Commit a6c7c7aa authored by yupeng's avatar yupeng Committed by David S. Miller
Browse files

net: add document for several snmp counters

add document for below counters:
Signed-off-by: default avataryupeng <>
Signed-off-by: default avatarDavid S. Miller <>
parent 26fc181e
......@@ -336,7 +336,26 @@ time client replies ACK, this socket will get another chance to move
to the accept queue.
TCP Fast Open
* TcpEstabResets
Defined in `RFC1213 tcpEstabResets`_.
.. _RFC1213 tcpEstabResets:
* TcpAttemptFails
Defined in `RFC1213 tcpAttemptFails`_.
.. _RFC1213 tcpAttemptFails:
* TcpOutRsts
Defined in `RFC1213 tcpOutRsts`_. The RFC says this counter indicates
the 'segments sent containing the RST flag', but in linux kernel, this
couner indicates the segments kerenl tried to send. The sending
process might be failed due to some errors (e.g. memory alloc failed).
.. _RFC1213 tcpOutRsts:
TCP Fast Path
When kernel receives a TCP packet, it has two paths to handler the
packet, one is fast path, another is slow path. The comment in kernel
......@@ -383,8 +402,6 @@ increase 1.
TCP abort
* TcpExtTCPAbortOnData
It means TCP layer has data in flight, but need to close the
connection. So TCP layer sends a RST to the other side, indicate the
......@@ -545,7 +562,6 @@ packet yet, the sender would know packet 4 is out of order. The TCP
stack of kernel will increase TcpExtTCPSACKReorder for both of the
above scenarios.
The DSACK is defined in `RFC2883`_. The receiver uses DSACK to report
......@@ -566,13 +582,63 @@ The TCP stack receives an out of order duplicate packet, so it sends a
DSACK to the sender.
The TCP stack receives a DSACK, which indicate an acknowledged
The TCP stack receives a DSACK, which indicates an acknowledged
duplicate packet is received.
The TCP stack receives a DSACK, which indicate an out of order
duplicate packet is received.
invalid SACK and DSACK
When a SACK (or DSACK) block is invalid, a corresponding counter would
be updated. The validation method is base on the start/end sequence
number of the SACK block. For more details, please refer the comment
of the function tcp_is_sackblock_valid in the kernel source code. A
SACK option could have up to 4 blocks, they are checked
individually. E.g., if 3 blocks of a SACk is invalid, the
corresponding counter would be updated 3 times. The comment of the
`Add counters for discarded SACK blocks`_ patch has additional
.. _Add counters for discarded SACK blocks:
* TcpExtTCPSACKDiscard
This counter indicates how many SACK blocks are invalid. If the invalid
SACK block is caused by ACK recording, the TCP stack will only ignore
it and won't update this counter.
* TcpExtTCPDSACKIgnoredOld and TcpExtTCPDSACKIgnoredNoUndo
When a DSACK block is invalid, one of these two counters would be
updated. Which counter will be updated depends on the undo_marker flag
of the TCP socket. If the undo_marker is not set, the TCP stack isn't
likely to re-transmit any packets, and we still receive an invalid
DSACK block, the reason might be that the packet is duplicated in the
middle of the network. In such scenario, TcpExtTCPDSACKIgnoredNoUndo
will be updated. If the undo_marker is set, TcpExtTCPDSACKIgnoredOld
will be updated. As implied in its name, it might be an old packet.
SACK shift
The linux networking stack stores data in sk_buff struct (skb for
short). If a SACK block acrosses multiple skb, the TCP stack will try
to re-arrange data in these skb. E.g. if a SACK block acknowledges seq
10 to 15, skb1 has seq 10 to 13, skb2 has seq 14 to 20. The seq 14 and
15 in skb2 would be moved to skb1. This operation is 'shift'. If a
SACK block acknowledges seq 10 to 20, skb1 has seq 10 to 13, skb2 has
seq 14 to 20. All data in skb2 will be moved to skb1, and skb2 will be
discard, this operation is 'merge'.
* TcpExtTCPSackShifted
A skb is shifted
* TcpExtTCPSackMerged
A skb is merged
* TcpExtTCPSackShiftFallback
A skb should be shifted or merged, but the TCP stack doesn't do it for
some reasons.
TCP out of order
* TcpExtTCPOFOQueue
......@@ -662,6 +728,60 @@ unacknowledged number (more strict than `RFC 5961 section 5.2`_).
.. _RFC 5961 section 4.2:
.. _RFC 5961 section 5.2:
TCP receive window
* TcpExtTCPWantZeroWindowAdv
Depending on current memory usage, the TCP stack tries to set receive
window to zero. But the receive window might still be a no-zero
value. For example, if the previous window size is 10, and the TCP
stack receives 3 bytes, the current window size would be 7 even if the
window size calculated by the memory usage is zero.
* TcpExtTCPToZeroWindowAdv
The TCP receive window is set to zero from a no-zero value.
* TcpExtTCPFromZeroWindowAdv
The TCP receive window is set to no-zero value from zero.
Delayed ACK
The TCP Delayed ACK is a technique which is used for reducing the
packet count in the network. For more details, please refer the
`Delayed ACK wiki`_
.. _Delayed ACK wiki:
* TcpExtDelayedACKs
A delayed ACK timer expires. The TCP stack will send a pure ACK packet
and exit the delayed ACK mode.
* TcpExtDelayedACKLocked
A delayed ACK timer expires, but the TCP stack can't send an ACK
immediately due to the socket is locked by a userspace program. The
TCP stack will send a pure ACK later (after the userspace program
unlock the socket). When the TCP stack sends the pure ACK later, the
TCP stack will also update TcpExtDelayedACKs and exit the delayed ACK
* TcpExtDelayedACKLost
It will be updated when the TCP stack receives a packet which has been
ACKed. A Delayed ACK loss might cause this issue, but it would also be
triggered by other reasons, such as a packet is duplicated in the
Tail Loss Probe (TLP)
TLP is an algorithm which is used to detect TCP packet loss. For more
details, please refer the `TLP paper`_.
.. _TLP paper:
* TcpExtTCPLossProbes
A TLP probe packet is sent.
* TcpExtTCPLossProbeRecovery
A packet loss is detected and recovered by TLP.
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