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[ Upstream commit 2570284060 ]
there is a problem with the CWR flag set in an incoming ACK segment
and it leads to the situation when the ECE flag is latched forever
the following packetdrill script shows what happens:
// Stack receives incoming segments with CE set
+0.1 <[ect0] . 11001:12001(1000) ack 1001 win 65535
+0.0 <[ce] . 12001:13001(1000) ack 1001 win 65535
+0.0 <[ect0] P. 13001:14001(1000) ack 1001 win 65535
// Stack repsonds with ECN ECHO
+0.0 >[noecn] . 1001:1001(0) ack 12001
+0.0 >[noecn] E. 1001:1001(0) ack 13001
+0.0 >[noecn] E. 1001:1001(0) ack 14001
// Write a packet
+0.1 write(3, ..., 1000) = 1000
+0.0 >[ect0] PE. 1001:2001(1000) ack 14001
// Pure ACK received
+0.01 <[noecn] W. 14001:14001(0) ack 2001 win 65535
// Since CWR was sent, this packet should NOT have ECE set
+0.1 write(3, ..., 1000) = 1000
+0.0 >[ect0] P. 2001:3001(1000) ack 14001
// but Linux will still keep ECE latched here, with packetdrill
// flagging a missing ECE flag, expecting
// >[ect0] PE. 2001:3001(1000) ack 14001
// in the script
In the situation above we will continue to send ECN ECHO packets
and trigger the peer to reduce the congestion window. To avoid that
we can check CWR on pure ACKs received.
v3:
- Add a sequence check to avoid sending an ACK to an ACK
v2:
- Adjusted the comment
- move CWR check before checking for unacknowledged packets
Signed-off-by: Denis Kirjanov <denis.kirjanov@suse.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.