===========
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 will be increased even if the packet is dropped
later (e.g. due to the IP header is invalid or the checksum is wrong
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

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.

* 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.

.. _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

.. _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
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
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).

.. _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
.. _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.

General TCP counters
==================
* TcpInSegs
Defined in `RFC1213 tcpInSegs`_

.. _RFC1213 tcpInSegs: https://tools.ietf.org/html/rfc1213#page-48

The number of packets received by the TCP layer. As mentioned in
RFC1213, it includes the packets received in error, such as checksum
error, invalid TCP header and so on. Only one error won't be included:
if the layer 2 destination address is not the NIC's layer 2
address. It might happen if the packet is a multicast or broadcast
packet, or the NIC is in promiscuous mode. In these situations, the
packets would be delivered to the TCP layer, but the TCP layer will discard
these packets before increasing TcpInSegs. The TcpInSegs counter
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

The number of packets sent by the TCP layer. As mentioned in RFC1213,
it excludes the retransmitted packets. But it includes the SYN, ACK
and RST packets. Doesn't like TcpInSegs, the TcpOutSegs is aware of
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

It means the TCP layer sends a SYN, and come into the SYN-SENT
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

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
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::

  TCPOrigDataSent: number of outgoing packets with original data (excluding
  retransmission but including data-in-SYN). This counter is different from
  TcpOutSegs because TcpOutSegs also tracks pure ACKs. TCPOrigDataSent is
  more useful to track the TCP retransmission rate.

* TCPSynRetrans
This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
explaination below::

  TCPSynRetrans: number of SYN and SYN/ACK retransmits to break down
  retransmissions into SYN, fast-retransmits, timeout retransmits, etc.

* TCPFastOpenActiveFail
This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
explaination below::

  TCPFastOpenActiveFail: Fast Open attempts (SYN/data) failed because
  the remote does not accept it or the attempts timed out.

.. _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
TCP socket is in LISTEN state, and kernel need to drop a packet,
kernel would always add 1 to TcpExtListenDrops. So increase
TcpExtListenOverflows would let TcpExtListenDrops increasing at the
same time, but TcpExtListenDrops would also increase without
TcpExtListenOverflows increasing, e.g. a memory allocation fail would
also let TcpExtListenDrops increase.

Note: The above explanation is based on kernel 4.10 or above version, on
an old kernel, the TCP stack has different behavior when TCP accept
queue is full. On the old kernel, TCP stack won't drop the SYN, it
would complete the 3-way handshake. As the accept queue is full, TCP
stack will keep the socket in the TCP half-open queue. As it is in the
half open queue, TCP stack will send SYN+ACK on an exponential backoff
timer, after client replies ACK, TCP stack checks whether the accept
queue is still full, if it is not full, moves the socket to the accept
queue, if it is full, keeps the socket in the half-open queue, at next
time client replies ACK, this socket will get another chance to move
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::

  It is split into a fast path and a slow path. The fast path is
  disabled when:

  - A zero window was announced from us
  - zero window probing
    is only handled properly on the slow path.
  - Out of order segments arrived.
  - Urgent data is expected.
  - There is no buffer space left
  - Unexpected TCP flags/window values/header lengths are received
    (detected by checking the TCP header against pred_flags)
  - Data is sent in both directions. The fast path only supports pure senders
    or pure receivers (this means either the sequence number or the ack
    value must stay constant)
  - Unexpected TCP option.

