svn commit: r327559 - in head: . sys/net

[hiren panchasara]

On 01/04/18 at 09:52P, Steven Hartland wrote:
> On 04/01/2018 20:50, Eugene Grosbein wrote:
> > 05.01.2018 3:05, Steven Hartland wrote:
> >
> >> Author: smh
> >> Date: Thu Jan  4 20:05:47 2018
> >> New Revision: 327559
> >> URL: https://svnweb.freebsd.org/changeset/base/327559
> >>
> >> Log:
> >>    Disabled the use of flowid for lagg by default
> >>    
> >>    Disabled the use of RSS hash from the network card aka flowid for
> >>    lagg(4) interfaces by default as it's currently incompatible with
> >>    the lacp and loadbalance protocols.
> >>    
> >>    The incompatibility is due to the fact that the flowid isn't know
> >>    for the first packet of a new outbound stream which can result in
> >>    the hash calculation method changing and hence a stream being
> >>    incorrectly split across multiple interfaces during normal
> >>    operation.
> >>    
> >>    This can be re-enabled by setting the following in loader.conf:
> >>    net.link.lagg.default_use_flowid="1"
> >>    
> >>    Discussed with: kmacy
> >>    Sponsored by:	Multiplay
> > RSS by definition has meaning to received stream. What is "outbound" stream
> > in this context, why can the hash calculatiom method change and what exactly
> > does it mean "a stream being incorrectly split"?
> Yes RSS is indeed a received stream but that is used by lagg for lacp 
> and loadbalance protocols to decide which port of the lagg to "send" the 
> packet out of. As the flowid is not known when a new "output" stream is 
> instigated the current code falls back to manual hash calculation to 
> determine which port to send the initial packet from. Once a response is 
> received a tx then uses the flowid. This change of hash calculation 
> method can result in the initial packet being sent from a different port 
> than the rest of the stream; this is what I meant by "incorrectly split".

For my understanding, is this just an issue for the first packet when we
originate the flow? Once we have a response and if flowid is there, we'd
use it, right? OR am I missing something? 

And with this change, we'd always go and do manual calculation even when
we have a valid flowid (i.e. we didn't initiate a connection)?

Thanks,
Hiren
> 
> See the following:
> https://github.com/freebsd/freebsd/blob/master/sys/net/if_lagg.c#L2066
> https://github.com/freebsd/freebsd/blob/master/sys/net/ieee8023ad_lacp.c#L846
> 
> > Defaults should not be changed so easily just because they are not optimal
> > for some specific case. Each lagg has its own setting for flowid usage
> > and why one cannot just use "ifconfig lagg0 -use_flowid" for such cases?
> >
> Yes we're already using -use_flowid to mitigate the problem, but the 
> defaults should never result in broken behavior hence the change, at 
> least for now.
> 
> For reference I did look at keeping the default of 1 but only using that 
> for protocols which weren't effected by the issue, and introducing a 2 
> to force those that are, but as its defined as acting on creation and we 
> always create lagg interfaces as failover and then amend them that 
> wasn't possible without making more invasive changes.
> 
>  ??? Regards
>  ??? Steve
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