datapath: Consider tunnels to have no MTU, fixing jumbo frame support.

Until now, tunnel vports have had a specific MTU, in the same way that
ordinary network devices have an MTU, but treating them this way does not
always make sense.  For example, consider a datapath that has three ports:
the local port, a GRE tunnel to another host, and a physical port.  If
the physical port is configured with a jumbo MTU, it should be possible to
send jumbo packets across the tunnel: the tunnel can do fragmentation or
the physical port traversed by the tunnel might have a jumbo MTU.

However, until now, tunnels always had a 1500-byte MTU by default.  It
could be adjusted using ODP_VPORT_MTU_SET, but nothing actually did this.
One alternative would be to make ovs-vswitchd able to set the vport's MTU.
This commit, however, takes a different approach, of dropping the concept
of MTU entirely for tunnel vports.  This also solves the problem described
above, without making any additional work for anyone.

I tested that, without this change, I could not send 1600-byte "pings"
between two machines whose NICs had 2000-byte MTUs that were connected to
vswitches that were in turn connected over GRE tunnels with the default
1500-byte MTU.  With this change, it worked OK, regardless of the MTU of
the network traversed by the GRE tunnel.

This patch also makes "patch" ports MTU-less.

It might make sense to remove vport_set_mtu() and the associated callback
now, since ordinary network devices are the only vports that support it
now.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Suggested-by: Jesse Gross <jesse@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Bug #3728.

1 files changed, 5 insertions, 2 deletions

diff --git a/lib/netdev-provider.h b/lib/netdev-provider.h
index 4181758f..be51c48a 100644
--- a/lib/netdev-provider.h
+++ b/lib/netdev-provider.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009, 2010 Nicira Networks.
+ * Copyright (c) 2009, 2010, 2011 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -237,7 +237,10 @@ struct netdev_class {
      *
      * The MTU is the maximum size of transmitted (and received) packets, in
      * bytes, not including the hardware header; thus, this is typically 1500
-     * bytes for Ethernet devices.*/
+     * bytes for Ethernet devices.
+     *
+     * If 'netdev' does not have an MTU (e.g. as some tunnels do not), then
+     * this function should set '*mtup' to INT_MAX. */
     int (*get_mtu)(const struct netdev *netdev, int *mtup);
 
     /* Returns the ifindex of 'netdev', if successful, as a positive number.