Kernel will try to use fast path unless any of the above conditions
are satisfied. If the packets are out of order, kernel will handle
them in slow path, which means the performance might be not very
good. Kernel would also come into slow path if the "Delayed ack" is
used, because when using "Delayed ack", the data is sent in both
directions. When the TCP window scale option is not used, kernel will
try to enable fast path immediately when the connection comes into the
established state, but if the TCP window scale option is used, kernel
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
counter is using the SO_LINGER option. Please refer to the SO_LINGER
section of the `socket man page`_:

.. _socket man page: http://man7.org/linux/man-pages/man7/socket.7.html

By default, when an application closes a connection, the close function
will return immediately and kernel will try to send the in-flight data
async. If you use the SO_LINGER option, set l_onoff to 1, and l_linger
to a positive number, the close function won't return immediately, but
wait for the in-flight data are acked by the other side, the max wait
time is l_linger seconds. If set l_onoff to 1 and set l_linger to 0,
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
side of the connection, then the app has no relationship with the
socket any more, but kernel need to keep the socket, this socket
becomes an orphan socket, kernel waits for the reply of the other side,
and would come to the TIME_WAIT state finally. When kernel has no
enough memory to keep the orphan socket, kernel would send an RST to
the other side, and delete the socket, in such situation, kernel will
increase 1 to the TcpExtTCPAbortOnMemory. Two conditions would trigger
TcpExtTCPAbortOnMemory:

1. the memory used by the TCP protocol is higher than the third value of
the tcp_mem. Please refer the tcp_mem section in the `TCP man page`_:

.. _TCP man page: http://man7.org/linux/man-pages/man7/tcp.7.html

2. the orphan socket count is higher than net.ipv4.tcp_max_orphans


* 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
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.

.. _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
approached. The two pieces of information are ACK train length and
increase in packet delay. For detail information, please refer the
`Hybrid Slow Start paper`_. Either ACK train length or packet delay
hits a specific threshold, the congestion control algorithm will come
into the Congestion Avoidance state. Until v4.20, two congestion
control algorithms are using Hybrid Slow Start, they are cubic (the
default congestion control algorithm) and cdg. Four snmp counters
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
recovery. The SACK is a TCP option, which is defined in `RFC2018`_,
the fast recovery is defined in `RFC6582`_, which is also called
'Reno'.

The TCP congestion control is a big and complex topic. To understand
the related snmp counter, we need to know the states of the congestion
control state machine. There are 5 states: Open, Disorder, CWR,
Recovery and Loss. For details about these states, please refer page 5
and page 6 of this document:
https://pdfs.semanticscholar.org/0e9c/968d09ab2e53e24c4dca5b2d67c7f7140f8e.pdf

.. _RFC2018: https://tools.ietf.org/html/rfc2018
.. _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
could drop a packet which has been acknowledged by SACK, although it is
unusual, it is allowed by the TCP protocol. The sender doesn't really
know what happened on the receiver side. The sender just waits until
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
the original retransmitted packet is not lost, it is just out of
order, the receiver would acknowledge multiple times, one for the
retransmitted packet, another for the arriving of the original out of
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
fill the hole), two conditions will let TcpExtTCPTSReorder increase
1: (1) if the packet 3 is not re-retransmitted yet. (2) if the packet
3 is retransmitted but the timestamp of the packet 3's ACK is earlier
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
is the sender believes an out of order packet is lost so it sends the
packet again. (2) Assume packet 1,2,3,4,5 are sent by the sender, and
the sender has received SACKs for packet 2 and 5, now the sender
receives SACK for packet 4 and the sender doesn't retransmit the
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
duplicate packets to the sender. There are two kinds of
duplications: (1) a packet which has been acknowledged is
duplicate. (2) an out of order packet is duplicate. The TCP stack
counts these two kinds of duplications on both receiver side and
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`_
and the `RFC of PAWS`_.

.. _RFC of PAWS: https://tools.ietf.org/html/rfc1323#page-17
.. _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
due to tcp_invalid_ratelimit, kernel would update one of below
counters to indicate the ACK is skipped in which scenario. The ACK
would only be skipped if the received packet is either a SYN packet or
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
in the Syn-Recv status. But in several scenarios, the TCP stack need
to send an ACK. E.g., the TCP stack receives the same SYN packet
repeately, the received packet does not pass the PAWS check, or the
received packet sequence number is out of window. In these scenarios,
the TCP stack needs to send ACK. If the ACk sending frequency is higher than
tcp_invalid_ratelimit allows, the TCP stack will skip sending ACK and
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
TcpExtTCPACKSkippedSynRecv, TcpExtTCPACKSkippedFinWait2 or
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
three scenarios, In some TCP status, the linux TCP stack would also
send challenge ACKs if the ACK number is before the first
unacknowledged number (more strict than `RFC 5961 section 5.2`_).

