1This document describes the interfaces /proc/net/tcp and /proc/net/tcp6.
2Note that these interfaces are deprecated in favor of tcp_diag.
3
4These /proc interfaces provide information about currently active TCP 
5connections, and are implemented by tcp4_seq_show() in net/ipv4/tcp_ipv4.c
6and tcp6_seq_show() in net/ipv6/tcp_ipv6.c, respectively.
7
8It will first list all listening TCP sockets, and next list all established
9TCP connections. A typical entry of /proc/net/tcp would look like this (split 
10up into 3 parts because of the length of the line):
11
12   46: 010310AC:9C4C 030310AC:1770 01 
13   |      |      |      |      |   |--> connection state
14   |      |      |      |      |------> remote TCP port number
15   |      |      |      |-------------> remote IPv4 address
16   |      |      |--------------------> local TCP port number
17   |      |---------------------------> local IPv4 address
18   |----------------------------------> number of entry
19
20   00000150:00000000 01:00000019 00000000  
21      |        |     |     |       |--> number of unrecovered RTO timeouts
22      |        |     |     |----------> number of jiffies until timer expires
23      |        |     |----------------> timer_active (see below)
24      |        |----------------------> receive-queue
25      |-------------------------------> transmit-queue
26
27   1000        0 54165785 4 cd1e6040 25 4 27 3 -1
28    |          |    |     |    |     |  | |  | |--> slow start size threshold, 
29    |          |    |     |    |     |  | |  |      or -1 if the threshold
30    |          |    |     |    |     |  | |  |      is >= 0xFFFF
31    |          |    |     |    |     |  | |  |----> sending congestion window
32    |          |    |     |    |     |  | |-------> (ack.quick<<1)|ack.pingpong
33    |          |    |     |    |     |  |---------> Predicted tick of soft clock
34    |          |    |     |    |     |              (delayed ACK control data)
35    |          |    |     |    |     |------------> retransmit timeout
36    |          |    |     |    |------------------> location of socket in memory
37    |          |    |     |-----------------------> socket reference count
38    |          |    |-----------------------------> inode
39    |          |----------------------------------> unanswered 0-window probes
40    |---------------------------------------------> uid
41
42timer_active:
43  0  no timer is pending
44  1  retransmit-timer is pending
45  2  another timer (e.g. delayed ack or keepalive) is pending
46  3  this is a socket in TIME_WAIT state. Not all fields will contain 
47     data (or even exist)
48  4  zero window probe timer is pending
49