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Fix documentation markup warnings in snmp_counter.rst: Documentation/networking/snmp_counter.rst:416: WARNING: Title underline too short. Documentation/networking/snmp_counter.rst:684: WARNING: Bullet list ends without a blank line; unexpected unindent. Documentation/networking/snmp_counter.rst:693: WARNING: Title underline too short. Documentation/networking/snmp_counter.rst:707: WARNING: Bullet list ends without a blank line; unexpected unindent. Documentation/networking/snmp_counter.rst:712: WARNING: Bullet list ends without a blank line; unexpected unindent. Documentation/networking/snmp_counter.rst:722: WARNING: Title underline too short. Documentation/networking/snmp_counter.rst:733: WARNING: Bullet list ends without a blank line; unexpected unindent. Documentation/networking/snmp_counter.rst:736: WARNING: Bullet list ends without a blank line; unexpected unindent. Documentation/networking/snmp_counter.rst:739: WARNING: Bullet list ends without a blank line; unexpected unindent. Fixes:80cc49507b
("net: Add part of TCP counts explanations in snmp_counters.rst") Fixes:8e2ea53a83
("add snmp counters document") Fixes:a6c7c7aac2
("net: add document for several snmp counters") Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Cc: yupeng <yupeng0921@gmail.com>
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============
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SNMP counter
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============
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This document explains the meaning of SNMP counters.
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General IPv4 counters
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=====================
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All layer 4 packets and ICMP packets will change these counters, but
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these counters won't be changed by layer 2 packets (such as STP) or
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ARP packets.
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* IpInReceives
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Defined in `RFC1213 ipInReceives`_
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.. _RFC1213 ipInReceives: https://tools.ietf.org/html/rfc1213#page-26
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The number of packets received by the IP layer. It gets increasing at the
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beginning of ip_rcv function, always be updated together with
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IpExtInOctets. It will be increased even if the packet is dropped
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later (e.g. due to the IP header is invalid or the checksum is wrong
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and so on). It indicates the number of aggregated segments after
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GRO/LRO.
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* IpInDelivers
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Defined in `RFC1213 ipInDelivers`_
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.. _RFC1213 ipInDelivers: https://tools.ietf.org/html/rfc1213#page-28
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The number of packets delivers to the upper layer protocols. E.g. TCP, UDP,
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ICMP and so on. If no one listens on a raw socket, only kernel
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supported protocols will be delivered, if someone listens on the raw
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socket, all valid IP packets will be delivered.
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* IpOutRequests
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Defined in `RFC1213 ipOutRequests`_
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.. _RFC1213 ipOutRequests: https://tools.ietf.org/html/rfc1213#page-28
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The number of packets sent via IP layer, for both single cast and
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multicast packets, and would always be updated together with
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IpExtOutOctets.
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* IpExtInOctets and IpExtOutOctets
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They are Linux kernel extensions, no RFC definitions. Please note,
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RFC1213 indeed defines ifInOctets and ifOutOctets, but they
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are different things. The ifInOctets and ifOutOctets include the MAC
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layer header size but IpExtInOctets and IpExtOutOctets don't, they
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only include the IP layer header and the IP layer data.
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* IpExtInNoECTPkts, IpExtInECT1Pkts, IpExtInECT0Pkts, IpExtInCEPkts
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They indicate the number of four kinds of ECN IP packets, please refer
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`Explicit Congestion Notification`_ for more details.
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.. _Explicit Congestion Notification: https://tools.ietf.org/html/rfc3168#page-6
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These 4 counters calculate how many packets received per ECN
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status. They count the real frame number regardless the LRO/GRO. So
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for the same packet, you might find that IpInReceives count 1, but
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IpExtInNoECTPkts counts 2 or more.
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* IpInHdrErrors
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Defined in `RFC1213 ipInHdrErrors`_. It indicates the packet is
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dropped due to the IP header error. It might happen in both IP input
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and IP forward paths.
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.. _RFC1213 ipInHdrErrors: https://tools.ietf.org/html/rfc1213#page-27
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* IpInAddrErrors
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Defined in `RFC1213 ipInAddrErrors`_. It will be increased in two
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scenarios: (1) The IP address is invalid. (2) The destination IP
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address is not a local address and IP forwarding is not enabled
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.. _RFC1213 ipInAddrErrors: https://tools.ietf.org/html/rfc1213#page-27
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* IpExtInNoRoutes
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This counter means the packet is dropped when the IP stack receives a
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packet and can't find a route for it from the route table. It might
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happen when IP forwarding is enabled and the destination IP address is
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not a local address and there is no route for the destination IP
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address.
