Subject: mic comments on presentation of draft-pfister-6man-sadr-ra-00

From: Alexandru Petrescu <[email-masked]>
Date: Mon, 23 Mar 2015 17:42:55 +0100

Hello,

Please allow me to reiterate the mic comments I had during the
presentation of draft-pfister-6man-sadr-ra-00.

Encoding source prefix in an RA, along with RIO, is making sense.

But when it comes to specifying the Host (and Router, for that matter)
behaviour do source-based routing, things are less clear.

I think we do not have a specification of dst-based routing in first
place, and as such adding src-based routing could only be less clear.

What's the place of a default route in the dst-based longest match
followed by the src-based longest match?  If src-based longest prefix
match search finds nothing, will it hit a 'src-based' default route?  Or
hit the existing dst-based default route?  If the former, would it take
precedence over the dst-based def route?

How will it behave when the unspecified address is used as src?

Will it be CIDR/SADR?  Or search only the first 64 bits?

An earlier commenter asked about the Default Router Preference, and
Lifetimes in the RA - will this apply to src-based routing as well?
Will we have a src-based default router address in an RA?

Alex

---

From: Sowmini Varadhan <[email-masked]>
Date: Mon, 23 Mar 2015 13:27:15 -0400

> precedence over the dst-based def route?

As Erik mentioned, we had to tackle some of these questions for 
Solaris some years ago. There was an administrative knob that allowed
something between srong and weak ES, if set, this would give the following
behavior
	Equivalent to the weak end-system model in
        receive behavior, i.e., a packet will be accepted
        on any interface, as long as the IP destination
        of the packet is configured on one of the host's
        interfaces.  When transmitting a packet, if the
        multiple routes for the IP destination in the
        packet are available, the system will prefer
        routes where the IP source address in the packet
        is configured on the outgoing interface. If no
        such route is available, the system will fall
        back to selecting the "best" route as with the
        weak ES case.

Thus in answer to the questions above,
if src-based fib lookup failed (the default route would end up being
the "longest matching prefix" if there was no other route with mask > 0)
it would fall back to existing ECMP rules for selecting between
available default routes. 

The answer to "If the former, would it take precedence over the dst-based
def route?" is more ambivalent: if the longer matching prefixes 
do not satisfy the "src-based" criteria, but a shorter prefix does,
what should be chosen? I believe that this situation cannot happen,
for the current homenet arch being discussed?

> How will it behave when the unspecified address is used as src?

In our Solaris experiment, if the src is unspec, then the default
fib lookups would have applied, i.e., longest prefix, with whatever
ECMP policies were in place for multiple routes to the same dst/mask
but varying gw, interface etc.

--Sowmini

---

From: Ole Troan <[email-masked]>
Date: Mon, 23 Mar 2015 13:56:37 -0500

Alex, et al,

> Will it be CIDR/SADR?  Or search only the first 64 bits?

the forwarding lookup and to the extent that hosts use the same algorithm is outside the scope of 6MAN.
that’s in the routing area. see e.g.: https://tools.ietf.org/html/draft-baker-ipv6-isis-dst-src-routing-02

cheers,
Ole