.. _RFC 5961 section 3.2: https://tools.ietf.org/html/rfc5961#page-7
.. _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


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.

tcp 3-way handshake
------------------
On server side, we run::

  nstatuser@nstat-b:~$ nc -lknv 0.0.0.0 9000
  Listening on [0.0.0.0] (family 0, port 9000)

On client side, we run::

  nstatuser@nstat-a:~$ nc -nv 192.168.122.251 9000
  Connection to 192.168.122.251 9000 port [tcp/*] succeeded!

The server listened on tcp 9000 port, the client connected to it, they
completed the 3-way handshake.

On server side, we can find below nstat output::

  nstatuser@nstat-b:~$ nstat | grep -i tcp
  TcpPassiveOpens                 1                  0.0
  TcpInSegs                       2                  0.0
  TcpOutSegs                      1                  0.0
  TcpExtTCPPureAcks               1                  0.0

On client side, we can find below nstat output::

  nstatuser@nstat-a:~$ nstat | grep -i tcp
  TcpActiveOpens                  1                  0.0
  TcpInSegs                       1                  0.0
  TcpOutSegs                      2                  0.0

When the server received the first SYN, it replied a SYN+ACK, and came into
SYN-RCVD state, so TcpPassiveOpens increased 1. The server received
SYN, sent SYN+ACK, received ACK, so server sent 1 packet, received 2
packets, TcpInSegs increased 2, TcpOutSegs increased 1. The last ACK
of the 3-way handshake is a pure ACK without data, so
TcpExtTCPPureAcks increased 1.

When the client sent SYN, the client came into the SYN-SENT state, so
TcpActiveOpens increased 1, the client sent SYN, received SYN+ACK, sent
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
  Listening on [0.0.0.0] (family 0, port 9000)

Run nc on client::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000
  Connection to nstat-b 9000 port [tcp/*] succeeded!

Input a string in the nc client ('hello' in our example)::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000
  Connection to nstat-b 9000 port [tcp/*] succeeded!
  hello

The client side nstat output::

  nstatuser@nstat-a:~$ nstat
  #kernel
  IpInReceives                    1                  0.0
  IpInDelivers                    1                  0.0
  IpOutRequests                   1                  0.0
  TcpInSegs                       1                  0.0
  TcpOutSegs                      1                  0.0
  TcpExtTCPPureAcks               1                  0.0
  TcpExtTCPOrigDataSent           1                  0.0
  IpExtInOctets                   52                 0.0
  IpExtOutOctets                  58                 0.0
  IpExtInNoECTPkts                1                  0.0

The server side nstat output::

  nstatuser@nstat-b:~$ nstat
  #kernel
  IpInReceives                    1                  0.0
  IpInDelivers                    1                  0.0
  IpOutRequests                   1                  0.0
  TcpInSegs                       1                  0.0
  TcpOutSegs                      1                  0.0
  IpExtInOctets                   58                 0.0
  IpExtOutOctets                  52                 0.0
  IpExtInNoECTPkts                1                  0.0

Input a string in nc client side again ('world' in our exmaple)::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000
  Connection to nstat-b 9000 port [tcp/*] succeeded!
  hello
  world

Client side nstat output::

  nstatuser@nstat-a:~$ nstat
  #kernel
  IpInReceives                    1                  0.0
  IpInDelivers                    1                  0.0
  IpOutRequests                   1                  0.0
  TcpInSegs                       1                  0.0
  TcpOutSegs                      1                  0.0
  TcpExtTCPHPAcks                 1                  0.0
  TcpExtTCPOrigDataSent           1                  0.0
  IpExtInOctets                   52                 0.0
  IpExtOutOctets                  58                 0.0
  IpExtInNoECTPkts                1                  0.0


