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Commit ae5220c6 authored by Randy Dunlap's avatar Randy Dunlap Committed by David S. Miller
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networking: Documentation: fix snmp_counters.rst Sphinx warnings 

Fix over 100 documentation warnings in snmp_counter.rst by
extending the underline string lengths and inserting a blank line
after bullet items.

Examples:

Documentation/networking/snmp_counter.rst:1: WARNING: Title overline too short.
Documentation/networking/snmp_counter.rst:14: WARNING: Bullet list ends without a blank line; unexpected unindent.

Fixes: 2b965472 ("add document for TCP OFO, PAWS and skip ACK counters")
Fixes: 8e2ea53a ("add snmp counters document")
Fixes: 712ee16c ("add documents for snmp counters")
Fixes: 80cc4950 ("net: Add part of TCP counts explanations in snmp_counters.rst")
Fixes: b08794a9 ("documentation of some IP/ICMP snmp counters")
Signed-off-by: default avatarRandy Dunlap <rdunlap@infradead.org>
Cc: yupeng <yupeng0921@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent bb3e16ad
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Documentation/networking/snmp_counter.rst
View file @ ae5220c6
===========
============
SNMP counter
===========
============
This document explains the meaning of SNMP counters.
General IPv4 counters
====================
=====================
All layer 4 packets and ICMP packets will change these counters, but
these counters won't be changed by layer 2 packets (such as STP) or
ARP packets.
* IpInReceives
Defined in `RFC1213 ipInReceives`_
.. _RFC1213 ipInReceives: https://tools.ietf.org/html/rfc1213#page-26
......@@ -23,6 +24,7 @@ and so on). It indicates the number of aggregated segments after
GRO/LRO.
* IpInDelivers
Defined in `RFC1213 ipInDelivers`_
.. _RFC1213 ipInDelivers: https://tools.ietf.org/html/rfc1213#page-28
......@@ -33,6 +35,7 @@ supported protocols will be delivered, if someone listens on the raw
socket, all valid IP packets will be delivered.
* IpOutRequests
Defined in `RFC1213 ipOutRequests`_
.. _RFC1213 ipOutRequests: https://tools.ietf.org/html/rfc1213#page-28
......@@ -42,6 +45,7 @@ multicast packets, and would always be updated together with
IpExtOutOctets.
* IpExtInOctets and IpExtOutOctets
They are Linux kernel extensions, no RFC definitions. Please note,
RFC1213 indeed defines ifInOctets and ifOutOctets, but they
are different things. The ifInOctets and ifOutOctets include the MAC
......@@ -49,6 +53,7 @@ layer header size but IpExtInOctets and IpExtOutOctets don't, they
only include the IP layer header and the IP layer data.
* IpExtInNoECTPkts, IpExtInECT1Pkts, IpExtInECT0Pkts, IpExtInCEPkts
They indicate the number of four kinds of ECN IP packets, please refer
`Explicit Congestion Notification`_ for more details.
......@@ -60,6 +65,7 @@ for the same packet, you might find that IpInReceives count 1, but
IpExtInNoECTPkts counts 2 or more.
* IpInHdrErrors
Defined in `RFC1213 ipInHdrErrors`_. It indicates the packet is
dropped due to the IP header error. It might happen in both IP input
and IP forward paths.
......@@ -67,6 +73,7 @@ and IP forward paths.
.. _RFC1213 ipInHdrErrors: https://tools.ietf.org/html/rfc1213#page-27
* IpInAddrErrors
Defined in `RFC1213 ipInAddrErrors`_. It will be increased in two
scenarios: (1) The IP address is invalid. (2) The destination IP
address is not a local address and IP forwarding is not enabled
......@@ -74,6 +81,7 @@ address is not a local address and IP forwarding is not enabled
.. _RFC1213 ipInAddrErrors: https://tools.ietf.org/html/rfc1213#page-27
* IpExtInNoRoutes
This counter means the packet is dropped when the IP stack receives a
packet and can't find a route for it from the route table. It might
happen when IP forwarding is enabled and the destination IP address is
......@@ -81,6 +89,7 @@ not a local address and there is no route for the destination IP
address.
