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

[Radia Perlman]

No. I just forgot that bit.

So, we need
a) ingress RBridge
b) egress RBridge
(where both above fields are the same size)
c) TTL (6 bits ought to be enough)
d) multicast flag (just a single bit)

Anything else?

Radia



Dinesh G Dutt wrote:

> I hit "send" too soon. We talked about an additional bit in the header 
> to identify a multicast frame. Did you see no need for that ?
>
> Dinesh
> 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
>>
>>   
>
>