RE: [dhcwg] dhcpv6-24: use of anycast

["Bound, Jim" <[e-mail]>]

Ralph,

I don't believe we have any but will check for well known unicast.  But why not use well known multicast address on the link.  We do that today.

Either way it does not break any API or implementation.  

But using multicast there is well known way to join that group and we just say what it is reserved for?

Good idea.

regards,
/jim

> -----Original Message-----
> From: Ralph Droms [mailto:[e-mail]]
> Sent: Sunday, May 12, 2002 12:45 PM
> To: Bound, Jim
> Cc: Bernie Volz (EUD); Thomas Narten; Ole Troan; [e-mail]
> Subject: RE: [dhcwg] dhcpv6-24: use of anycast 
> 
> 
> 
> Jim makes a good good point about anycast.  Given the current
> discussion about anycast addresses and hosts on the ipngwg mailing
> list, it would be premature to specify site-local anycast for DHCPv6.
> 
> What about link-local only "anycast" - perhaps that's not really
> "anycast" because there's no routing?
> 
> Instead of an anycast address, suppose we request the reservation of a
> link-scoped unicast address for DHCP?  If we use a unicast 
> address, only
> one relay agent or server on a link can be listening on that address,
> right?  For point-to-point links that's not a problem - there 
> can only be
> one relay agent or server at the other end of the point-to-point link.
> I'm still not clear about multiple-access links and multicast - are
> there any multiple access link technologies that don't 
> provide or emulate
> multicast?
> 
> Is there a precedent for the reservation of a link-scoped, well-known
> unicast address?
> 
> Using a link-scoped unicast address, the first relay agent or 
> server to
> bind to the address gets to use it.   I'm not familiar with 
> the IPv6 API;
> I assume that an application (with sufficient privilege) can make a
> request to assign a specific address to an interface.  I 
> guess the stack
> does DAD on the requested address and the request fails if 
> the address is
> already in use?
> 
> I'm not thrilled with my idea - it sounds fairly ugly.  Does 
> it break or
> bend fewer existing standards, or require the definition of fewer new
> standards than the use of an anycast address?  Does the 
> assignment of a
> well-known link-scoped unicast address for DHCP set a precedent we'd
> rather not set?
> 
> The use of the unicast address could be further discouraged 
> by including a
> restriction that relay agents and servers MUST NOT listen on 
> the unicast
> address and clients MUST NOT send messages to the unicast address on a
> link that supports multicast.
> 
> - Ralph
> 
>  On Sun, 12 May 2002, Bound, Jim wrote:
> 
> > Thomas,
> >
> > I am fine with this too.  But what we are permitting the 
> server to have an anycast address and the addr arch says 
> don't do this?  Will it not be kicked  back again by the IESG 
> because we did that?
> >
> > thanks
> > /jim
> >
> > -----Original Message-----
> > From: Bernie Volz (EUD) [mailto:[e-mail]]
> > Sent: Thursday, May 09, 2002 9:27 AM
> > To: 'Thomas Narten'; Bernie Volz (EUD)
> > Cc: Ole Troan; Ralph Droms; [e-mail]
> > Subject: RE: [dhcwg] dhcpv6-24: use of anycast
> >
> >
> > >> What we probably should say is that the client MUST use the
> > >> multicast address if the interface on which it is sending the
> > >> message supports multicast. Otherwise, it may use the anycast
> > >> address.
> > >
> > >This is OK with me. Just so long as it's clear that the 
> anycast usage
> > >is only link-local and is only for reaching servers (and 
> relay agents)
> > >on the same link. This seems like a straightforward thing to get
> > >right.
> > Sounds fine to me.
> > - Bernie
> > -----Original Message-----
> > From: Thomas Narten [mailto:[e-mail]]
> > Sent: Thursday, May 09, 2002 9:23 AM
> > To: Bernie Volz (EUD)
> > Cc: Ole Troan; Ralph Droms; [e-mail]
> > Subject: Re: [dhcwg] dhcpv6-24: use of anycast
> >
> >
> > > The only issue with this is that DHCP usually runs as an 
> application
> > > and is therefore only able to access that information which the
> > > lower layers (via APIs) make available.
> > Understood.
> > > Determining whether multicast is supported or not is 
> pretty standard
> > > from the socket API. However, determining that a particular
> > > interface is a specific media type may be difficult.
> > OK.
> > > Therefore, there is some value in providing a clear direction here
> > >  that is also relatively easy to implement.
> > Agreed.
> > > If all interface types provide multicast (either inherently or via
> > > some type of emulation), there is little need to ever use the
> > > anycast address and so what harm is there in specifying it.
> > The harm is that a) we may define something that isn't useful (which
> > may cause problems when deployed because some implementations choose
> > use the feature even if it doesn't work as intended). b) clients may
> > choose to send to anycast addresses, but if servers aren't 
> configured
> > to deal with them, we don't get interoperability, so specifying that
> > clients MAY do something really implies (to me) that server 
> SHOULD (or
> > maybe even MUST) be prepared to handle such packets from clients. If
> > the client  can't expect a server to handle something, 
> there is little
> > point in clients implementing the feature either.
> > My general opinion: if its not clear something is useful, 
> and it's not
> > obvious what the details should be, including the feature distracts
> > from the overall goal of getting the spec done. The more one can
> > remove, the fewer details that have to be gotten right.
> > > What we probably should say is that the client MUST use the
> > > multicast address if the interface on which it is sending the
> > > message supports multicast. Otherwise, it may use the anycast
> > > address.
> > This is OK with me. Just so long as it's clear that the 
> anycast usage
> > is only link-local and is only for reaching servers (and 
> relay agents)
> > on the same link. This seems like a straightforward thing to get
> > right.
> > Thomas
> >
> 
> 
> 

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