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* IpInUnknownProtos
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Defined in `RFC1213 ipInUnknownProtos`_. It will be increased if the
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layer 4 protocol is unsupported by kernel. If an application is using
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raw socket, kernel will always deliver the packet to the raw socket
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and this counter won't be increased.
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.. _RFC1213 ipInUnknownProtos: https://tools.ietf.org/html/rfc1213#page-27
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* IpExtInTruncatedPkts
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For IPv4 packet, it means the actual data size is smaller than the
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"Total Length" field in the IPv4 header.
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* IpInDiscards
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Defined in `RFC1213 ipInDiscards`_. It indicates the packet is dropped
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in the IP receiving path and due to kernel internal reasons (e.g. no
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enough memory).
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.. _RFC1213 ipInDiscards: https://tools.ietf.org/html/rfc1213#page-28
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* IpOutDiscards
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Defined in `RFC1213 ipOutDiscards`_. It indicates the packet is
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dropped in the IP sending path and due to kernel internal reasons.
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.. _RFC1213 ipOutDiscards: https://tools.ietf.org/html/rfc1213#page-28
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* IpOutNoRoutes
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Defined in `RFC1213 ipOutNoRoutes`_. It indicates the packet is
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dropped in the IP sending path and no route is found for it.
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.. _RFC1213 ipOutNoRoutes: https://tools.ietf.org/html/rfc1213#page-29
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ICMP counters
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=============
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* IcmpInMsgs and IcmpOutMsgs
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Defined by `RFC1213 icmpInMsgs`_ and `RFC1213 icmpOutMsgs`_
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.. _RFC1213 icmpInMsgs: https://tools.ietf.org/html/rfc1213#page-41
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.. _RFC1213 icmpOutMsgs: https://tools.ietf.org/html/rfc1213#page-43
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As mentioned in the RFC1213, these two counters include errors, they
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would be increased even if the ICMP packet has an invalid type. The
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ICMP output path will check the header of a raw socket, so the
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IcmpOutMsgs would still be updated if the IP header is constructed by
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a userspace program.
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* ICMP named types
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| These counters include most of common ICMP types, they are:
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| IcmpInDestUnreachs: `RFC1213 icmpInDestUnreachs`_
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| IcmpInTimeExcds: `RFC1213 icmpInTimeExcds`_
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| IcmpInParmProbs: `RFC1213 icmpInParmProbs`_
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| IcmpInSrcQuenchs: `RFC1213 icmpInSrcQuenchs`_
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| IcmpInRedirects: `RFC1213 icmpInRedirects`_
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| IcmpInEchos: `RFC1213 icmpInEchos`_
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| IcmpInEchoReps: `RFC1213 icmpInEchoReps`_
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| IcmpInTimestamps: `RFC1213 icmpInTimestamps`_
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| IcmpInTimestampReps: `RFC1213 icmpInTimestampReps`_
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| IcmpInAddrMasks: `RFC1213 icmpInAddrMasks`_
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| IcmpInAddrMaskReps: `RFC1213 icmpInAddrMaskReps`_
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| IcmpOutDestUnreachs: `RFC1213 icmpOutDestUnreachs`_
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| IcmpOutTimeExcds: `RFC1213 icmpOutTimeExcds`_
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| IcmpOutParmProbs: `RFC1213 icmpOutParmProbs`_
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| IcmpOutSrcQuenchs: `RFC1213 icmpOutSrcQuenchs`_
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| IcmpOutRedirects: `RFC1213 icmpOutRedirects`_
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| IcmpOutEchos: `RFC1213 icmpOutEchos`_
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| IcmpOutEchoReps: `RFC1213 icmpOutEchoReps`_
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| IcmpOutTimestamps: `RFC1213 icmpOutTimestamps`_
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| IcmpOutTimestampReps: `RFC1213 icmpOutTimestampReps`_
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| IcmpOutAddrMasks: `RFC1213 icmpOutAddrMasks`_
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| IcmpOutAddrMaskReps: `RFC1213 icmpOutAddrMaskReps`_
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.. _RFC1213 icmpInDestUnreachs: https://tools.ietf.org/html/rfc1213#page-41
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.. _RFC1213 icmpInTimeExcds: https://tools.ietf.org/html/rfc1213#page-41
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.. _RFC1213 icmpInParmProbs: https://tools.ietf.org/html/rfc1213#page-42
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.. _RFC1213 icmpInSrcQuenchs: https://tools.ietf.org/html/rfc1213#page-42
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.. _RFC1213 icmpInRedirects: https://tools.ietf.org/html/rfc1213#page-42
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.. _RFC1213 icmpInEchos: https://tools.ietf.org/html/rfc1213#page-42
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.. _RFC1213 icmpInEchoReps: https://tools.ietf.org/html/rfc1213#page-42