* IpInUnknownProtos
Defined in `RFC1213 ipInUnknownProtos`_. It will be increased if the
layer 4 protocol is unsupported by kernel. If an application is using
raw socket, kernel will always deliver the packet to the raw socket
......@@ -89,10 +98,12 @@ and this counter won't be increased.
.. _RFC1213 ipInUnknownProtos: https://tools.ietf.org/html/rfc1213#page-27
* IpExtInTruncatedPkts
For IPv4 packet, it means the actual data size is smaller than the
"Total Length" field in the IPv4 header.
* IpInDiscards
Defined in `RFC1213 ipInDiscards`_. It indicates the packet is dropped
in the IP receiving path and due to kernel internal reasons (e.g. no
enough memory).
......@@ -100,20 +111,23 @@ enough memory).
.. _RFC1213 ipInDiscards: https://tools.ietf.org/html/rfc1213#page-28
* IpOutDiscards
Defined in `RFC1213 ipOutDiscards`_. It indicates the packet is
dropped in the IP sending path and due to kernel internal reasons.
.. _RFC1213 ipOutDiscards: https://tools.ietf.org/html/rfc1213#page-28
* IpOutNoRoutes
Defined in `RFC1213 ipOutNoRoutes`_. It indicates the packet is
dropped in the IP sending path and no route is found for it.
.. _RFC1213 ipOutNoRoutes: https://tools.ietf.org/html/rfc1213#page-29
ICMP counters
============
=============
* IcmpInMsgs and IcmpOutMsgs
Defined by `RFC1213 icmpInMsgs`_ and `RFC1213 icmpOutMsgs`_
.. _RFC1213 icmpInMsgs: https://tools.ietf.org/html/rfc1213#page-41
......@@ -126,6 +140,7 @@ IcmpOutMsgs would still be updated if the IP header is constructed by
a userspace program.
* ICMP named types
| These counters include most of common ICMP types, they are:
| IcmpInDestUnreachs: `RFC1213 icmpInDestUnreachs`_
| IcmpInTimeExcds: `RFC1213 icmpInTimeExcds`_
......@@ -180,6 +195,7 @@ straightforward. The 'In' counter means kernel receives such a packet
and the 'Out' counter means kernel sends such a packet.
* ICMP numeric types
They are IcmpMsgInType[N] and IcmpMsgOutType[N], the [N] indicates the
ICMP type number. These counters track all kinds of ICMP packets. The
ICMP type number definition could be found in the `ICMP parameters`_
......@@ -192,12 +208,14 @@ IcmpMsgOutType8 would increase 1. And if kernel gets an ICMP Echo Reply
packet, IcmpMsgInType0 would increase 1.
* IcmpInCsumErrors
This counter indicates the checksum of the ICMP packet is
wrong. Kernel verifies the checksum after updating the IcmpInMsgs and
before updating IcmpMsgInType[N]. If a packet has bad checksum, the
IcmpInMsgs would be updated but none of IcmpMsgInType[N] would be updated.
* IcmpInErrors and IcmpOutErrors
Defined by `RFC1213 icmpInErrors`_ and `RFC1213 icmpOutErrors`_
.. _RFC1213 icmpInErrors: https://tools.ietf.org/html/rfc1213#page-41
......@@ -209,7 +227,7 @@ and the sending packet path use IcmpOutErrors. When IcmpInCsumErrors
is increased, IcmpInErrors would always be increased too.
relationship of the ICMP counters
-------------------------------
---------------------------------
The sum of IcmpMsgOutType[N] is always equal to IcmpOutMsgs, as they
are updated at the same time. The sum of IcmpMsgInType[N] plus
IcmpInErrors should be equal or larger than IcmpInMsgs. When kernel
......@@ -229,8 +247,9 @@ IcmpInMsgs should be less than the sum of IcmpMsgOutType[N] plus
IcmpInErrors.
