From: Petr Machata <petrm@mellanox.com>
Date: Fri, 3 Apr 2020 23:05:30 +0000 (+0300)
Subject: man: tc-pedit: Add examples for dsfield and retain
X-Git-Tag: v5.7.0~44
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f91f788c70e92a80b53f8e6c83ac3059aa0b9ddc;p=thirdparty%2Fiproute2.git

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@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
---

diff --git a/man/man8/tc-pedit.8 b/man/man8/tc-pedit.8
index 3f6baa3dd..d2b37ef07 100644
--- a/man/man8/tc-pedit.8
+++ b/man/man8/tc-pedit.8
@@ -369,6 +369,28 @@ 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),