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.. _RFC1213 icmpInTimestamps: https://tools.ietf.org/html/rfc1213#page-42
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.. _RFC1213 icmpInTimestampReps: https://tools.ietf.org/html/rfc1213#page-43
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.. _RFC1213 icmpInAddrMasks: https://tools.ietf.org/html/rfc1213#page-43
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.. _RFC1213 icmpInAddrMaskReps: https://tools.ietf.org/html/rfc1213#page-43
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.. _RFC1213 icmpOutDestUnreachs: https://tools.ietf.org/html/rfc1213#page-44
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.. _RFC1213 icmpOutTimeExcds: https://tools.ietf.org/html/rfc1213#page-44
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.. _RFC1213 icmpOutParmProbs: https://tools.ietf.org/html/rfc1213#page-44
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.. _RFC1213 icmpOutSrcQuenchs: https://tools.ietf.org/html/rfc1213#page-44
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.. _RFC1213 icmpOutRedirects: https://tools.ietf.org/html/rfc1213#page-44
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.. _RFC1213 icmpOutEchos: https://tools.ietf.org/html/rfc1213#page-45
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.. _RFC1213 icmpOutEchoReps: https://tools.ietf.org/html/rfc1213#page-45
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.. _RFC1213 icmpOutTimestamps: https://tools.ietf.org/html/rfc1213#page-45
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.. _RFC1213 icmpOutTimestampReps: https://tools.ietf.org/html/rfc1213#page-45
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.. _RFC1213 icmpOutAddrMasks: https://tools.ietf.org/html/rfc1213#page-45
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.. _RFC1213 icmpOutAddrMaskReps: https://tools.ietf.org/html/rfc1213#page-46
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Every ICMP type has two counters: 'In' and 'Out'. E.g., for the ICMP
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Echo packet, they are IcmpInEchos and IcmpOutEchos. Their meanings are
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straightforward. The 'In' counter means kernel receives such a packet
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and the 'Out' counter means kernel sends such a packet.
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* ICMP numeric types
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They are IcmpMsgInType[N] and IcmpMsgOutType[N], the [N] indicates the
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ICMP type number. These counters track all kinds of ICMP packets. The
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ICMP type number definition could be found in the `ICMP parameters`_
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document.
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.. _ICMP parameters: https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml
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For example, if the Linux kernel sends an ICMP Echo packet, the
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IcmpMsgOutType8 would increase 1. And if kernel gets an ICMP Echo Reply
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packet, IcmpMsgInType0 would increase 1.
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* IcmpInCsumErrors
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This counter indicates the checksum of the ICMP packet is
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wrong. Kernel verifies the checksum after updating the IcmpInMsgs and
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before updating IcmpMsgInType[N]. If a packet has bad checksum, the
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IcmpInMsgs would be updated but none of IcmpMsgInType[N] would be updated.
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* IcmpInErrors and IcmpOutErrors
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Defined by `RFC1213 icmpInErrors`_ and `RFC1213 icmpOutErrors`_
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.. _RFC1213 icmpInErrors: https://tools.ietf.org/html/rfc1213#page-41
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.. _RFC1213 icmpOutErrors: https://tools.ietf.org/html/rfc1213#page-43
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When an error occurs in the ICMP packet handler path, these two
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counters would be updated. The receiving packet path use IcmpInErrors
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and the sending packet path use IcmpOutErrors. When IcmpInCsumErrors
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is increased, IcmpInErrors would always be increased too.
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relationship of the ICMP counters
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---------------------------------
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The sum of IcmpMsgOutType[N] is always equal to IcmpOutMsgs, as they
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are updated at the same time. The sum of IcmpMsgInType[N] plus
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IcmpInErrors should be equal or larger than IcmpInMsgs. When kernel
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receives an ICMP packet, kernel follows below logic:
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1. increase IcmpInMsgs
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2. if has any error, update IcmpInErrors and finish the process
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3. update IcmpMsgOutType[N]
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4. handle the packet depending on the type, if has any error, update
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IcmpInErrors and finish the process
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So if all errors occur in step (2), IcmpInMsgs should be equal to the
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sum of IcmpMsgOutType[N] plus IcmpInErrors. If all errors occur in
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step (4), IcmpInMsgs should be equal to the sum of
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IcmpMsgOutType[N]. If the errors occur in both step (2) and step (4),
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IcmpInMsgs should be less than the sum of IcmpMsgOutType[N] plus
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IcmpInErrors.
