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| -rw-r--r-- | Documentation/networking/tcp-thin.txt | 47 | ||||
| -rw-r--r-- | include/net/tcp.h | 8 |
2 files changed, 55 insertions, 0 deletions
diff --git a/Documentation/networking/tcp-thin.txt b/Documentation/networking/tcp-thin.txt new file mode 100644 index 00000000000..151e229980f --- /dev/null +++ b/Documentation/networking/tcp-thin.txt @@ -0,0 +1,47 @@ +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 diff --git a/include/net/tcp.h b/include/net/tcp.h index 75a00c80bdd..0bdc3f64024 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1386,6 +1386,14 @@ static inline void tcp_highest_sack_combine(struct sock *sk, tcp_sk(sk)->highest_sack = new; } +/* Determines whether this is a thin stream (which may suffer from + * increased latency). Used to trigger latency-reducing mechanisms. + */ +static inline unsigned int tcp_stream_is_thin(struct tcp_sock *tp) +{ + return tp->packets_out < 4 && !tcp_in_initial_slowstart(tp); +} + /* /proc */ enum tcp_seq_states { TCP_SEQ_STATE_LISTENING, |