Server side nstat output::

  nstatuser@nstat-b:~$ nstat
  #kernel
  IpInReceives                    1                  0.0
  IpInDelivers                    1                  0.0
  IpOutRequests                   1                  0.0
  TcpInSegs                       1                  0.0
  TcpOutSegs                      1                  0.0
  TcpExtTCPHPHits                 1                  0.0
  IpExtInOctets                   58                 0.0
  IpExtOutOctets                  52                 0.0
  IpExtInNoECTPkts                1                  0.0

Compare the first client-side nstat and the second client-side nstat,
we could find one difference: the first one had a 'TcpExtTCPPureAcks',
but the second one had a 'TcpExtTCPHPAcks'. The first server-side
nstat and the second server-side nstat had a difference too: the
second server-side nstat had a TcpExtTCPHPHits, but the first
server-side nstat didn't have it. The network traffic patterns were
exactly the same: the client sent a packet to the server, the server
replied an ACK. But kernel handled them in different ways. When the
TCP window scale option is not used, kernel will try to enable fast
path immediately when the connection comes into the established state,
but if the TCP window scale option is used, kernel will disable the
fast path at first, and try to enable it after kerenl receives
packets. We could use the 'ss' command to verify whether the window
scale option is used. e.g. run below command on either server or
client::

  nstatuser@nstat-a:~$ ss -o state established -i '( dport = :9000 or sport = :9000 )
  Netid    Recv-Q     Send-Q            Local Address:Port             Peer Address:Port
  tcp      0          0               192.168.122.250:40654         192.168.122.251:9000
             ts sack cubic wscale:7,7 rto:204 rtt:0.98/0.49 mss:1448 pmtu:1500 rcvmss:536 advmss:1448 cwnd:10 bytes_acked:1 segs_out:2 segs_in:1 send 118.2Mbps lastsnd:46572 lastrcv:46572 lastack:46572 pacing_rate 236.4Mbps rcv_space:29200 rcv_ssthresh:29200 minrtt:0.98

The 'wscale:7,7' means both server and client set the window scale
option to 7. Now we could explain the nstat output in our test:

In the first nstat output of client side, the client sent a packet, server
reply an ACK, when kernel handled this ACK, the fast path was not
enabled, so the ACK was counted into 'TcpExtTCPPureAcks'.

In the second nstat output of client side, the client sent a packet again,
and received another ACK from the server, in this time, the fast path is
enabled, and the ACK was qualified for fast path, so it was handled by
the fast path, so this ACK was counted into TcpExtTCPHPAcks.

In the first nstat output of server side, fast path was not enabled,
so there was no 'TcpExtTCPHPHits'.

In the second nstat output of server side, the fast path was enabled,
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
  import time

  port = 9000

  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.bind(('0.0.0.0', port))
  s.listen(1)
  sock, addr = s.accept()
  while True:
      time.sleep(9999999)

This python script listen on 9000 port, but doesn't read anything from
the connection.

On the client side, we send the string "hello" by nc::

  nstatuser@nstat-a:~$ echo "hello" | nc nstat-b 9000

Then, we come back to the server side, the server has received the "hello"
packet, and the TCP layer has acked this packet, but the application didn't
read it yet. We type Ctrl-C to terminate the server script. Then we
could find TcpExtTCPAbortOnClose increased 1 on the server side::

  nstatuser@nstat-b:~$ nstat | grep -i abort
  TcpExtTCPAbortOnClose           1                  0.0

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::

  sudo bash -c "echo 10 > /proc/sys/net/ipv4/tcp_max_orphans"

Client code (create 64 connection to server)::

  nstatuser@nstat-a:~$ cat client_orphan.py
  import socket
  import time

  server = 'nstat-b' # server address
  port = 9000

  count = 64

  connection_list = []

  for i in range(64):
      s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
      s.connect((server, port))
      connection_list.append(s)
      print("connection_count: %d" % len(connection_list))

  while True:
      time.sleep(99999)

Server code (accept 64 connection from client)::

  nstatuser@nstat-b:~$ cat server_orphan.py
  import socket
  import time

  port = 9000
  count = 64

  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.bind(('0.0.0.0', port))
  s.listen(count)
  connection_list = []
  while True:
      sock, addr = s.accept()
      connection_list.append((sock, addr))
      print("connection_count: %d" % len(connection_list))

Run the python scripts on server and client.

