interfaces: T3090: add new TCP MSS clamping CLI commands

1 files changed, 14 insertions, 0 deletions

diff --git a/docs/_include/interface-ipv6.txt b/docs/_include/interface-ipv6.txt
index e03817cf..d1ed8837 100644
--- a/docs/_include/interface-ipv6.txt
+++ b/docs/_include/interface-ipv6.txt
@@ -53,3 +53,17 @@
   .. code-block:: none
 
     set interfaces {{ var0 }} {{ var1 }} {{ var2 }} {{ var4 }} {{ var5 }} {{ var7 }} ipv6 disable-forwarding
+
+.. cfgcmd:: set interfaces {{ var0 }} <interface> {{ var2 }} {{ var3 }}
+  {{ var5 }} {{ var6 }} ipv6 adjust-mss <mss>
+
+  As Internet wide PMTU discovery rarely works, we sometimes need to clamp our
+  TCP MSS value to a specific value. This is a field in the TCP options part of
+  a SYN packet. By setting the MSS value, you are telling the remote side
+  unequivocally 'do not try to send me packets bigger than this value'.
+
+  .. note:: This command was introduced in VyOS 1.4 - it was previously called:
+    ``set firewall options interface <name> adjust-mss6 <value>``
+
+  .. hint:: MSS value = MTU - 40 (IPv6 header) - 20 (TCP header), resulting in
+    1432 bytes on a 1492 byte MTU.