net: add document for several snmp counters
add document for below counters: TcpEstabResets TcpAttemptFails TcpOutRsts TcpExtTCPSACKDiscard TcpExtTCPDSACKIgnoredOld TcpExtTCPDSACKIgnoredNoUndo TcpExtTCPSackShifted TcpExtTCPSackMerged TcpExtTCPSackShiftFallback TcpExtTCPWantZeroWindowAdv TcpExtTCPToZeroWindowAdv TcpExtTCPFromZeroWindowAdv TcpExtDelayedACKs TcpExtDelayedACKLocked TcpExtDelayedACKLost TcpExtTCPLossProbes TcpExtTCPLossProbeRecovery Signed-off-by: yupeng <yupeng0921@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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yupeng
David S. Miller
parent
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commit
a6c7c7aac2
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@ -336,7 +336,26 @@ 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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* 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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TCP Fast Path
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============
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When kernel receives a TCP packet, it has two paths to handler the
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packet, one is fast path, another is slow path. The comment in kernel
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@ -383,8 +402,6 @@ increase 1.
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TCP abort
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========
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* TcpExtTCPAbortOnData
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It means TCP layer has data in flight, but need to close the
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connection. So TCP layer sends a RST to the other side, indicate the
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@ -545,7 +562,6 @@ packet yet, the sender would know packet 4 is out of order. The TCP
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stack of kernel will increase TcpExtTCPSACKReorder for both of the
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above scenarios.
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DSACK
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=====
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The DSACK is defined in `RFC2883`_. The receiver uses DSACK to report
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@ -566,13 +582,63 @@ The TCP stack receives an out of order duplicate packet, so it sends a
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DSACK to the sender.
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* TcpExtTCPDSACKRecv
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The TCP stack receives a DSACK, which indicate an acknowledged
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The TCP stack receives a DSACK, which indicates an acknowledged
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duplicate packet is received.
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* TcpExtTCPDSACKOfoRecv
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The TCP stack receives a DSACK, which indicate an out of order
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duplicate packet is received.
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invalid SACK and DSACK
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====================
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When a SACK (or DSACK) block is invalid, a corresponding counter would
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be updated. The validation method is base on the start/end sequence
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number of the SACK block. For more details, please refer the comment
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of the function tcp_is_sackblock_valid in the kernel source code. A
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SACK option could have up to 4 blocks, they are checked
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individually. E.g., if 3 blocks of a SACk is invalid, the
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corresponding counter would be updated 3 times. The comment of the
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`Add counters for discarded SACK blocks`_ patch has additional
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explaination:
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.. _Add counters for discarded SACK blocks: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=18f02545a9a16c9a89778b91a162ad16d510bb32
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* TcpExtTCPSACKDiscard
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This counter indicates how many SACK blocks are invalid. If the invalid
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SACK block is caused by ACK recording, the TCP stack will only ignore
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it and won't update this counter.
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* TcpExtTCPDSACKIgnoredOld and TcpExtTCPDSACKIgnoredNoUndo
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When a DSACK block is invalid, one of these two counters would be
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updated. Which counter will be updated depends on the undo_marker flag
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of the TCP socket. If the undo_marker is not set, the TCP stack isn't
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likely to re-transmit any packets, and we still receive an invalid
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DSACK block, the reason might be that the packet is duplicated in the
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middle of the network. In such scenario, TcpExtTCPDSACKIgnoredNoUndo
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will be updated. If the undo_marker is set, TcpExtTCPDSACKIgnoredOld
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will be updated. As implied in its name, it might be an old packet.
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SACK shift
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=========
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The linux networking stack stores data in sk_buff struct (skb for
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short). If a SACK block acrosses multiple skb, the TCP stack will try
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to re-arrange data in these skb. E.g. if a SACK block acknowledges seq
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10 to 15, skb1 has seq 10 to 13, skb2 has seq 14 to 20. The seq 14 and
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15 in skb2 would be moved to skb1. This operation is 'shift'. If a
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SACK block acknowledges seq 10 to 20, skb1 has seq 10 to 13, skb2 has
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seq 14 to 20. All data in skb2 will be moved to skb1, and skb2 will be
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discard, this operation is 'merge'.
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* TcpExtTCPSackShifted
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A skb is shifted
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* TcpExtTCPSackMerged
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A skb is merged
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* TcpExtTCPSackShiftFallback
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A skb should be shifted or merged, but the TCP stack doesn't do it for
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some reasons.
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TCP out of order
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===============
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* TcpExtTCPOFOQueue
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@ -662,6 +728,60 @@ unacknowledged number (more strict than `RFC 5961 section 5.2`_).
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.. _RFC 5961 section 4.2: https://tools.ietf.org/html/rfc5961#page-9
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.. _RFC 5961 section 5.2: https://tools.ietf.org/html/rfc5961#page-11
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TCP receive window
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=================
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* TcpExtTCPWantZeroWindowAdv
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Depending on current memory usage, the TCP stack tries to set receive
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window to zero. But the receive window might still be a no-zero
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value. For example, if the previous window size is 10, and the TCP
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stack receives 3 bytes, the current window size would be 7 even if the
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window size calculated by the memory usage is zero.
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* TcpExtTCPToZeroWindowAdv
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The TCP receive window is set to zero from a no-zero value.
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* TcpExtTCPFromZeroWindowAdv
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The TCP receive window is set to no-zero value from zero.
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Delayed ACK
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==========
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The TCP Delayed ACK is a technique which is used for reducing the
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packet count in the network. For more details, please refer the
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`Delayed ACK wiki`_
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.. _Delayed ACK wiki: https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment
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* TcpExtDelayedACKs
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A delayed ACK timer expires. The TCP stack will send a pure ACK packet
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and exit the delayed ACK mode.
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* TcpExtDelayedACKLocked
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A delayed ACK timer expires, but the TCP stack can't send an ACK
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immediately due to the socket is locked by a userspace program. The
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TCP stack will send a pure ACK later (after the userspace program
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unlock the socket). When the TCP stack sends the pure ACK later, the
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TCP stack will also update TcpExtDelayedACKs and exit the delayed ACK
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mode.
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* TcpExtDelayedACKLost
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It will be updated when the TCP stack receives a packet which has been
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ACKed. A Delayed ACK loss might cause this issue, but it would also be
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triggered by other reasons, such as a packet is duplicated in the
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network.
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Tail Loss Probe (TLP)
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===================
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TLP is an algorithm which is used to detect TCP packet loss. For more
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details, please refer the `TLP paper`_.
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.. _TLP paper: https://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01
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* TcpExtTCPLossProbes
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A TLP probe packet is sent.
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* TcpExtTCPLossProbeRecovery
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A packet loss is detected and recovered by TLP.
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examples
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=======
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