On server::

  python3 server_orphan.py

On client::

  python3 client_orphan.py

Run iptables on server::

  sudo iptables -A INPUT -i ens3 -p tcp --destination-port 9000 -j DROP

Type Ctrl-C on client, stop client_orphan.py.

Check TcpExtTCPAbortOnMemory on client::

  nstatuser@nstat-a:~$ nstat | grep -i abort
  TcpExtTCPAbortOnMemory          54                 0.0

Check orphane socket count on client::

  nstatuser@nstat-a:~$ ss -s
  Total: 131 (kernel 0)
  TCP:   14 (estab 1, closed 0, orphaned 10, synrecv 0, timewait 0/0), ports 0

  Transport Total     IP        IPv6
  *         0         -         -
  RAW       1         0         1
  UDP       1         1         0
  TCP       14        13        1
  INET      16        14        2
  FRAG      0         0         0

The explanation of the test: after run server_orphan.py and
client_orphan.py, we set up 64 connections between server and
client. Run the iptables command, the server will drop all packets from
the client, type Ctrl-C on client_orphan.py, the system of the client
would try to close these connections, and before they are closed
gracefully, these connections became orphan sockets. As the iptables
of the server blocked packets from the client, the server won't receive fin
from the client, so all connection on clients would be stuck on FIN_WAIT_1
stage, so they will keep as orphan sockets until timeout. We have echo
10 to /proc/sys/net/ipv4/tcp_max_orphans, so the client system would
only keep 10 orphan sockets, for all other orphan sockets, the client
system sent RST for them and delete them. We have 64 connections, so
the 'ss -s' command shows the system has 10 orphan sockets, and the
value of TcpExtTCPAbortOnMemory was 54.

An additional explanation about orphan socket count: You could find the
exactly orphan socket count by the 'ss -s' command, but when kernel
decide whither increases TcpExtTCPAbortOnMemory and sends RST, kernel
doesn't always check the exactly orphan socket count. For increasing
performance, kernel checks an approximate count firstly, if the
approximate count is more than tcp_max_orphans, kernel checks the
exact count again. So if the approximate count is less than
tcp_max_orphans, but exactly count is more than tcp_max_orphans, you
would find TcpExtTCPAbortOnMemory is not increased at all. If
tcp_max_orphans is large enough, it won't occur, but if you decrease
tcp_max_orphans to a small value like our test, you might find this
issue. So in our test, the client set up 64 connections although the
tcp_max_orphans is 10. If the client only set up 11 connections, we
can't find the change of TcpExtTCPAbortOnMemory.

Continue the previous test, we wait for several minutes. Because of the
iptables on the server blocked the traffic, the server wouldn't receive
fin, and all the client's orphan sockets would timeout on the
FIN_WAIT_1 state finally. So we wait for a few minutes, we could find
10 timeout on the client::

  nstatuser@nstat-a:~$ nstat | grep -i abort
  TcpExtTCPAbortOnTimeout         10                 0.0

TcpExtTCPAbortOnLinger
---------------------
The server side code::

  nstatuser@nstat-b:~$ cat server_linger.py
  import socket
  import time

  port = 9000

  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.bind(('0.0.0.0', port))
  s.listen(1)
  sock, addr = s.accept()
  while True:
      time.sleep(9999999)

The client side code::

  nstatuser@nstat-a:~$ cat client_linger.py
  import socket
  import struct

  server = 'nstat-b' # server address
  port = 9000

  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack('ii', 1, 10))
  s.setsockopt(socket.SOL_TCP, socket.TCP_LINGER2, struct.pack('i', -1))
  s.connect((server, port))
  s.close()

