| 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 |