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Diffstat (limited to 'Documentation/networking/snmp_counter.rst')
| -rw-r--r-- | Documentation/networking/snmp_counter.rst | 130 |
1 files changed, 125 insertions, 5 deletions
diff --git a/Documentation/networking/snmp_counter.rst b/Documentation/networking/snmp_counter.rst index b0dfdaaca512..fe8f741193be 100644 --- a/Documentation/networking/snmp_counter.rst +++ b/Documentation/networking/snmp_counter.rst @@ -336,7 +336,26 @@ time client replies ACK, this socket will get another chance to move to the accept queue. -TCP Fast Open +* TcpEstabResets +Defined in `RFC1213 tcpEstabResets`_. + +.. _RFC1213 tcpEstabResets: https://tools.ietf.org/html/rfc1213#page-48 + +* TcpAttemptFails +Defined in `RFC1213 tcpAttemptFails`_. + +.. _RFC1213 tcpAttemptFails: https://tools.ietf.org/html/rfc1213#page-48 + +* TcpOutRsts +Defined in `RFC1213 tcpOutRsts`_. The RFC says this counter indicates +the 'segments sent containing the RST flag', but in linux kernel, this +couner indicates the segments kerenl tried to send. The sending +process might be failed due to some errors (e.g. memory alloc failed). + +.. _RFC1213 tcpOutRsts: https://tools.ietf.org/html/rfc1213#page-52 + + +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 @@ -383,8 +402,6 @@ 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 @@ -545,7 +562,6 @@ packet yet, the sender would know packet 4 is out of order. The TCP stack of kernel will increase TcpExtTCPSACKReorder for both of the above scenarios. - DSACK ===== The DSACK is defined in `RFC2883`_. The receiver uses DSACK to report @@ -566,13 +582,63 @@ The TCP stack receives an out of order duplicate packet, so it sends a DSACK to the sender. * TcpExtTCPDSACKRecv -The TCP stack receives a DSACK, which indicate an acknowledged +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 @@ -662,6 +728,60 @@ unacknowledged number (more strict than `RFC 5961 section 5.2`_). .. _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. examples ======= |