Run server_linger.py on server::

  nstatuser@nstat-b:~$ python3 server_linger.py

Run client_linger.py on client::

  nstatuser@nstat-a:~$ python3 client_linger.py

After run client_linger.py, check the output of nstat::

  nstatuser@nstat-a:~$ nstat | grep -i abort
  TcpExtTCPAbortOnLinger          1                  0.0

TcpExtTCPRcvCoalesce
-------------------
On the server, we run a program which listen on TCP port 9000, but
doesn't read any data::

  import socket
  import time
  port = 9000
  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.bind(('0.0.0.0', port))
  s.listen(1)
  sock, addr = s.accept()
  while True:
      time.sleep(9999999)

Save the above code as server_coalesce.py, and run::

  python3 server_coalesce.py

On the client, save below code as client_coalesce.py::

  import socket
  server = 'nstat-b'
  port = 9000
  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
  s.connect((server, port))

Run::

  nstatuser@nstat-a:~$ python3 -i client_coalesce.py

We use '-i' to come into the interactive mode, then a packet::

  >>> s.send(b'foo')
  3

Send a packet again::

  >>> s.send(b'bar')
  3

On the server, run nstat::

  ubuntu@nstat-b:~$ nstat
  #kernel
  IpInReceives                    2                  0.0
  IpInDelivers                    2                  0.0
  IpOutRequests                   2                  0.0
  TcpInSegs                       2                  0.0
  TcpOutSegs                      2                  0.0
  TcpExtTCPRcvCoalesce            1                  0.0
  IpExtInOctets                   110                0.0
  IpExtOutOctets                  104                0.0
  IpExtInNoECTPkts                2                  0.0

The client sent two packets, server didn't read any data. When
the second packet arrived at server, the first packet was still in
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
  Listening on [0.0.0.0] (family 0, port 9000)

On client, run 3 nc commands in different terminals::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000
  Connection to nstat-b 9000 port [tcp/*] succeeded!

The nc command only accepts 1 connection, and the accept queue length
is 1. On current linux implementation, set queue length to n means the
actual queue length is n+1. Now we create 3 connections, 1 is accepted
by nc, 2 in accepted queue, so the accept queue is full.

Before running the 4th nc, we clean the nstat history on the server::

  nstatuser@nstat-b:~$ nstat -n

Run the 4th nc on the client::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000

If the nc server is running on kernel 4.10 or higher version, you
won't see the "Connection to ... succeeded!" string, because kernel
will drop the SYN if the accept queue is full. If the nc client is running
on an old kernel, you would see that the connection is succeeded,
because kernel would complete the 3 way handshake and keep the socket
on half open queue. I did the test on kernel 4.15. Below is the nstat
on the server::

  nstatuser@nstat-b:~$ nstat
  #kernel
  IpInReceives                    4                  0.0
  IpInDelivers                    4                  0.0
  TcpInSegs                       4                  0.0
  TcpExtListenOverflows           4                  0.0
  TcpExtListenDrops               4                  0.0
  IpExtInOctets                   240                0.0
  IpExtInNoECTPkts                4                  0.0

Both TcpExtListenOverflows and TcpExtListenDrops were 4. If the time
between the 4th nc and the nstat was longer, the value of
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::

  $ sudo ip route add 8.8.8.8/32 via 192.168.122.251

Prepare on server B, disable send_redirects for all interfaces::

  $ sudo sysctl -w net.ipv4.conf.all.send_redirects=0
  $ sudo sysctl -w net.ipv4.conf.ens3.send_redirects=0
  $ sudo sysctl -w net.ipv4.conf.lo.send_redirects=0
  $ sudo sysctl -w net.ipv4.conf.default.send_redirects=0

We want to let sever A send a packet to 8.8.8.8, and route the packet
to server B. When server B receives such packet, it might send a ICMP
Redirect message to server A, set send_redirects to 0 will disable
this behavior.

