## 3 Algorithm Design and Analysis
### TCP-SCTP Method
#### 3.3.3 Algorithm Design
Different from the TCP-SCTP switch process, the MS-SCTP method switched the primary path to improve the network condition instead of adjusting the current network congestion by switching another protocol. After the path switch, MS-SCTP found a path that was better than the current path. Figure 7c displays that when the PER reached 20%, the MS-SCTP method could keep the throughput stable around 20 KBps, while the throughputs of the TCP protocol and the SCTP native protocol were only about half of this scheme. From Figure 7d, the throughputs of the TCP protocol and the SCTP native protocol were significantly reduced or even close to zero. However, the throughput of the MS-SCTP scheme was only slightly lower than the PER when the PER was 20%, and it still stayed around 10 KBps stably, not declining rapidly due to the deterioration of the link condition. When the PER went up to 40% in Figure 7e, the real-time throughput curve under the MS-SCTP scheme still existed. However, at this point, the throughput under the TCP and the native SCTP protocols had begun to approach zero. Even when the PER was as high as 50%, as shown in Figure 7f, the real-time throughput of MS-SCTP was also 1 KBps. Figure 8 shows the comparison of other metrics among the three schemes. Figure 8a shows the comparison of average throughput with the three schemes at different PERs. At the PER of 0%, the throughput curve looked the same when using the MS-SCTP schemes as it did when using the SCTP native protocols for data transfer. However, the average throughput of the SCTP multipath switching protocol was slightly higher. As the PER increased, the average throughput of the three schemes decreased rapidly. However, from the range of 0% to 10%, the decline trend of average throughput was much slower when the MS-SCTP scheme was used. This shows that the scheme played an important role in the improvement of data transmission. When the PER reached 30%, the average throughput of the MS-SCTP scheme still had obvious advantages. After the PER reached 50%, the condition of each link became very bad, and the average throughput approached zero. Figure 7: Real-time throughput comparison when the error rate was 0-50%. (**a**) PER: 0%; (**b**) PER: 10%; (**c**) PER: 20%; (**d**) PER: 30%; (**e**) PER: 40%; (**f**) PER: 50%. From Figure 8b, when using the MS-SCTP scheme for data transmission, the throughput variance variation trend was similar to that when using SCTP. The scheme was an improvement on the SCTP protocol, and it still retained some features of SCTP. SCTP was more volatile in throughput changes than TCP and remained a feature of MS-SCTP. The MS-SCTP had a smaller variance in throughput

Figure 8: Comparison of three schemes’ performances. (**a**) Average throughput; (**b**) throughput variance; (**c**) packet loss rate. than the SCTP native protocol, but it was larger than TCP. All three curves showed an upward trend before the PER reached 10%. When the PER reached 20%, the throughput fluctuation range of the MS-SCTP was close to TCP. At this point, the throughputs of the TCP and the native SCTP protocols were greatly reduced. However, the MS-SCTP also had the advantage of relatively large throughput and relative stability while maintaining a large throughput, which was the advantage of this scheme. When the PER reached 40%, the throughput variance curve of MS-SCTP was at the bottom, and its throughput was still the highest. The MS-SCTP was indeed a great improvement over SCTP, which not only improved throughput but also ensured better stability to some extent. As shown in Figure 8c, the packet loss rate of the three schemes had the same trend under different PERs, which all increased with the rise of PER. However, in each case of the same packet loss rate, the packet loss rate of the MS-SCTP scheme was lower than that when using the TCP and the SCTP native protocols.