bfd: Send BFD packets with DSCP CS6

Send BFD packets with TOS value equivalent to DSCP CS6 so that the network
can apply the right QoS for those packets. This can help avoid BFD flaps due
to network congestion.

For a reference on this being the right choice, here is a short
declaration:

http://www.ciscopress.com/articles/article.asp?p=357102&seqNum=4

A long dissertation:

https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book/QoSIntro.html

But in a nutshell:

Network engineers create various queue/drop policies based upon precedence.
Routing protocols are considered high priority/high precedence.  During link
saturation events, packets will get dropped. By creating an egress policy
where packets marked by CS6 are allowed front-of-the-queue status, one can be
sure that hello's from the various protocols arrive when they need to, without
delay and without loss.  On the other hand, if the hellos are dropped as part
of normal traffic operations, then traffic routing will flap, leading to
further congestion and drops.

CS6 is a 'well known' marker to network engineers. In many vendor's gear, it
is automatically assigned to routing protocol packets.

Since OVS does not perform queuing, and leaves that to the kernel edge
operations, the queue policies can be used to ensure timely egress of the BFD
packets during high utilization events.

See also:
https://mail.openvswitch.org/pipermail/ovs-dev/2017-October/339784.html
https://mail.openvswitch.org/pipermail/ovs-dev/2017-October/339785.html

Thanks to Raymond Burkholder <ray@oneunified.net> for much of the above
information.

Signed-off-by: Venkatesan Pradeep <venkatesan.pradeep@ericsson.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/bfd.c b/lib/bfd.c
index 482e779ea..55b7453d1 100644
--- a/lib/bfd.c
+++ b/lib/bfd.c
@@ -613,7 +613,7 @@ bfd_put_packet(struct bfd *bfd, struct dp_packet *p,
     ip->ip_ihl_ver = IP_IHL_VER(5, 4);
     ip->ip_tot_len = htons(sizeof *ip + sizeof *udp + sizeof *msg);
     ip->ip_ttl = MAXTTL;
-    ip->ip_tos = IPTOS_LOWDELAY | IPTOS_THROUGHPUT;
+    ip->ip_tos = IPTOS_PREC_INTERNETCONTROL;
     ip->ip_proto = IPPROTO_UDP;
     put_16aligned_be32(&ip->ip_src, bfd->ip_src);
     put_16aligned_be32(&ip->ip_dst, bfd->ip_dst);