documentation of some IP/ICMP snmp counters

The snmp_counter.rst explains the meanings of snmp counters. It also
provides a set of experiments (only 1 for this initial patch),
combines the experiments' resutls and the snmp counters'
meanings. This is an initial path, only explains a part of IP/ICMP
counters and provide a simple ping test.
Signed-off-by: yupeng <yupeng0921@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -31,6 +31,7 @@ Contents:
    net_failover
    alias
    bridge
+   snmp_counter
 
 .. only::  subproject
 

Documentation/networking/snmp_counter.rst

+===========
+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
+
+The number of packets received by the IP layer. It gets increasing at the
+beginning of ip_rcv function, always be updated together with
+IpExtInOctets. 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
+
+The number of packets delivers to the upper layer protocols. E.g. TCP, UDP,
+ICMP and so on. If no one listens on a raw socket, only kernel
+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
+
+The number of packets sent via IP layer, for both single cast and
+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
+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.
+
+.. _Explicit Congestion Notification: https://tools.ietf.org/html/rfc3168#page-6
+
+These 4 counters calculate how many packets received per ECN
+status. They count the real frame number regardless the LRO/GRO. So
+for the same packet, you might find that IpInReceives count 1, but
+IpExtInNoECTPkts counts 2 or more.
+
+ICMP counters
+============
+* IcmpInMsgs and IcmpOutMsgs
+Defined by `RFC1213 icmpInMsgs`_ and `RFC1213 icmpOutMsgs`_
+
+.. _RFC1213 icmpInMsgs: https://tools.ietf.org/html/rfc1213#page-41
+.. _RFC1213 icmpOutMsgs: https://tools.ietf.org/html/rfc1213#page-43
+
+As mentioned in the RFC1213, these two counters include errors, they
+would be increased even if the ICMP packet has an invalid type. The
+ICMP output path will check the header of a raw socket, so the
+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`_
+| IcmpInParmProbs: `RFC1213 icmpInParmProbs`_
+| IcmpInSrcQuenchs: `RFC1213 icmpInSrcQuenchs`_
+| IcmpInRedirects: `RFC1213 icmpInRedirects`_
+| IcmpInEchos: `RFC1213 icmpInEchos`_
+| IcmpInEchoReps: `RFC1213 icmpInEchoReps`_
+| IcmpInTimestamps: `RFC1213 icmpInTimestamps`_
+| IcmpInTimestampReps: `RFC1213 icmpInTimestampReps`_
+| IcmpInAddrMasks: `RFC1213 icmpInAddrMasks`_
+| IcmpInAddrMaskReps: `RFC1213 icmpInAddrMaskReps`_
+| IcmpOutDestUnreachs: `RFC1213 icmpOutDestUnreachs`_
+| IcmpOutTimeExcds: `RFC1213 icmpOutTimeExcds`_
+| IcmpOutParmProbs: `RFC1213 icmpOutParmProbs`_
+| IcmpOutSrcQuenchs: `RFC1213 icmpOutSrcQuenchs`_
+| IcmpOutRedirects: `RFC1213 icmpOutRedirects`_
+| IcmpOutEchos: `RFC1213 icmpOutEchos`_
+| IcmpOutEchoReps: `RFC1213 icmpOutEchoReps`_
+| IcmpOutTimestamps: `RFC1213 icmpOutTimestamps`_
+| IcmpOutTimestampReps: `RFC1213 icmpOutTimestampReps`_
+| IcmpOutAddrMasks: `RFC1213 icmpOutAddrMasks`_
+| IcmpOutAddrMaskReps: `RFC1213 icmpOutAddrMaskReps`_
+
+.. _RFC1213 icmpInDestUnreachs: https://tools.ietf.org/html/rfc1213#page-41
+.. _RFC1213 icmpInTimeExcds: https://tools.ietf.org/html/rfc1213#page-41
+.. _RFC1213 icmpInParmProbs: https://tools.ietf.org/html/rfc1213#page-42
+.. _RFC1213 icmpInSrcQuenchs: https://tools.ietf.org/html/rfc1213#page-42
+.. _RFC1213 icmpInRedirects: https://tools.ietf.org/html/rfc1213#page-42
+.. _RFC1213 icmpInEchos: https://tools.ietf.org/html/rfc1213#page-42
+.. _RFC1213 icmpInEchoReps: https://tools.ietf.org/html/rfc1213#page-42
+.. _RFC1213 icmpInTimestamps: https://tools.ietf.org/html/rfc1213#page-42
+.. _RFC1213 icmpInTimestampReps: https://tools.ietf.org/html/rfc1213#page-43
+.. _RFC1213 icmpInAddrMasks: https://tools.ietf.org/html/rfc1213#page-43
+.. _RFC1213 icmpInAddrMaskReps: https://tools.ietf.org/html/rfc1213#page-43
+
