Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+============
+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
+explanation 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
+explanation 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
+explanation 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
+=============
+* 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
+counter indicates the segments kernel tried to send. The sending
+process might be failed due to some errors (e.g. memory alloc failed).
+
+.. _RFC1213 tcpOutRsts: https://tools.ietf.org/html/rfc1213#page-52
+
+* 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
+=============
+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.
+
+* 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
+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 indicates an acknowledged
+duplicate packet is received.
+
+* TcpExtTCPDSACKOfoRecv
+
+The TCP stack receives a DSACK, which indicate an out of order
+duplicate packet is received.
+
+invalid SACK and DSACK
+======================
+When a SACK (or DSACK) block is invalid, a corresponding counter would
+be updated. The validation method is base on the start/end sequence
+number of the SACK block. For more details, please refer the comment
+of the function tcp_is_sackblock_valid in the kernel source code. A
+SACK option could have up to 4 blocks, they are checked
+individually. E.g., if 3 blocks of a SACk is invalid, the
+corresponding counter would be updated 3 times. The comment of the
+`Add counters for discarded SACK blocks`_ patch has additional
+explanation:
+
+.. _Add counters for discarded SACK blocks: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=18f02545a9a16c9a89778b91a162ad16d510bb32
+
+* TcpExtTCPSACKDiscard
+
+This counter indicates how many SACK blocks are invalid. If the invalid
+SACK block is caused by ACK recording, the TCP stack will only ignore
+it and won't update this counter.
+
+* TcpExtTCPDSACKIgnoredOld and TcpExtTCPDSACKIgnoredNoUndo
+
+When a DSACK block is invalid, one of these two counters would be
+updated. Which counter will be updated depends on the undo_marker flag
+of the TCP socket. If the undo_marker is not set, the TCP stack isn't
+likely to re-transmit any packets, and we still receive an invalid
+DSACK block, the reason might be that the packet is duplicated in the
+middle of the network. In such scenario, TcpExtTCPDSACKIgnoredNoUndo
+will be updated. If the undo_marker is set, TcpExtTCPDSACKIgnoredOld
+will be updated. As implied in its name, it might be an old packet.
+
+SACK shift
+==========
+The linux networking stack stores data in sk_buff struct (skb for
+short). If a SACK block acrosses multiple skb, the TCP stack will try
+to re-arrange data in these skb. E.g. if a SACK block acknowledges seq
+10 to 15, skb1 has seq 10 to 13, skb2 has seq 14 to 20. The seq 14 and
+15 in skb2 would be moved to skb1. This operation is 'shift'. If a
+SACK block acknowledges seq 10 to 20, skb1 has seq 10 to 13, skb2 has
+seq 14 to 20. All data in skb2 will be moved to skb1, and skb2 will be
+discard, this operation is 'merge'.
+
+* TcpExtTCPSackShifted
+
+A skb is shifted
+
+* TcpExtTCPSackMerged
+
+A skb is merged
+
+* TcpExtTCPSackShiftFallback
+
+A skb should be shifted or merged, but the TCP stack doesn't do it for
+some reasons.
+
+TCP out of order
+================
+* TcpExtTCPOFOQueue
+
+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.rst
+
+* 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
+
+The 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
+
+TCP receive window
+==================
+* TcpExtTCPWantZeroWindowAdv
+
+Depending on current memory usage, the TCP stack tries to set receive
+window to zero. But the receive window might still be a no-zero
+value. For example, if the previous window size is 10, and the TCP
+stack receives 3 bytes, the current window size would be 7 even if the
+window size calculated by the memory usage is zero.
+
+* TcpExtTCPToZeroWindowAdv
+
+The TCP receive window is set to zero from a no-zero value.
+
+* TcpExtTCPFromZeroWindowAdv
+
+The TCP receive window is set to no-zero value from zero.
+
+
+Delayed ACK
+===========
+The TCP Delayed ACK is a technique which is used for reducing the
+packet count in the network. For more details, please refer the
+`Delayed ACK wiki`_
+
+.. _Delayed ACK wiki: https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment
+
+* TcpExtDelayedACKs
+
+A delayed ACK timer expires. The TCP stack will send a pure ACK packet
+and exit the delayed ACK mode.
+
+* TcpExtDelayedACKLocked
+
+A delayed ACK timer expires, but the TCP stack can't send an ACK
+immediately due to the socket is locked by a userspace program. The
+TCP stack will send a pure ACK later (after the userspace program
+unlock the socket). When the TCP stack sends the pure ACK later, the
+TCP stack will also update TcpExtDelayedACKs and exit the delayed ACK
+mode.
+
+* TcpExtDelayedACKLost
+
+It will be updated when the TCP stack receives a packet which has been
+ACKed. A Delayed ACK loss might cause this issue, but it would also be
+triggered by other reasons, such as a packet is duplicated in the
+network.
+
+Tail Loss Probe (TLP)
+=====================
+TLP is an algorithm which is used to detect TCP packet loss. For more
+details, please refer the `TLP paper`_.
+
+.. _TLP paper: https://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01
+
+* TcpExtTCPLossProbes
+
+A TLP probe packet is sent.
+
+* TcpExtTCPLossProbeRecovery
+
+A packet loss is detected and recovered by TLP.
+
+TCP Fast Open description
+=========================
+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 explanation 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 skb,
+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
+========
+
+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 example)::
+
+  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 kernel 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 orphaned 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 counter 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 captured 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, change 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