General TCP counters
==================
====================
* TcpInSegs
Defined in `RFC1213 tcpInSegs`_
.. _RFC1213 tcpInSegs: https://tools.ietf.org/html/rfc1213#page-48
......@@ -247,6 +266,7 @@ isn't aware of GRO. So if two packets are merged by GRO, the TcpInSegs
counter would only increase 1.
* TcpOutSegs
Defined in `RFC1213 tcpOutSegs`_
.. _RFC1213 tcpOutSegs: https://tools.ietf.org/html/rfc1213#page-48
......@@ -258,6 +278,7 @@ GSO, so if a packet would be split to 2 by GSO, TcpOutSegs will
increase 2.
* TcpActiveOpens
Defined in `RFC1213 tcpActiveOpens`_
.. _RFC1213 tcpActiveOpens: https://tools.ietf.org/html/rfc1213#page-47
......@@ -267,6 +288,7 @@ state. Every time TcpActiveOpens increases 1, TcpOutSegs should always
increase 1.
* TcpPassiveOpens
Defined in `RFC1213 tcpPassiveOpens`_
.. _RFC1213 tcpPassiveOpens: https://tools.ietf.org/html/rfc1213#page-47
......@@ -275,6 +297,7 @@ It means the TCP layer receives a SYN, replies a SYN+ACK, come into
the SYN-RCVD state.
* TcpExtTCPRcvCoalesce
When packets are received by the TCP layer and are not be read by the
application, the TCP layer will try to merge them. This counter
indicate how many packets are merged in such situation. If GRO is
......@@ -282,12 +305,14 @@ enabled, lots of packets would be merged by GRO, these packets
wouldn't be counted to TcpExtTCPRcvCoalesce.
* TcpExtTCPAutoCorking
When sending packets, the TCP layer will try to merge small packets to
a bigger one. This counter increase 1 for every packet merged in such
situation. Please refer to the LWN article for more details:
https://lwn.net/Articles/576263/
* TcpExtTCPOrigDataSent
This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
explaination below::
......@@ -297,6 +322,7 @@ explaination below::
more useful to track the TCP retransmission rate.
* TCPSynRetrans
This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
explaination below::
......@@ -304,6 +330,7 @@ explaination below::
retransmissions into SYN, fast-retransmits, timeout retransmits, etc.
* TCPFastOpenActiveFail
This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
explaination below::
......@@ -313,6 +340,7 @@ explaination below::
.. _kernel commit f19c29e3e391: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f19c29e3e391a66a273e9afebaf01917245148cd
* TcpExtListenOverflows and TcpExtListenDrops
When kernel receives a SYN from a client, and if the TCP accept queue
is full, kernel will drop the SYN and add 1 to TcpExtListenOverflows.
At the same time kernel will also add 1 to TcpExtListenDrops. When a
......@@ -337,7 +365,7 @@ to the accept queue.
TCP Fast Open
============
=============
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
code provides a good explanation of them, I pasted them below::
......@@ -370,22 +398,24 @@ will disable the fast path at first, and try to enable it after kernel
receives packets.
* TcpExtTCPPureAcks and TcpExtTCPHPAcks
If a packet set ACK flag and has no data, it is a pure ACK packet, if
kernel handles it in the fast path, TcpExtTCPHPAcks will increase 1,
if kernel handles it in the slow path, TcpExtTCPPureAcks will
increase 1.
* TcpExtTCPHPHits
If a TCP packet has data (which means it is not a pure ACK packet),
and this packet is handled in the fast path, TcpExtTCPHPHits will
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
connection is not closed very graceful. An easy way to increase this
......@@ -404,11 +434,13 @@ when the application closes a connection, kernel will send a RST
immediately and increase the TcpExtTCPAbortOnData counter.
* TcpExtTCPAbortOnClose
This counter means the application has unread data in the TCP layer when
the application wants to close the TCP connection. In such a situation,
kernel will send a RST to the other side of the TCP connection.
