[PATCH net] add document for several snmp counters

[yupeng <yupeng0921@gmail.com>]

add document for below counters:
TcpEstabResets
TcpAttemptFails
TcpOutRsts
TcpExtTCPSACKDiscard
TcpExtTCPDSACKIgnoredOld
TcpExtTCPDSACKIgnoredNoUndo
TcpExtTCPSackShifted
TcpExtTCPSackMerged
TcpExtTCPSackShiftFallback
TcpExtTCPWantZeroWindowAdv
TcpExtTCPToZeroWindowAdv
TcpExtTCPFromZeroWindowAdv
TcpExtDelayedACKs
TcpExtDelayedACKLocked
TcpExtDelayedACKLost
TcpExtTCPLossProbes
TcpExtTCPLossProbeRecovery

Signed-off-by: yupeng <yupeng0921@gmail.com>
---
 Documentation/networking/snmp_counter.rst | 130 +++++++++++++++++++++-
 1 file changed, 125 insertions(+), 5 deletions(-)

diff --git a/Documentation/networking/snmp_counter.rst b/Documentation/networking/snmp_counter.rst
index b0dfdaaca512..fe8f741193be 100644
--- a/Documentation/networking/snmp_counter.rst
+++ b/Documentation/networking/snmp_counter.rst
@@ -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: https://tools.ietf.org/html/rfc1213#page-48
+
+* TcpAttemptFails
+Defined in `RFC1213 tcpAttemptFails`_.
+
+.. _RFC1213 tcpAttemptFails: https://tools.ietf.org/html/rfc1213#page-48
+
+* 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: https://tools.ietf.org/html/rfc1213#page-52
+
+
+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.
 
-
 DSACK
 =====
 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.
 
 * TcpExtTCPDSACKRecv
-The TCP stack receives a DSACK, which indicate an acknowledged
+The TCP stack receives a DSACK, which indicates an acknowledged
 duplicate packet is received.
 
 * TcpExtTCPDSACKOfoRecv
 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
+explaination:
+
+.. _Add counters for discarded SACK blocks: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=18f02545a9a16c9a89778b91a162ad16d510bb32
+
+* 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: https://tools.ietf.org/html/rfc5961#page-9
 .. _RFC 5961 section 5.2: https://tools.ietf.org/html/rfc5961#page-11
 
+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: https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment
+
+* 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
+mode.
+
+* 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
+network.
+
+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: https://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01
+
+* TcpExtTCPLossProbes
+A TLP probe packet is sent.
+
+* TcpExtTCPLossProbeRecovery
+A packet loss is detected and recovered by TLP.
 
 examples
 =======
-- 
2.17.1