netdev - [PATCH iproute2-next 2/3] man: tc-pedit: Add examples for dsfield and retain

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Message-Id: <837315b74b1fdfc4a08362c18f3a1544d8fe1092.1585331173.git.petrm@mellanox.com>
Date:   Fri, 27 Mar 2020 20:55:09 +0300
From:   Petr Machata <petrm@...lanox.com>
To:     netdev@...r.kernel.org
Cc:     Petr Machata <petrm@...lanox.com>, David Ahern <dsahern@...il.com>
Subject: [PATCH iproute2-next 2/3] man: tc-pedit: Add examples for dsfield and retain

Describe a way to update just the DSCP and just the ECN part of the
dsfield. That is useful on its own, but also it shows how retain works.

Signed-off-by: Petr Machata <petrm@...lanox.com>
---
 man/man8/tc-pedit.8 | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/man/man8/tc-pedit.8 b/man/man8/tc-pedit.8
index b44b0263..54b91d3d 100644
--- a/man/man8/tc-pedit.8
+++ b/man/man8/tc-pedit.8
@@ -377,6 +377,28 @@ tc filter add dev eth0 parent ffff: u32 \\
 	action pedit ex munge tcp dport set 22
 .EE
 .RE
+
+To rewrite just part of a field, use the
+.B retain
+directive. E.g. to overwrite the DSCP part of a dsfield with $DSCP, without
+touching ECN:
+
+.RS
+.EX
+tc filter add dev eth0 ingress flower ... \\
+	action pedit ex munge ip dsfield set $((DSCP << 2)) retain 0xfc
+.EE
+.RE
+
+And vice versa, to set ECN to e.g. 1 without impacting DSCP:
+
+.RS
+.EX
+tc filter add dev eth0 ingress flower ... \\
+	action pedit ex munge ip dsfield set 1 retain 0x3
+.EE
+.RE
+
 .SH SEE ALSO
 .BR tc (8),
 .BR tc-htb (8),
-- 
2.20.1