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General TCP counters
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====================
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* TcpInSegs
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Defined in `RFC1213 tcpInSegs`_
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.. _RFC1213 tcpInSegs: https://tools.ietf.org/html/rfc1213#page-48
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The number of packets received by the TCP layer. As mentioned in
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RFC1213, it includes the packets received in error, such as checksum
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error, invalid TCP header and so on. Only one error won't be included:
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if the layer 2 destination address is not the NIC's layer 2
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address. It might happen if the packet is a multicast or broadcast
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packet, or the NIC is in promiscuous mode. In these situations, the
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packets would be delivered to the TCP layer, but the TCP layer will discard
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these packets before increasing TcpInSegs. The TcpInSegs counter
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isn't aware of GRO. So if two packets are merged by GRO, the TcpInSegs
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counter would only increase 1.
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* TcpOutSegs
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Defined in `RFC1213 tcpOutSegs`_
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.. _RFC1213 tcpOutSegs: https://tools.ietf.org/html/rfc1213#page-48
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The number of packets sent by the TCP layer. As mentioned in RFC1213,
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it excludes the retransmitted packets. But it includes the SYN, ACK
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and RST packets. Doesn't like TcpInSegs, the TcpOutSegs is aware of
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GSO, so if a packet would be split to 2 by GSO, TcpOutSegs will
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increase 2.
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* TcpActiveOpens
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Defined in `RFC1213 tcpActiveOpens`_
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.. _RFC1213 tcpActiveOpens: https://tools.ietf.org/html/rfc1213#page-47
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It means the TCP layer sends a SYN, and come into the SYN-SENT
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state. Every time TcpActiveOpens increases 1, TcpOutSegs should always
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increase 1.
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* TcpPassiveOpens
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Defined in `RFC1213 tcpPassiveOpens`_
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.. _RFC1213 tcpPassiveOpens: https://tools.ietf.org/html/rfc1213#page-47
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It means the TCP layer receives a SYN, replies a SYN+ACK, come into
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the SYN-RCVD state.
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* TcpExtTCPRcvCoalesce
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When packets are received by the TCP layer and are not be read by the
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application, the TCP layer will try to merge them. This counter
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indicate how many packets are merged in such situation. If GRO is
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enabled, lots of packets would be merged by GRO, these packets
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wouldn't be counted to TcpExtTCPRcvCoalesce.
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* TcpExtTCPAutoCorking
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When sending packets, the TCP layer will try to merge small packets to
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a bigger one. This counter increase 1 for every packet merged in such
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situation. Please refer to the LWN article for more details:
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https://lwn.net/Articles/576263/
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* TcpExtTCPOrigDataSent
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This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
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explaination below::
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TCPOrigDataSent: number of outgoing packets with original data (excluding
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retransmission but including data-in-SYN). This counter is different from
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TcpOutSegs because TcpOutSegs also tracks pure ACKs. TCPOrigDataSent is
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more useful to track the TCP retransmission rate.
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* TCPSynRetrans
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This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
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explaination below::
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TCPSynRetrans: number of SYN and SYN/ACK retransmits to break down
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retransmissions into SYN, fast-retransmits, timeout retransmits, etc.
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* TCPFastOpenActiveFail
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This counter is explained by `kernel commit f19c29e3e391`_, I pasted the
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explaination below::
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TCPFastOpenActiveFail: Fast Open attempts (SYN/data) failed because
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the remote does not accept it or the attempts timed out.
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.. _kernel commit f19c29e3e391: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f19c29e3e391a66a273e9afebaf01917245148cd
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* TcpExtListenOverflows and TcpExtListenDrops
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When kernel receives a SYN from a client, and if the TCP accept queue
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is full, kernel will drop the SYN and add 1 to TcpExtListenOverflows.
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At the same time kernel will also add 1 to TcpExtListenDrops. When a
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TCP socket is in LISTEN state, and kernel need to drop a packet,
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kernel would always add 1 to TcpExtListenDrops. So increase
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TcpExtListenOverflows would let TcpExtListenDrops increasing at the
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same time, but TcpExtListenDrops would also increase without
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TcpExtListenOverflows increasing, e.g. a memory allocation fail would
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also let TcpExtListenDrops increase.
