A tricky problem concerned with fragmentation and Logical link Layer service.

[lostlander]

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

[swengi...@gmail.com]

Layer 7 and layer 2 are completely independent of each other so layer
7 fragments its PDU into 2 and invokes the layer 6 send primitive 2
times with what layer 6 sees as 2 distinct SDUs. In other words the
layer n PDU becomes the layer n-1 PDU. When your hypothetical layer 7
receives the first SDU, after it makes its way down the stack, it
sends it to the peer layer 7 who sends the appropriate ACK for that
PDU and sends it up the stack to eventually reach layer 7. Notice that
the PDU has already been acknowledged at layer 2 by the time the layer
above gets it so now layer 2 is free to send the next PDU which is the
second layer 7 fragment. Notice that layer 2 knows nothing about thes
PDUs being related to each other in any way and in fact the layer 2
send primitive must be invoked 2 times for this to operate as you have
described.

> 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.
>

This is the same as above. Layer 7 and layer 2 are completely
independent of each other.

> 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

[lostlander]

On Aug 6, 1:48 am, "swengineer...@gmail.com" <swengineer...@gmail.com>
wrote:

> > Regards. Tom- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -

Thanks really for replying!!

I'm sorry, maybe I didn't clearly describe the scenario.
1, There is no layer6/5/4/3, only layer7 and layer2;
2, The layer2 uses Type3 service of 802.2, which means if Layer7 want
to exchange data, receiver's ack should be piggybacked with the
response from peer layer7, layer2 won't send an independent ack!!

802.2 standard:
http://standards.ieee.org/getieee802/download/802.2-1998.pdf