diff options
| -rw-r--r-- | Documentation/networking/snmp_counter.rst | 12 |
1 files changed, 9 insertions, 3 deletions
diff --git a/Documentation/networking/snmp_counter.rst b/Documentation/networking/snmp_counter.rst index 52b026be028f..38a4edc4522b 100644 --- a/Documentation/networking/snmp_counter.rst +++ b/Documentation/networking/snmp_counter.rst @@ -413,7 +413,7 @@ 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:: @@ -681,6 +681,7 @@ 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. @@ -690,7 +691,7 @@ 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 @@ -704,11 +705,13 @@ 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 @@ -719,7 +722,7 @@ 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 @@ -730,12 +733,15 @@ 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. |