03 1e−05 MED AMI ● DED ● ●
Page 1 and 2: Chapter 3 CHAPTER 3. TIME-TO-LIVE C
Page 3 and 4: CHAPTER 3. TIME-TO-LIVE COVERT CHAN
Page 5 and 6: Proportion ≤≤ x 1.0 0.8 0.6 0.4
Page 11 and 12: Capacity (bits/packet) 1.00 0.95 0.
Page 13 and 14: 0 1 1-p L 1-p L p L p L CHAPTER 3.
Page 21 and 22: Algorithm 3.3 Block decoding algori
Page 23 and 24: Error rate 1e−02 1e−04 1e−06
Page 27: Proportion ≤ x 1.0 0.8 0.6 0.4 0.
Page 31 and 32: Throughput (bits/s) 60 50 40 30 20
Page 33 and 34: Throughput (bits/s) 200 150 100 50

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