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5aa4b32f AP |
1 | Thin-streams and TCP |
2 | ==================== | |
3 | A wide range of Internet-based services that use reliable transport | |
4 | protocols display what we call thin-stream properties. This means | |
5 | that the application sends data with such a low rate that the | |
6 | retransmission mechanisms of the transport protocol are not fully | |
7 | effective. In time-dependent scenarios (like online games, control | |
8 | systems, stock trading etc.) where the user experience depends | |
9 | on the data delivery latency, packet loss can be devastating for | |
10 | the service quality. Extreme latencies are caused by TCP's | |
11 | dependency on the arrival of new data from the application to trigger | |
12 | retransmissions effectively through fast retransmit instead of | |
13 | waiting for long timeouts. | |
14 | ||
15 | After analysing a large number of time-dependent interactive | |
16 | applications, we have seen that they often produce thin streams | |
17 | and also stay with this traffic pattern throughout its entire | |
18 | lifespan. The combination of time-dependency and the fact that the | |
19 | streams provoke high latencies when using TCP is unfortunate. | |
20 | ||
21 | In order to reduce application-layer latency when packets are lost, | |
22 | a set of mechanisms has been made, which address these latency issues | |
23 | for thin streams. In short, if the kernel detects a thin stream, | |
24 | the retransmission mechanisms are modified in the following manner: | |
25 | ||
26 | 1) If the stream is thin, fast retransmit on the first dupACK. | |
27 | 2) If the stream is thin, do not apply exponential backoff. | |
28 | ||
29 | These enhancements are applied only if the stream is detected as | |
30 | thin. This is accomplished by defining a threshold for the number | |
31 | of packets in flight. If there are less than 4 packets in flight, | |
32 | fast retransmissions can not be triggered, and the stream is prone | |
33 | to experience high retransmission latencies. | |
34 | ||
35 | Since these mechanisms are targeted at time-dependent applications, | |
36 | they must be specifically activated by the application using the | |
37 | TCP_THIN_LINEAR_TIMEOUTS and TCP_THIN_DUPACK IOCTLS or the | |
38 | tcp_thin_linear_timeouts and tcp_thin_dupack sysctls. Both | |
39 | modifications are turned off by default. | |
40 | ||
41 | References | |
42 | ========== | |
43 | More information on the modifications, as well as a wide range of | |
44 | experimental data can be found here: | |
45 | "Improving latency for interactive, thin-stream applications over | |
46 | reliable transport" | |
47 | http://simula.no/research/nd/publications/Simula.nd.477/simula_pdf_file |