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

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

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

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.

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.

  • 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

  • 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.

  • 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).

  • IpOutDiscards

Defined in RFC1213 ipOutDiscards. It indicates the packet is dropped in the IP sending path and due to kernel internal reasons.

  • IpOutNoRoutes

Defined in RFC1213 ipOutNoRoutes. It indicates the packet is dropped in the IP sending path and no route is found for it.

ICMP counters

  • IcmpInMsgs and IcmpOutMsgs

Defined by RFC1213 icmpInMsgs and RFC1213 icmpOutMsgs

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

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.

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

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

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

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

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

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.
  • 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.

  • TcpEstabResets

Defined in RFC1213 tcpEstabResets.

  • TcpAttemptFails

Defined in RFC1213 tcpAttemptFails.

  • TcpOutRsts

Defined in RFC1213 tcpOutRsts. The RFC says this counter indicates the 'segments sent containing the RST flag', but in linux kernel, this couner indicates the segments kerenl tried to send. The sending process might be failed due to some errors (e.g. memory alloc failed).

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:

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:

  1. 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.

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.

  • 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

  • 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.

  • 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 explaination:

  • 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.

  • 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.

  • 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).

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

  • 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.

  • TcpExtTCPLossProbes

A TLP probe packet is sent.

  • TcpExtTCPLossProbeRecovery

A packet loss is detected and recovered by TLP.

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