1.. SPDX-License-Identifier: GPL-2.0
2
3============================================
4The proc/net/tcp and proc/net/tcp6 variables
5============================================
6
7This document describes the interfaces /proc/net/tcp and /proc/net/tcp6.
8Note that these interfaces are deprecated in favor of tcp_diag.
9
10These /proc interfaces provide information about currently active TCP
11connections, and are implemented by tcp4_seq_show() in net/ipv4/tcp_ipv4.c
12and tcp6_seq_show() in net/ipv6/tcp_ipv6.c, respectively.
13
14It will first list all listening TCP sockets, and next list all established
15TCP connections. A typical entry of /proc/net/tcp would look like this (split
16up into 3 parts because of the length of the line)::
17
18   46: 010310AC:9C4C 030310AC:1770 01
19   |      |      |      |      |   |--> connection state
20   |      |      |      |      |------> remote TCP port number
21   |      |      |      |-------------> remote IPv4 address
22   |      |      |--------------------> local TCP port number
23   |      |---------------------------> local IPv4 address
24   |----------------------------------> number of entry
25
26   00000150:00000000 01:00000019 00000000
27      |        |     |     |       |--> number of unrecovered RTO timeouts
28      |        |     |     |----------> number of jiffies until timer expires
29      |        |     |----------------> timer_active (see below)
30      |        |----------------------> receive-queue
31      |-------------------------------> transmit-queue
32
33   1000        0 54165785 4 cd1e6040 25 4 27 3 -1
34    |          |    |     |    |     |  | |  | |--> slow start size threshold,
35    |          |    |     |    |     |  | |  |      or -1 if the threshold
36    |          |    |     |    |     |  | |  |      is >= 0xFFFF
37    |          |    |     |    |     |  | |  |----> sending congestion window
38    |          |    |     |    |     |  | |-------> (ack.quick<<1)|ack.pingpong
39    |          |    |     |    |     |  |---------> Predicted tick of soft clock
40    |          |    |     |    |     |              (delayed ACK control data)
41    |          |    |     |    |     |------------> retransmit timeout
42    |          |    |     |    |------------------> location of socket in memory
43    |          |    |     |-----------------------> socket reference count
44    |          |    |-----------------------------> inode
45    |          |----------------------------------> unanswered 0-window probes
46    |---------------------------------------------> uid
47
48timer_active:
49
50 ==  ================================================================
51  0  no timer is pending
52  1  retransmit-timer is pending
53  2  another timer (e.g. delayed ack or keepalive) is pending
54  3  this is a socket in TIME_WAIT state. Not all fields will contain
55     data (or even exist)
56  4  zero window probe timer is pending
57 ==  ================================================================
58