A tricky problem concerned with fragmentation and Logical link Layer service.
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Posted by lander on August 5, 2007, 2:51 am
Please log in for more thread options The following statements are based on DSRC protocol(see the end), however, I'm sure it also fits into other protocol as long as it has following characteristic: The upper layer supports fragmentation but the layer right under it is a stop-wait protocol(one sequence number). ========================================== Background knowledge(you don't need to understand, just for convenience and clarification): RSU: road side unit, a equipment located on the road side. OBU: on-board unit, a equipment located on the cars. DSRC: a protocol used to communicate between OBU and RSU. RSU and OBU both have Layer2(link layer) and Layer7(application layer) entity. Layer7: Application layer which supports fragmentation Layer2: Logical link layer which is a stop-wait protocol with one sequence number. (802.2LLC type3) Graph Description: RSU OBU ********* ********** *Layer7* <------------> *Layer7
********* **********
*Layer2* <------------> *Layer2
********* **********
*Physical(antenna) Physical(antena) ============================================================= Problem Description: 1, Layer2 is rightly under layer7, layer2 uses a stop-wait protocol, which has one sequence number. Layer7 supports fragmentation. Now there are two peers: RSU and OBU. RSU's Layer7 fragment a PDU into 2 fragments. So layer7 call layer2 primitive to send fragment 1 with sequence number 0. OBU receives fragment 1, and OBU's layer7 finds it's a fragmented PDU, so it waits for fragment2. However, RSU's layer7 won't send fragment2 since RSU's layer2 is waiting for response from OBU(layer2 is a stop-wait protocol). This scenario will lead to deadlock!!!!!! 2, Also, if RSU's layer7 sends a non-fragmented PDU to OBU, and OBU needs to send back 2 fragments, this will also leads to incorrect sequence number maintenance since Layer2 is a stop-wait protocol with one sequence number maintained. Am I right? How to deal with such problem? ================================================================ Related resources: 1, 802.2 Standard. (the layer2 uses type3 service of LLC) 2, STF282(DSRC Conformance Testing in support of Interoperability) http://portal.etsi.org/stfs/STF_HomePages/STF282/STF282.asp 3, DSRC Home: http://www.leearmstrong.com/DSRC/DSRCHomeset.htm Thanks very much for your attention!!!! Regards. Tom | |||||||||||||
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