scale factor for portion of window reserved for buffering overhead

$cat /proc/sys/net/ipv4/tcp_app_win
31
$

Scale factor for portion of window reserved for buffering overhead. Reserve max(window / 2 ^ tcp_app_win, mss) of window for application buffer. Value 0 is special, it means that nothing is reserved. The default value is 31.
[ wireshark ]
Frame 1 (1496 bytes on wire, 1496 bytes captured)
Ethernet II, Src: Netgear_b4:91:1a (00:1f:33:b4:91:1a), Dst: Giga-Byt_f2:71:b6 (00:1a:4d:f2:71:b6)
Internet Protocol, Src: 122.166.110.18 (122.166.110.18), Dst: 10.0.0.2 (10.0.0.2)
Transmission Control Protocol, Src Port: http (80), Dst Port: 51385 (51385), Seq: 1, Ack: 1, Len: 1430
Flags: 0x10 (ACK)

Window size: 514

Checksum: 0x9e3e [validation disabled]
Options: (12 bytes)
SEQ/ACK analysis
Hypertext Transfer Protocol
grepping through linux kernel source 2.6.32

sysctl_net_ipv4.c:              .procname       = "tcp_app_win",
sysctl_net_ipv4.c:              .data           = &sysctl_tcp_app_win,

tcp_input.c:int sysctl_tcp_app_win __read_mostly = 31;