TCP Client SYN Retries

Number of retries

#define TCP_SYN_RETRIES	 6	/* This is how many retries are done
				 * when active opening a connection.
				 * RFC1122 says the minimum retry MUST
				 * be at least 180secs.  Nevertheless
				 * this value is corresponding to
				 * 63secs of retransmission with the
				 * current initial RTO.
				 */

Timeout

#define TCP_TIMEOUT_INIT ((unsigned)(1*HZ))	/* RFC6298 2.1 initial RTO value	*/

net.ipv4.tcp_syn_linear_timeouts - INTEGER (since Linux 6.5)

The number of times for an active TCP connection to retransmit SYNs with a linear backoff timeout before defaulting to an exponential backoff timeout. This has no effect on SYNACK at the passive TCP side.

With an initial RTO of 1 and tcp_syn_linear_timeouts = 4 we would expect SYN RTOs to be: 1, 1, 1, 1, 1, 2, 4, … (4 linear timeouts, and the first exponential backoff using 2^0 * initial_RTO). Default: 4

Referenes

• https://github.com/torvalds/linux/blob/795c58e4c7fc6163d8fb9f2baa86cfe898fa4b19/include/net/tcp.h#L110-L117
• https://github.com/torvalds/linux/blob/795c58e4c7fc6163d8fb9f2baa86cfe898fa4b19/include/net/tcp.h#L152
• https://github.com/torvalds/linux/blob/795c58e4c7fc6163d8fb9f2baa86cfe898fa4b19/net/ipv4/tcp_timer.c#L260-L262
• https://docs.kernel.org/networking/ip-sysctl.html

Comments on Kernel Source Code

• 3WHS means three-way handshake.
• tcp_init_transfer and subsequently tcp_init_metrics are only called after the connection has been established, so TCP_TIMEOUT_FALLBACK is not used during the three-way handshake.