First, generate InAddrErrors. On server B, we disable IP forwarding::

  $ sudo sysctl -w net.ipv4.conf.all.forwarding=0

On server A, we send packets to 8.8.8.8::

  $ nc -v 8.8.8.8 53

On server B, we check the output of nstat::

  $ nstat
  #kernel
  IpInReceives                    3                  0.0
  IpInAddrErrors                  3                  0.0
  IpExtInOctets                   180                0.0
  IpExtInNoECTPkts                3                  0.0

As we have let server A route 8.8.8.8 to server B, and we disabled IP
forwarding on server B, Server A sent packets to server B, then server B
dropped packets and increased IpInAddrErrors. As the nc command would
re-send the SYN packet if it didn't receive a SYN+ACK, we could find
multiple IpInAddrErrors.

Second, generate IpExtInNoRoutes. On server B, we enable IP
forwarding::

  $ sudo sysctl -w net.ipv4.conf.all.forwarding=1

Check the route table of server B and remove the default route::

  $ ip route show
  default via 192.168.122.1 dev ens3 proto static
  192.168.122.0/24 dev ens3 proto kernel scope link src 192.168.122.251
  $ sudo ip route delete default via 192.168.122.1 dev ens3 proto static

On server A, we contact 8.8.8.8 again::

  $ nc -v 8.8.8.8 53
  nc: connect to 8.8.8.8 port 53 (tcp) failed: Network is unreachable

On server B, run nstat::

  $ nstat
  #kernel
  IpInReceives                    1                  0.0
  IpOutRequests                   1                  0.0
  IcmpOutMsgs                     1                  0.0
  IcmpOutDestUnreachs             1                  0.0
  IcmpMsgOutType3                 1                  0.0
  IpExtInNoRoutes                 1                  0.0
  IpExtInOctets                   60                 0.0
  IpExtOutOctets                  88                 0.0
  IpExtInNoECTPkts                1                  0.0

We enabled IP forwarding on server B, when server B received a packet
which destination IP address is 8.8.8.8, server B will try to forward
this packet. We have deleted the default route, there was no route for
8.8.8.8, so server B increase IpExtInNoRoutes and sent the "ICMP
Destination Unreachable" message to server A.

Third, generate IpOutNoRoutes. Run ping command on server B::

  $ ping -c 1 8.8.8.8
  connect: Network is unreachable

Run nstat on server B::

  $ nstat
  #kernel
  IpOutNoRoutes                   1                  0.0

We have deleted the default route on server B. Server B couldn't find
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
again, and record the reply time (the duplicate ACK reply time). The
third SYN will let server check the previous duplicate ACK reply time,
and decide to skip the duplicate ACK, then increase the
TcpExtTCPACKSkippedSynRecv counter.

Run tcpdump to capture a SYN packet::

  nstatuser@nstat-a:~$ sudo tcpdump -c 1 -w /tmp/syn.pcap port 9000
  tcpdump: listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes

Open another terminal, run nc command::

  nstatuser@nstat-a:~$ nc nstat-b 9000

As the nstat-b didn't listen on port 9000, it should reply a RST, and
the nc command exited immediately. It was enough for the tcpdump
command to capture a SYN packet. A linux server might use hardware
offload for the TCP checksum, so the checksum in the /tmp/syn.pcap
might be not correct. We call tcprewrite to fix it::

  nstatuser@nstat-a:~$ tcprewrite --infile=/tmp/syn.pcap --outfile=/tmp/syn_fixcsum.pcap --fixcsum

On nstat-b, we run nc to listen on port 9000::

  nstatuser@nstat-b:~$ nc -lkv 9000
  Listening on [0.0.0.0] (family 0, port 9000)

On nstat-a, we blocked the packet from port 9000, or nstat-a would send
RST to nstat-b::

  nstatuser@nstat-a:~$ sudo iptables -A INPUT -p tcp --sport 9000 -j DROP

Send 3 SYN repeatly to nstat-b::

  nstatuser@nstat-a:~$ for i in {1..3}; do sudo tcpreplay -i ens3 /tmp/syn_fixcsum.pcap; done

