[ovs-dev] [PATCH] bfd: Send BFD packets with DSCP CS6

Raymond Burkholder ray at oneunified.net
Tue Jan 23 02:23:29 UTC 2018


On 01/22/2018 03:36 PM, Ben Pfaff wrote:
> Can you provide a reference to something that helps me understand why
> the new value is the "correct" one?  This is not my area of expertise
> and all I learn from the commit message is an assertion that the new
> value is better.

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.


> 
> Thanks,
> 
> Ben.
> 
> On Mon, Dec 25, 2017 at 04:59:22PM +0000, Venkatesan Pradeep wrote:
>> 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.
>>
>> Signed-off-by: Venkatesan Pradeep <venkatesan.pradeep at ericsson.com>
>> ---
>>   lib/bfd.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/bfd.c b/lib/bfd.c
>> index 40cd0be..8d291bb 100644
>> --- a/lib/bfd.c
>> +++ b/lib/bfd.c
>> @@ -612,7 +612,7 @@ 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);
>> _______________________________________________
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>> dev at openvswitch.org
>> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
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-- 
Raymond Burkholder
ray at oneunified.net
https://blog.raymond.burkholder.net

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