[ovs-dev] [PATCH v2 1/2] dpif-netlink-rtnl: Use 65000 instead of 65535 as tunnel MTU.

Ben Pfaff blp at ovn.org
Wed Jan 17 17:58:48 UTC 2018


Most of the existing tunnels accept 65535 for MTU and internally reduce it
to the maximum value actually supported.  However, in RTM_SETLINK calls,
at least GRE tunnels reject MTU larger than actually supported.  This
commit changes the MTU used in RTM_NEWLINK calls to use a value that should
be acceptable to all tunnels and yet does not noticeably reduce
performance.

(This code doesn't actually use RTM_SETLINK to change MTU yet, but that's
coming up.)

Suggested-by: Eric Garver <e at erig.me>
Suggested-at: https://mail.openvswitch.org/pipermail/ovs-dev/2018-January/343304.html
Signed-off-by: Ben Pfaff <blp at ovn.org>
---
 lib/dpif-netlink-rtnl.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/dpif-netlink-rtnl.c b/lib/dpif-netlink-rtnl.c
index fe9c8ed7104f..37451b80de0e 100644
--- a/lib/dpif-netlink-rtnl.c
+++ b/lib/dpif-netlink-rtnl.c
@@ -277,6 +277,15 @@ dpif_netlink_rtnl_create(const struct netdev_tunnel_config *tnl_cfg,
                          const char *name, enum ovs_vport_type type,
                          const char *kind, uint32_t flags)
 {
+    enum {
+        /* For performance, we want to use the largest MTU that the system
+         * supports.  Most existing tunnels will accept UINT16_MAX, treating it
+         * as the actual max MTU, but some do not.  Thus, we use a slightly
+         * smaller value, that should always be safe yet does not noticeably
+         * reduce performance. */
+        MAX_MTU = 65000
+    };
+
     size_t linkinfo_off, infodata_off;
     struct ifinfomsg *ifinfo;
     struct ofpbuf request;
@@ -287,7 +296,7 @@ dpif_netlink_rtnl_create(const struct netdev_tunnel_config *tnl_cfg,
     ifinfo = ofpbuf_put_zeros(&request, sizeof(struct ifinfomsg));
     ifinfo->ifi_change = ifinfo->ifi_flags = IFF_UP;
     nl_msg_put_string(&request, IFLA_IFNAME, name);
-    nl_msg_put_u32(&request, IFLA_MTU, UINT16_MAX);
+    nl_msg_put_u32(&request, IFLA_MTU, MAX_MTU);
     linkinfo_off = nl_msg_start_nested(&request, IFLA_LINKINFO);
     nl_msg_put_string(&request, IFLA_INFO_KIND, kind);
     infodata_off = nl_msg_start_nested(&request, IFLA_INFO_DATA);
-- 
2.10.2



More information about the dev mailing list