Check snmp cunter on nstat-b::

  nstatuser@nstat-b:~$ nstat | grep -i skip
  TcpExtTCPACKSkippedSynRecv      1                  0.0

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::

  nstatuser@nstat-b:~$ nc -lkv 9000
  Listening on [0.0.0.0] (family 0, port 9000)

On nstat-a, run tcpdump to capture a SYN::

  nstatuser@nstat-a:~$ sudo tcpdump -w /tmp/paws_pre.pcap -c 1 port 9000
  tcpdump: listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes

On nstat-a, run nc as a client to connect nstat-b::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000
  Connection to nstat-b 9000 port [tcp/*] succeeded!

Now the tcpdump has captured the SYN and exit. We should fix the
checksum::

  nstatuser@nstat-a:~$ tcprewrite --infile /tmp/paws_pre.pcap --outfile /tmp/paws.pcap --fixcsum

Send the SYN packet twice::

  nstatuser@nstat-a:~$ for i in {1..2}; do sudo tcpreplay -i ens3 /tmp/paws.pcap; done

On nstat-b, check the snmp counter::

  nstatuser@nstat-b:~$ nstat | grep -i skip
  TcpExtTCPACKSkippedPAWS         1                  0.0

We sent two SYN via tcpreplay, both of them would let PAWS check
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
data, so we need a pure ACK packet. To generate such a packet, we
could create two sockets: one on port 9000, another on port 9001. Then
we capture an ACK on port 9001, change the source/destination port
numbers to match the port 9000 socket. Then we could trigger
TcpExtTCPACKSkippedSeq via this packet.

On nstat-b, open two terminals, run two nc commands to listen on both
port 9000 and port 9001::

  nstatuser@nstat-b:~$ nc -lkv 9000
  Listening on [0.0.0.0] (family 0, port 9000)

  nstatuser@nstat-b:~$ nc -lkv 9001
  Listening on [0.0.0.0] (family 0, port 9001)

On nstat-a, run two nc clients::

  nstatuser@nstat-a:~$ nc -v nstat-b 9000
  Connection to nstat-b 9000 port [tcp/*] succeeded!

  nstatuser@nstat-a:~$ nc -v nstat-b 9001
  Connection to nstat-b 9001 port [tcp/*] succeeded!

On nstat-a, run tcpdump to capture an ACK::

  nstatuser@nstat-a:~$ sudo tcpdump -w /tmp/seq_pre.pcap -c 1 dst port 9001
  tcpdump: listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes

On nstat-b, send a packet via the port 9001 socket. E.g. we sent a
string 'foo' in our example::

  nstatuser@nstat-b:~$ nc -lkv 9001
  Listening on [0.0.0.0] (family 0, port 9001)
  Connection from nstat-a 42132 received!
  foo

On nstat-a, the tcpdump should have caputred the ACK. We should check
the source port numbers of the two nc clients::

  nstatuser@nstat-a:~$ ss -ta '( dport = :9000 || dport = :9001 )' | tee
  State  Recv-Q   Send-Q         Local Address:Port           Peer Address:Port
  ESTAB  0        0            192.168.122.250:50208       192.168.122.251:9000
  ESTAB  0        0            192.168.122.250:42132       192.168.122.251:9001

Run tcprewrite, change port 9001 to port 9000, chagne port 42132 to
port 50208::

  nstatuser@nstat-a:~$ tcprewrite --infile /tmp/seq_pre.pcap --outfile /tmp/seq.pcap -r 9001:9000 -r 42132:50208 --fixcsum

Now the /tmp/seq.pcap is the packet we need. Send it to nstat-b::

  nstatuser@nstat-a:~$ for i in {1..2}; do sudo tcpreplay -i ens3 /tmp/seq.pcap; done

Check TcpExtTCPACKSkippedSeq on nstat-b::

  nstatuser@nstat-b:~$ nstat | grep -i skip
  TcpExtTCPACKSkippedSeq          1                  0.0
