dhcpcd-discuss

On Tue, 14 Feb 2017 22:26:04 +0000
Roy Marples <roy@xxxxxxxxxxxx> wrote:

> Hi
> 
> On 13/02/2017 16:31, Shahid Mahmood wrote:
> > On Sun, Feb 12, 2017 at 5:12 PM, Roy Marples <roy@xxxxxxxxxxxx
> > <mailto:roy@xxxxxxxxxxxx>> wrote:
> > 
> >     Hi Shahid
> > 
> >     On 10/02/17 15:37, Shahid Mahmood wrote:
> >     > Hello Roy,
> >     > Just a quick question on section 17.1.3 of RFC 3315. It states 3
> >     options
> >     > available to the client in order to select and Advertise message:
> >     >
> >     > --- quote
> >     >
> >     >    Upon receipt of one or more valid Advertise messages, the client
> >     >    selects one or more Advertise messages based upon the following
> >     >    criteria.
> >     >
> >     >    -  Those Advertise messages with the highest server preference
> >     value
> >     >       are preferred over all other Advertise messages.
> >     >
> >     >    -  Within a group of Advertise messages with the same server
> >     >       preference value, a client MAY select those servers whose
> >     >       Advertise messages advertise information of interest to the
> >     >       client.  For example, the client may choose a server that
> >     returned
> >     >       an advertisement with configuration options of interest to the
> >     >       client.
> >     >
> >     >    -  The client MAY choose a less-preferred server if that server
> >     has a
> >     >       better set of advertised parameters, such as the available
> >     >       addresses advertised in IAs.
> >     >
> >     > --- end quote
> >     >
> >     > Specifically, when does client pick the last option:
> >     >
> >     >    -  The client MAY choose a less-preferred server if that server
> >     has a
> >     >       better set of advertised parameters, such as the available
> >     >       addresses advertised in IAs.
> >     >
> >     > Also, can application influence this choice (with parameter or
> >     something)?
> > 
> >     Currently there is no preference selection available.
> >     dhcpcd binds the first one received and keeps it. It does not change to
> >     another lease as such.
> > 
> >  
> > It is observed that dhcpc picks the first server if two happen to send
> > Advertize almost simultaneously.
> > The first advertize has Pref-Value 250, 2nd one 253. It does not seem to
> > follow 1st bullet in sec 17.1.3 in RFC 3315.
> >  
> > 
> > 
> >     Because dhcpcd can trivially remove the lease it applied, changing to a
> >     new lease from a new server wouldn't be too onerous. How would you say
> >     to prefer one over another?
> > 
> >  
> > At this point, I just wanted to confirm if the behavior is in accordance
> > with the RFC.
> 
> Section 17.1.3 says the client MAY select a better server, not MUST.
> So yes, in the strict sense dhcpcd is in compliance even though it
> ignores the option.
> 
> Making dhcpcd react to a more preferred server right away would be quite
> easy.
> Storing the adverts for a period of time and then choosing the more
> preferred one to bind to would be trickier.
> Right now, I'm not sure which is the better solution.

Or maybe something in the middle. Save the first advert received. Then for 2-5 seconds, receive an advert, compare it with the saved advert, and save the 'better' one. At timeout, use the saved advert. Of course, determining 'better' may be the real challenge.

Does it make sense to choose a route now, then later change if a better offer comes along?

N