+.. _RFC1213 icmpOutDestUnreachs: https://tools.ietf.org/html/rfc1213#page-44
+.. _RFC1213 icmpOutTimeExcds: https://tools.ietf.org/html/rfc1213#page-44
+.. _RFC1213 icmpOutParmProbs: https://tools.ietf.org/html/rfc1213#page-44
+.. _RFC1213 icmpOutSrcQuenchs: https://tools.ietf.org/html/rfc1213#page-44
+.. _RFC1213 icmpOutRedirects: https://tools.ietf.org/html/rfc1213#page-44
+.. _RFC1213 icmpOutEchos: https://tools.ietf.org/html/rfc1213#page-45
+.. _RFC1213 icmpOutEchoReps: https://tools.ietf.org/html/rfc1213#page-45
+.. _RFC1213 icmpOutTimestamps: https://tools.ietf.org/html/rfc1213#page-45
+.. _RFC1213 icmpOutTimestampReps: https://tools.ietf.org/html/rfc1213#page-45
+.. _RFC1213 icmpOutAddrMasks: https://tools.ietf.org/html/rfc1213#page-45
+.. _RFC1213 icmpOutAddrMaskReps: https://tools.ietf.org/html/rfc1213#page-46
+
+Every ICMP type has two counters: 'In' and 'Out'. E.g., for the ICMP
+Echo packet, they are IcmpInEchos and IcmpOutEchos. Their meanings are
+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`_
+document.
+
+.. _ICMP parameters: https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml
+
+For example, if the Linux kernel sends an ICMP Echo packet, the
+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
+.. _RFC1213 icmpOutErrors: https://tools.ietf.org/html/rfc1213#page-43
+
+When an error occurs in the ICMP packet handler path, these two
+counters would be updated. The receiving packet path use IcmpInErrors
+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
+receives an ICMP packet, kernel follows below logic:
+
+1. increase IcmpInMsgs
+2. if has any error, update IcmpInErrors and finish the process
+3. update IcmpMsgOutType[N]
+4. handle the packet depending on the type, if has any error, update
+   IcmpInErrors and finish the process
+
+So if all errors occur in step (2), IcmpInMsgs should be equal to the
+sum of IcmpMsgOutType[N] plus IcmpInErrors. If all errors occur in
+step (4), IcmpInMsgs should be equal to the sum of
+IcmpMsgOutType[N]. If the errors occur in both step (2) and step (4),
+IcmpInMsgs should be less than the sum of IcmpMsgOutType[N] plus
+IcmpInErrors.
+
+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
+  PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
+  64 bytes from 8.8.8.8: icmp_seq=1 ttl=119 time=17.8 ms
+
+  --- 8.8.8.8 ping statistics ---
+  1 packets transmitted, 1 received, 0% packet loss, time 0ms
+  rtt min/avg/max/mdev = 17.875/17.875/17.875/0.000 ms
+
+The nstayt result::
+
+  nstatuser@nstat-a:~$ nstat
+  #kernel
+  IpInReceives                    1                  0.0
+  IpInDelivers                    1                  0.0
+  IpOutRequests                   1                  0.0
+  IcmpInMsgs                      1                  0.0
+  IcmpInEchoReps                  1                  0.0
+  IcmpOutMsgs                     1                  0.0
+  IcmpOutEchos                    1                  0.0
+  IcmpMsgInType0                  1                  0.0
+  IcmpMsgOutType8                 1                  0.0
+  IpExtInOctets                   84                 0.0
+  IpExtOutOctets                  84                 0.0
+  IpExtInNoECTPkts                1                  0.0
+
+The Linux server sent an ICMP Echo packet, so IpOutRequests,
+IcmpOutMsgs, IcmpOutEchos and IcmpMsgOutType8 were increased 1. The
+server got ICMP Echo Reply from 8.8.8.8, so IpInReceives, IcmpInMsgs,
+IcmpInEchoReps and IcmpMsgInType0 were increased 1. The ICMP Echo Reply
+was passed to the ICMP layer via IP layer, so IpInDelivers was
+increased 1. The default ping data size is 48, so an ICMP Echo packet
+and its corresponding Echo Reply packet are constructed by:
+
+* 14 bytes MAC header
+* 20 bytes IP header
+* 16 bytes ICMP header
+* 48 bytes data (default value of the ping command)
+
+So the IpExtInOctets and IpExtOutOctets are 20+16+48=84.