* TcpExtTCPAbortOnMemory
When an application closes a TCP connection, kernel still need to track
the connection, let it complete the TCP disconnect process. E.g. an
app calls the close method of a socket, kernel sends fin to the other
......@@ -430,10 +462,12 @@ the tcp_mem. Please refer the tcp_mem section in the `TCP man page`_:
* TcpExtTCPAbortOnTimeout
This counter will increase when any of the TCP timers expire. In such
situation, kernel won't send RST, just give up the connection.
* TcpExtTCPAbortOnLinger
When a TCP connection comes into FIN_WAIT_2 state, instead of waiting
for the fin packet from the other side, kernel could send a RST and
delete the socket immediately. This is not the default behavior of
......@@ -441,6 +475,7 @@ Linux kernel TCP stack. By configuring the TCP_LINGER2 socket option,
you could let kernel follow this behavior.
* TcpExtTCPAbortFailed
The kernel TCP layer will send RST if the `RFC2525 2.17 section`_ is
satisfied. If an internal error occurs during this process,
TcpExtTCPAbortFailed will be increased.
......@@ -448,7 +483,7 @@ TcpExtTCPAbortFailed will be increased.
.. _RFC2525 2.17 section: https://tools.ietf.org/html/rfc2525#page-50
TCP Hybrid Slow Start
====================
=====================
The Hybrid Slow Start algorithm is an enhancement of the traditional
TCP congestion window Slow Start algorithm. It uses two pieces of
information to detect whether the max bandwidth of the TCP path is
......@@ -464,23 +499,27 @@ relate with the Hybrid Slow Start algorithm.
.. _Hybrid Slow Start paper: https://pdfs.semanticscholar.org/25e9/ef3f03315782c7f1cbcd31b587857adae7d1.pdf
* TcpExtTCPHystartTrainDetect
How many times the ACK train length threshold is detected
* TcpExtTCPHystartTrainCwnd
The sum of CWND detected by ACK train length. Dividing this value by
TcpExtTCPHystartTrainDetect is the average CWND which detected by the
ACK train length.
* TcpExtTCPHystartDelayDetect
How many times the packet delay threshold is detected.
* TcpExtTCPHystartDelayCwnd
The sum of CWND detected by packet delay. Dividing this value by
TcpExtTCPHystartDelayDetect is the average CWND which detected by the
packet delay.
TCP retransmission and congestion control
======================================
=========================================
The TCP protocol has two retransmission mechanisms: SACK and fast
recovery. They are exclusive with each other. When SACK is enabled,
the kernel TCP stack would use SACK, or kernel would use fast
......@@ -499,12 +538,14 @@ https://pdfs.semanticscholar.org/0e9c/968d09ab2e53e24c4dca5b2d67c7f7140f8e.pdf
.. _RFC6582: https://tools.ietf.org/html/rfc6582
* TcpExtTCPRenoRecovery and TcpExtTCPSackRecovery
When the congestion control comes into Recovery state, if sack is
used, TcpExtTCPSackRecovery increases 1, if sack is not used,
TcpExtTCPRenoRecovery increases 1. These two counters mean the TCP
stack begins to retransmit the lost packets.
* TcpExtTCPSACKReneging
A packet was acknowledged by SACK, but the receiver has dropped this
packet, so the sender needs to retransmit this packet. In this
situation, the sender adds 1 to TcpExtTCPSACKReneging. A receiver
......@@ -515,6 +556,7 @@ the RTO expires for this packet, then the sender assumes this packet
has been dropped by the receiver.
* TcpExtTCPRenoReorder
The reorder packet is detected by fast recovery. It would only be used
if SACK is disabled. The fast recovery algorithm detects recorder by
the duplicate ACK number. E.g., if retransmission is triggered, and
......@@ -525,6 +567,7 @@ order packet. Thus the sender would find more ACks than its
expectation, and the sender knows out of order occurs.
* TcpExtTCPTSReorder
The reorder packet is detected when a hole is filled. E.g., assume the
sender sends packet 1,2,3,4,5, and the receiving order is
1,2,4,5,3. When the sender receives the ACK of packet 3 (which will
......@@ -534,6 +577,7 @@ fill the hole), two conditions will let TcpExtTCPTSReorder increase
than the retransmission timestamp.