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Note: The above explanation is based on kernel 4.10 or above version, on
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an old kernel, the TCP stack has different behavior when TCP accept
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queue is full. On the old kernel, TCP stack won't drop the SYN, it
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would complete the 3-way handshake. As the accept queue is full, TCP
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stack will keep the socket in the TCP half-open queue. As it is in the
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half open queue, TCP stack will send SYN+ACK on an exponential backoff
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timer, after client replies ACK, TCP stack checks whether the accept
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queue is still full, if it is not full, moves the socket to the accept
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queue, if it is full, keeps the socket in the half-open queue, at next
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time client replies ACK, this socket will get another chance to move
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to the accept queue.
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TCP Fast Open
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=============
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* TcpEstabResets
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Defined in `RFC1213 tcpEstabResets`_.
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.. _RFC1213 tcpEstabResets: https://tools.ietf.org/html/rfc1213#page-48
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* TcpAttemptFails
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Defined in `RFC1213 tcpAttemptFails`_.
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.. _RFC1213 tcpAttemptFails: https://tools.ietf.org/html/rfc1213#page-48
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* TcpOutRsts
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Defined in `RFC1213 tcpOutRsts`_. The RFC says this counter indicates
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the 'segments sent containing the RST flag', but in linux kernel, this
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couner indicates the segments kerenl tried to send. The sending
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process might be failed due to some errors (e.g. memory alloc failed).
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.. _RFC1213 tcpOutRsts: https://tools.ietf.org/html/rfc1213#page-52
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* TcpExtTCPSpuriousRtxHostQueues
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|
|
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
|
|
explaination:
|
|
|
|
.. _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.txt
|
|
|
|
* TcpExtTCPACKSkippedSynRecv
|
|
|
|
The ACK is skipped in Syn-Recv status. The Syn-Recv status means the
|
|
TCP stack receives a SYN and replies SYN+ACK. Now the TCP stack is
|
|
waiting for an ACK. Generally, the TCP stack doesn't need to send ACK
|
|
in the Syn-Recv status. But in several scenarios, the TCP stack need
|
|
to send an ACK. E.g., the TCP stack receives the same SYN packet
|
|
repeately, the received packet does not pass the PAWS check, or the
|
|
received packet sequence number is out of window. In these scenarios,
|
|
the TCP stack needs to send ACK. If the ACk sending frequency is higher than
|
|
tcp_invalid_ratelimit allows, the TCP stack will skip sending ACK and
|
|
increase TcpExtTCPACKSkippedSynRecv.
|
|
|
|
|
|
* TcpExtTCPACKSkippedPAWS
|
|
|
|
The ACK is skipped due to PAWS (Protect Against Wrapped Sequence
|
|
numbers) check fails. If the PAWS check fails in Syn-Recv, Fin-Wait-2
|
|
or Time-Wait statuses, the skipped ACK would be counted to
|
|
TcpExtTCPACKSkippedSynRecv, TcpExtTCPACKSkippedFinWait2 or
|
|
TcpExtTCPACKSkippedTimeWait. In all other statuses, the skipped ACK
|
|
would be counted to TcpExtTCPACKSkippedPAWS.
|
|
|
|
* TcpExtTCPACKSkippedSeq
|
|
|
|
The sequence number is out of window and the timestamp passes the PAWS
|
|
check and the TCP status is not Syn-Recv, Fin-Wait-2, and Time-Wait.
|
|
|
|
* TcpExtTCPACKSkippedFinWait2
|
|
|
|
The ACK is skipped in Fin-Wait-2 status, the reason would be either
|
|
PAWS check fails or the received sequence number is out of window.
|
|
|
|
* TcpExtTCPACKSkippedTimeWait
|
|
|
|
Tha ACK is skipped in Time-Wait status, the reason would be either
|
|
PAWS check failed or the received sequence number is out of window.
|
|
|
|
* TcpExtTCPACKSkippedChallenge
|
|
|
|
The ACK is skipped if the ACK is a challenge ACK. The RFC 5961 defines
|
|
3 kind of challenge ACK, please refer `RFC 5961 section 3.2`_,
|
|
`RFC 5961 section 4.2`_ and `RFC 5961 section 5.2`_. Besides these
|
|
three scenarios, In some TCP status, the linux TCP stack would also
|
|
send challenge ACKs if the ACK number is before the first
|
|
unacknowledged number (more strict than `RFC 5961 section 5.2`_).
|
|
|
|
.. _RFC 5961 section 3.2: https://tools.ietf.org/html/rfc5961#page-7
|
|
.. _RFC 5961 section 4.2: https://tools.ietf.org/html/rfc5961#page-9
|
|
.. _RFC 5961 section 5.2: https://tools.ietf.org/html/rfc5961#page-11
|
|
|
|
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
|
|
=============
|
|
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 explaination 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 sbk,
|
|
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 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
|