Subject: Re: join(s,g) on shared tree

From: Arup Acharya <[email-masked]>
Date: Tue, 19 Jan 99 14:30:05 EST

- >> i don't follow your notation, but the answer is probably no.  the only 
- >> time an s,g join is sent _toward_ the RP is when rpf(s) == rpf(rp),
- >> however, it is still sent to S.
- 
-     Rusty, one might argue that an (S,G,RP-bit) join is sent up the RP tree
-     when it wants to undo previous sent (S,G,RP-bit) prunes. Even though
-     this can be undone (via the spec) by sending a (*,G) join without S
-     in the prune-list.
- 
- Dino
- 

Put in the above context, would PIM allow a DR to send a (S, G, RP-bit) join 
without having sent (S, G, RPbit) prune earlier ?....the intended result
would be to create a (S, G) path from the DR to either 1] a node
on the shared tree, or 2] a node on the SPT, if it exists
..depending on which node is closer to the dr. (apologies for the unclear
notation i used )

(The aim is to avoid triggering a SPT creation through
join(S, G, RP-bit) if the SPT didnt already exist). 

arup

-- 
- -the semantics of join(S,G, dest=RP)  that i had in  mind:
--
--a.  merge(join(S, G, dest=RP), join(S, *)) = join(*, G)   
--b.  merge(join(S, G, dest=RP), join(S,G, dest=S)) = join (S, G, dest=S)
--c.  merge(join(S,G, dest=RP), null) = create (S, G) state with iif=RPF to RP

---

From: [email-masked]
Date: Tue, 19 Jan 1999 12:08:33 -0800 (PST)

> join(S, G, RP-bit) if the SPT didnt already exist). 

ok, i think i understand what you're getting at.  within the context of
igmpv3, where a host specificly requests (s, g) traffic, you'd like to
be able to request only that source from the shared tree, without
having to send a (*, g) followed by (s,g) prunes for all else.  if it
happens to cross the spt along the way, then that's ok, is this
correct?

i think the pim rfc only directly concerns itself with group
membership as a whole, allowing for better service by switching to the
spt.  however, in light of Dino's message, the spec doesn't seem to 
prohibit this behavior either.  

maybe it'd be worthwhile to consider how igmpv3 will effect the
current pim spec (if at all).  i.e., is it as simple has having a DR
send (s,g) join/prunes in addition to the (*,g) joins it sends now?

is this the point you're making?

- rusty