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