Re: [manet] DSR and unidirectional links

[[e-mail] (Noah H. Miller)]

DSR Internet Draft 7 (http://www.ietf.org/internet-drafts/draft-ietf-manet-dsr-07.txt) specifies how it handles unidirectional links.  

To configure DSR to allow unidirectional links, you must use the DSR-level ACK REQ and ACK options for route maintenance, rather than MAC layer acknowledgements.  802.11 won't allow unidirectional links, but I suppose other radio links will.

given the network, where arrows show directionality:

A -> B <-> C
 |      |
<-D<-
 
For route discovery:
node A broadcasts an RREQ, and node B receives it.  Note that NO acknowledgement is necessary since the RREQ is a broadcast packet.  When the target C of the RREQ replies, it will piggyback its RREP on a new RREQ back to the initiator, node A, of the route discovery.  Two routes get set up:
A->B<->C  and
C<->B->D->A

For Source Routed data transmission:
Node A unicasts a packet to node B.  The network has been configured for the possibility of the link A->B being unidirectional, so it sends a DSR-level ACK either along a cached route back to A (such as B->D->A), or initiates route discovery back to A.  A thus receives next-hop confirmation through an indirect route.


Hope that clarifies,
Noah Miller
ISTS Dartmouth College


--- Sudarshan N Raghavan wrote:
DSR is supposed to work over unidirectional links. But how does DSR   
discover unidirectional links ?? Do we need to explicitly configure   
DSR to work over uni/bi directional links ??                          
                                                                      
Assume DSR is configured for uni-directional links:                   
1. A single RREQ broadcast can trigger multiple RREQ broadcasts from  
intermediate nodes that have a route to the destination in their cache
(assuming that the nodes are not able to find out that the source has 
already chosen a RREP)                                                
                                                                      
2. DSR expects each node along the source route to get an ack from the
next downstream node. If the links are unidirectional and the nodes do
not have a route to their upstream neighbor along the source route, it
can trigger N RREQ broadcats for a source route of length N           
                                                                      
3. The same is the case with route maintainance if the node reporting 
a link failure does not have a route to the source                    
                                                                      
This seems like an enormous number of broadcasts which has the        
potential to bring down the network if there is lot of network        
activity.                                                             
                                                                      
Have there been any studies/simulations to analyze chain reactions and
the performance of DSR when configured for unidirectional links ?     
                                                                      
Regards,                                                              
Sudarshan                                                             
                                                                      
****                                                                  
Sudarshan N Raghavan                                                  
Graduate Student, Computer Sc.                                        
North Carolina State University
--- end of quote ---

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