* TcpExtTCPSACKReorder
The reorder packet detected by SACK. The SACK has two methods to
detect reorder: (1) DSACK is received by the sender. It means the
sender sends the same packet more than one times. And the only reason
......@@ -558,39 +602,46 @@ sender side.
.. _RFC2883 : https://tools.ietf.org/html/rfc2883
* TcpExtTCPDSACKOldSent
The TCP stack receives a duplicate packet which has been acked, so it
sends a DSACK to the sender.
* TcpExtTCPDSACKOfoSent
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
duplicate packet is received.
* TcpExtTCPDSACKOfoRecv
The TCP stack receives a DSACK, which indicate an out of order
duplicate packet is received.
TCP out of order
===============
================
* TcpExtTCPOFOQueue
The TCP layer receives an out of order packet and has enough memory
to queue it.
* TcpExtTCPOFODrop
The TCP layer receives an out of order packet but doesn't have enough
memory, so drops it. Such packets won't be counted into
TcpExtTCPOFOQueue.
* TcpExtTCPOFOMerge
The received out of order packet has an overlay with the previous
packet. the overlay part will be dropped. All of TcpExtTCPOFOMerge
packets will also be counted into TcpExtTCPOFOQueue.
TCP PAWS
=======
========
PAWS (Protection Against Wrapped Sequence numbers) is an algorithm
which is used to drop old packets. It depends on the TCP
timestamps. For detail information, please refer the `timestamp wiki`_
......@@ -600,13 +651,15 @@ and the `RFC of PAWS`_.
.. _timestamp wiki: https://en.wikipedia.org/wiki/Transmission_Control_Protocol#TCP_timestamps
* TcpExtPAWSActive
Packets are dropped by PAWS in Syn-Sent status.
* TcpExtPAWSEstab
Packets are dropped by PAWS in any status other than Syn-Sent.
TCP ACK skip
===========
============
In some scenarios, kernel would avoid sending duplicate ACKs too
frequently. Please find more details in the tcp_invalid_ratelimit
section of the `sysctl document`_. When kernel decides to skip an ACK
......@@ -618,6 +671,7 @@ it has no data.
.. _sysctl document: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
* TcpExtTCPACKSkippedSynRecv
The ACK is skipped in Syn-Recv status. The Syn-Recv status means the
TCP stack receives a SYN and replies SYN+ACK. Now the TCP stack is
waiting for an ACK. Generally, the TCP stack doesn't need to send ACK
......@@ -631,6 +685,7 @@ increase TcpExtTCPACKSkippedSynRecv.
* TcpExtTCPACKSkippedPAWS
The ACK is skipped due to PAWS (Protect Against Wrapped Sequence
numbers) check fails. If the PAWS check fails in Syn-Recv, Fin-Wait-2
or Time-Wait statuses, the skipped ACK would be counted to
......@@ -639,18 +694,22 @@ TcpExtTCPACKSkippedTimeWait. In all other statuses, the skipped ACK
would be counted to TcpExtTCPACKSkippedPAWS.
* TcpExtTCPACKSkippedSeq
The sequence number is out of window and the timestamp passes the PAWS
check and the TCP status is not Syn-Recv, Fin-Wait-2, and Time-Wait.
* TcpExtTCPACKSkippedFinWait2
The ACK is skipped in Fin-Wait-2 status, the reason would be either
PAWS check fails or the received sequence number is out of window.
* TcpExtTCPACKSkippedTimeWait
Tha ACK is skipped in Time-Wait status, the reason would be either
PAWS check failed or the received sequence number is out of window.
