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add snmp counter document 

add document for tcp retransmission, tcp fast open, syn cookies,
challenge ack, prune and several general counters

Signed-off-by: yupeng <yupeng0921@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
yupeng 2019-02-09 14:46:18 -08:00 committed by David S. Miller
parent e240b7dbb7
commit 132c4e9e6a
1 changed files with 181 additions and 3 deletions
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184
Documentation/networking/snmp_counter.rst
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@ -367,16 +367,19 @@ 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
@ -384,6 +387,30 @@ process might be failed due to some errors (e.g. memory alloc failed).
.. _RFC1213 tcpOutRsts: https://tools.ietf.org/html/rfc1213#page-52
* TcpExtTCPSpuriousRtxHostQueues
When the TCP stack wants to retransmit a packet, and finds that packet
is not lost in the network, but the packet is not sent yet, the TCP
stack would give up the retransmission and update this counter. It
might happen if a packet stays too long time in a qdisc or driver
queue.
* TcpEstabResets
The socket receives a RST packet in Establish or CloseWait state.
* TcpExtTCPKeepAlive
This counter indicates many keepalive packets were sent. The keepalive
won't be enabled by default. A userspace program could enable it by
setting the SO_KEEPALIVE socket option.
* TcpExtTCPSpuriousRTOs
The spurious retransmission timeout detected by the `F-RTO`_
algorithm.
.. _F-RTO: https://tools.ietf.org/html/rfc5682
TCP Fast Path
============
@ -609,6 +636,29 @@ 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.
* TcpExtTCPSlowStartRetrans
The TCP stack wants to retransmit a packet and the congestion control
state is 'Loss'.
* TcpExtTCPFastRetrans
The TCP stack wants to retransmit a packet and the congestion control
state is not 'Loss'.
* TcpExtTCPLostRetransmit
A SACK points out that a retransmission packet is lost again.
* TcpExtTCPRetransFail
The TCP stack tries to deliver a retransmission packet to lower layers
but the lower layers return an error.
* TcpExtTCPSynRetrans
The TCP stack retransmits a SYN packet.
DSACK
=====
The DSACK is defined in `RFC2883`_. The receiver uses DSACK to report
@ -790,8 +840,9 @@ unacknowledged number (more strict than `RFC 5961 section 5.2`_).
.. _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
@ -799,14 +850,16 @@ 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`_
@ -814,10 +867,12 @@ packet count in the network. For more details, please refer the
.. _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
@ -826,24 +881,147 @@ 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.
TCP Fast Open
=============
TCP Fast Open is a technology which allows data transfer before the
3-way handshake complete. Please refer the `TCP Fast Open wiki`_ for a
general description.
.. _TCP Fast Open wiki: https://en.wikipedia.org/wiki/TCP_Fast_Open
* TcpExtTCPFastOpenActive
When the TCP stack receives an ACK packet in the SYN-SENT status, and
the ACK packet acknowledges the data in the SYN packet, the TCP stack
understand the TFO cookie is accepted by the other side, then it
updates this counter.
* TcpExtTCPFastOpenActiveFail
This counter indicates that the TCP stack initiated a TCP Fast Open,
but it failed. This counter would be updated in three scenarios: (1)
the other side doesn't acknowledge the data in the SYN packet. (2) The
SYN packet which has the TFO cookie is timeout at least once. (3)
after the 3-way handshake, the retransmission timeout happens
net.ipv4.tcp_retries1 times, because some middle-boxes may black-hole
fast open after the handshake.
* TcpExtTCPFastOpenPassive
This counter indicates how many times the TCP stack accepts the fast
open request.
* TcpExtTCPFastOpenPassiveFail
This counter indicates how many times the TCP stack rejects the fast
open request. It is caused by either the TFO cookie is invalid or the
TCP stack finds an error during the socket creating process.
* TcpExtTCPFastOpenListenOverflow
When the pending fast open request number is larger than
fastopenq->max_qlen, the TCP stack will reject the fast open request
and update this counter. When this counter is updated, the TCP stack
won't update TcpExtTCPFastOpenPassive or
TcpExtTCPFastOpenPassiveFail. The fastopenq->max_qlen is set by the
TCP_FASTOPEN socket operation and it could not be larger than
net.core.somaxconn. For example:
setsockopt(sfd, SOL_TCP, TCP_FASTOPEN, &qlen, sizeof(qlen));
* TcpExtTCPFastOpenCookieReqd
This counter indicates how many times a client wants to request a TFO
cookie.
SYN cookies
===========
SYN cookies are used to mitigate SYN flood, for details, please refer
the `SYN cookies wiki`_.
.. _SYN cookies wiki: https://en.wikipedia.org/wiki/SYN_cookies
* TcpExtSyncookiesSent
It indicates how many SYN cookies are sent.
* TcpExtSyncookiesRecv
How many reply packets of the SYN cookies the TCP stack receives.
* TcpExtSyncookiesFailed
The MSS decoded from the SYN cookie is invalid. When this counter is
updated, the received packet won't be treated as a SYN cookie and the
TcpExtSyncookiesRecv counter wont be updated.
Challenge ACK
=============
For details of challenge ACK, please refer the explaination of
TcpExtTCPACKSkippedChallenge.
* TcpExtTCPChallengeACK
The number of challenge acks sent.
* TcpExtTCPSYNChallenge
The number of challenge acks sent in response to SYN packets. After
updates this counter, the TCP stack might send a challenge ACK and
update the TcpExtTCPChallengeACK counter, or it might also skip to
send the challenge and update the TcpExtTCPACKSkippedChallenge.
prune
=====
When a socket is under memory pressure, the TCP stack will try to
reclaim memory from the receiving queue and out of order queue. One of
the reclaiming method is 'collapse', which means allocate a big sbk,
copy the contiguous skbs to the single big skb, and free these
contiguous skbs.
* TcpExtPruneCalled
The TCP stack tries to reclaim memory for a socket. After updates this
counter, the TCP stack will try to collapse the out of order queue and
the receiving queue. If the memory is still not enough, the TCP stack
will try to discard packets from the out of order queue (and update the
TcpExtOfoPruned counter)
* TcpExtOfoPruned
The TCP stack tries to discard packet on the out of order queue.
* TcpExtRcvPruned
After 'collapse' and discard packets from the out of order queue, if
the actually used memory is still larger than the max allowed memory,
this counter will be updated. It means the 'prune' fails.
* TcpExtTCPRcvCollapsed
This counter indicates how many skbs are freed during 'collapse'.
examples
========