[rbridge] New shim header proposal---without F-tag field

Tue Oct 31 11:36:19 PST 2006

Being able to send the frame on the (pruned) ingress rbridge tree
determined by the "egress" rbridge field of the shim header for a
multicast frame, rather than the "ingress" rbridge field of that shim
header.

I suppose there are actually three cases for multicast frames:

1) The shim ingress and "egress" rbridges are the same. This is the same
case as the current specification.

2) They are different but the ingress rbridge, which constructs this
encapsulation, is on the IRT of the selected "egress" (actually IRT
root) rbridge. Seems like you just need to add the usual bridge logic to
distinguish frames heading towards/away from the root.

3) I'm not sure if this case is needed but you could have a case where
the ingress rbridge which constructs the encapsulation is not on the IRT
for the designated "egress" rbridge. If this case can occur, while I
could imagine much more complex and optimal things to do, it seems
adequate to just use the "unicast" forwarding towards the "egress"
rbridge until you first hit an rbridge which is on the IRT rooted at the
"egress" rbridge at which point the case 2 logic can take over.

Donald 

-----Original Message-----
From: Gray, Eric [mailto:Eric.Gray at marconi.com] 
Sent: Tuesday, October 31, 2006 2:21 PM
To: Eastlake III Donald-LDE008; rbridge at postel.org
Subject: RE: [rbridge] New shim header proposal---without F-tag field

Donald,

	Sorry, I am confused by your response: when you say "very little
additional logic is required to support this" - what is "this"...

--
Eric

--> -----Original Message-----
--> From: rbridge-bounces at postel.org
--> [mailto:rbridge-bounces at postel.org] On Behalf Of Eastlake III 
--> Donald-LDE008
--> Sent: Tuesday, October 31, 2006 2:03 PM
--> To: rbridge at postel.org
--> Subject: Re: [rbridge] New shim header proposal---without F-tag 
--> field
--> 
--> Speaking as a WG member: Since rbridges need to be able to compute 
--> an ingress rbridge based tree for various other rbridges, it seems 
--> to me that very little additional logic is needed to support this.
--> Implementation would be relatively easy and would have to be 
--> mandatory to assure proper frame delivery. If it was optional, you 
--> have different nodes using different trees and it is trivial to come

--> up with cases where a frame is delivered zero times or twice when it

--> should be delivered once. Depending on implementation, it is also 
--> possible to create permanent loops if this is optional.
--> 
--> Donald
--> 
--> -----Original Message-----
--> From: rbridge-bounces at postel.org
--> [mailto:rbridge-bounces at postel.org] On Behalf Of Russ White
--> Sent: Tuesday, October 31, 2006 9:19 AM
--> To: Radia Perlman
--> Cc: rbridge at postel.org
--> Subject: Re: [rbridge] New shim header proposal---without F-tag 
--> field
--> 
--> -----BEGIN PGP SIGNED MESSAGE-----
--> Hash: SHA1
--> 
--> 
--> This sounds fine to me, as it also means that implementations that 
--> don't want to support this won't suffer from the additional space 
--> and work involved in the F-Tag. My primary question would be--what 
--> will happen if two implementations, one supporting, the other not, 
--> are used in the same network?
--> 
--> :-)
--> 
--> Russ
--> 
--> Radia Perlman wrote:
--> > After talking to Silvano and Dinesh to get them to
--> explain to me why
--> > the use of F-tag didn't multiply the number of trees that
--> RBridges
--> > needed to compute by the number of F-tag values, they
--> were able to
--> > explain to me why they wanted the F-tag. Here is a
--> proposal to get the
--> 
--> > functionality they want without the use of F-tag.
--> > 
--> > ***********************
--> > 
--> > Motivation for F-tag
--> > 
--> > They want to be able to do multipathing on multicast. We
--> can already
--> > do multipathing on unicast by having an RBridge keep
--> multiple equal
--> > cost (or near equal cost) paths to destination D, and
--> split traffic
--> > however it wants. But if we have a single tree for all
--> multicast with
--> > ingress RBridge R1, then all multicast traffic has to use
--> the same
--> > links. The original proposal was to use F-tag as a selector for a 
--> > tree. But the modified proposal is to use the "egress RBridge"
--> > field, which wasn't all that useful for multicast, as a "tree
--> selector".
--> > 
--> > ******************************
--> > 
--> > So the proposal is:
--> > 
--> > In the shim header will only be:
--> > 1) 16-bit ingress RBridge
--> > 2) 16-bit egress RBridge
--> > 3) TTL
--> > 
--> > 
--> > How these fields will be used in order to do multipathing:
--> > 
--> > a) RBridges calculate a single tree for each ingress RBridge
--> > 
--> > b) For unicast, an RBridge can calculate multiple
--> equal-cost paths to
--> > the destination (or near-equal cost), and split traffic. 
--> That is how
--> > we'd get multipathing for unicast. If S7 has two ways to get to 
--> > destination D, it can send some of its traffic one way,
--> some of it the
--> 
--> > other. This is solely S7's choice and doesn't need to be
--> coordinated
--> > with any other switches.
--> > 
--> > c) For multicast, we can provide multipathing by letting
--> the ingress
--> > RBridge choose the distribution tree.
--> > The tree used will be indicated by the "egress RBridge"
--> > field. So, if S3 receives a multicast packet from an
--> endnode, it puts
--> > "S3" as "ingress RBridge", but selects a tree by whatever
--> means it
--> > wants, and puts, say, "S1" into "egress RBridge". That
--> way the packet
--> > will be forwarded based on the tree calculated using S1
--> as the root.
--> > 
--> > With this proposal, we get multipathing for multicast without
--> configuration.
--> > The same space used for RBridge nicknames can be used for
--> choosing the
--> 
--> > tree for multicast packets. The number of trees to be
--> calculated does
--> > not change---it is still per ingress RBridge.
--> > 
--> > Radia
--> > 
--> > _______________________________________________
--> > rbridge mailing list
--> > rbridge at postel.org
--> > http://mailman.postel.org/mailman/listinfo/rbridge
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