The initial value of search_low to be used by the packetization layer

$cat /proc/sys/net/ipv4/tcp_base_mss
512
$

tcp_base_mss - INTEGER
        The initial value of search_low to be used by the packetization layer
        Path MTU discovery (MTU probing).  If MTU probing is enabled,
        this is the initial MSS used by the connection.

grepping through linux kernel source 2.6.32

sysctl_net_ipv4.c:              .procname       = "tcp_base_mss",
sysctl_net_ipv4.c:              .data           = &sysctl_tcp_base_mss,

tcp_output.c:int sysctl_tcp_base_mss __read_mostly = 512;