* TcpExtTCPACKSkippedChallenge
The ACK is skipped if the ACK is a challenge ACK. The RFC 5961 defines
3 kind of challenge ACK, please refer `RFC 5961 section 3.2`_,
`RFC 5961 section 4.2`_ and `RFC 5961 section 5.2`_. Besides these
......@@ -664,10 +723,10 @@ unacknowledged number (more strict than `RFC 5961 section 5.2`_).
examples
=======
========
ping test
--------
---------
Run the ping command against the public dns server 8.8.8.8::
nstatuser@nstat-a:~$ ping 8.8.8.8 -c 1
......@@ -711,7 +770,7 @@ and its corresponding Echo Reply packet are constructed by:
So the IpExtInOctets and IpExtOutOctets are 20+16+48=84.
tcp 3-way handshake
------------------
-------------------
On server side, we run::
nstatuser@nstat-b:~$ nc -lknv 0.0.0.0 9000
......@@ -753,7 +812,7 @@ ACK, so client sent 2 packets, received 1 packet, TcpInSegs increased
1, TcpOutSegs increased 2.
TCP normal traffic
-----------------
------------------
Run nc on server::
nstatuser@nstat-b:~$ nc -lkv 0.0.0.0 9000
......@@ -876,7 +935,7 @@ and the packet received from client qualified for fast path, so it
was counted into 'TcpExtTCPHPHits'.
TcpExtTCPAbortOnClose
--------------------
---------------------
On the server side, we run below python script::
import socket
......@@ -910,7 +969,7 @@ If we run tcpdump on the server side, we could find the server sent a
RST after we type Ctrl-C.
TcpExtTCPAbortOnMemory and TcpExtTCPAbortOnTimeout
-----------------------------------------------
---------------------------------------------------
Below is an example which let the orphan socket count be higher than
net.ipv4.tcp_max_orphans.
Change tcp_max_orphans to a smaller value on client::
......@@ -1032,7 +1091,7 @@ FIN_WAIT_1 state finally. So we wait for a few minutes, we could find
TcpExtTCPAbortOnTimeout 10 0.0
TcpExtTCPAbortOnLinger
---------------------
----------------------
The server side code::
nstatuser@nstat-b:~$ cat server_linger.py
......@@ -1077,7 +1136,7 @@ After run client_linger.py, check the output of nstat::
TcpExtTCPAbortOnLinger 1 0.0
TcpExtTCPRcvCoalesce
-------------------
--------------------
On the server, we run a program which listen on TCP port 9000, but
doesn't read any data::
......@@ -1137,7 +1196,7 @@ the receiving queue. So the TCP layer merged the two packets, and we
could find the TcpExtTCPRcvCoalesce increased 1.
TcpExtListenOverflows and TcpExtListenDrops
----------------------------------------
-------------------------------------------
On server, run the nc command, listen on port 9000::
nstatuser@nstat-b:~$ nc -lkv 0.0.0.0 9000
......@@ -1185,7 +1244,7 @@ TcpExtListenOverflows and TcpExtListenDrops would be larger, because
the SYN of the 4th nc was dropped, the client was retrying.
IpInAddrErrors, IpExtInNoRoutes and IpOutNoRoutes
----------------------------------------------
-------------------------------------------------
server A IP address: 192.168.122.250
server B IP address: 192.168.122.251
Prepare on server A, add a route to server B::
......@@ -1280,7 +1339,7 @@ a route for the 8.8.8.8 IP address, so server B increased
IpOutNoRoutes.
TcpExtTCPACKSkippedSynRecv
------------------------
--------------------------
In this test, we send 3 same SYN packets from client to server. The
first SYN will let server create a socket, set it to Syn-Recv status,
and reply a SYN/ACK. The second SYN will let server reply the SYN/ACK
......@@ -1328,7 +1387,7 @@ Check snmp cunter on nstat-b::
As we expected, TcpExtTCPACKSkippedSynRecv is 1.
TcpExtTCPACKSkippedPAWS
----------------------
-----------------------
To trigger PAWS, we could send an old SYN.
On nstat-b, let nc listen on port 9000::
......@@ -1365,7 +1424,7 @@ failed, the nstat-b replied an ACK for the first SYN, skipped the ACK
for the second SYN, and updated TcpExtTCPACKSkippedPAWS.
TcpExtTCPACKSkippedSeq
--------------------
----------------------
To trigger TcpExtTCPACKSkippedSeq, we send packets which have valid
timestamp (to pass PAWS check) but the sequence number is out of
window. The linux TCP stack would avoid to skip if the packet has
......
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