On Nov 8, 2009, at 6:10 AM, Linda Dunbar wrote: Dave and David, Hope you can help me to figure out the following issues with regard to “draft-ietf-bfd-multihop-8”: - Multiple BFD sessions between one pair of system was mentioned several times in the document. Is the purpose of having multiple BFD sessions between the same pair for traversing different alternative routes between the same pair? If yes, the document didn’t say how to enforce it. Even the different Destination/Source addresses being used can’t guarantee that those BFD sessions will traverse different paths. Is it out of the scope of this document?
The mention of multiple BFD sessions is primarily for discussion about how to demultiplex such sessions, rather than to attempt to deal with the liveness of alternate paths. As you point out, there is no mechanism for binding a BFD session to a path, only to its endpoints, and even that is somewhat fraught with subtlety (which is why the multipoint spec spends most of its time talking about how to demux BFD packets.)
As the whole point of BFD is to follow the same paths that data is following, sharing fate as much as possible, trying to run BFD over inactive alternate paths would be extremely difficult.
Multiple *active* paths may exist (such as an LSP and a routed path) and most of the verbiage is to provide a palette of mechanisms that can be used to sort the BFD sessions out. - What are the benefits of enforcing BFD sessions between a pair of nodes to traverse alternative paths between them? I guess by doing so, the two nodes get to know how many different paths are between them. Who will use this information?
See above. - Your BFD base draft (draft-ietf-bfd-base-09) does state that one of the goals of BFD is to traverse all possible paths between two nodes. Does it include the alternative paths between specific Ingress and Egress?
I don't believe that it ever says such a thing, and it isn't possible in any case. Can you point at the text that gave you this impression?
- Suggestion if you do think it is necessary to traverse all the alternative paths between two nodes as indicated in the BFD base document:
- Is it possible to create a special BFD session that all intermediate nodes will forward the BFD to all the paths in the ECMP group? Whenever it happens, the node will mark a bit to the Discriminator field. By receiving all the BFD frames with different Discriminator value, the egress node can figure out how many alternative paths are between them.
See above. Hope to have more discussion in Hiroshima. Linda Dunbar Advanced Technology Dept, Wireline Networks, Huawei Technologies, Inc.
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