RE: [RE] [EFM-P2MP] Definition of Timestamp in MPCP messages
Hello, Chan and Antti,
I agree with most of the Chan's opinion except the need for an upper
bound of the transmit + receive latency (25usec or 30usec). Chan pointed
out a case where longer frames are exchanged, but we are now focusing on
a discussion of TIMESTAMPs in MPCP messages, which are a fixed size of
64 Byte. So I think the issue can be simpler.
Let me summarize this issue briefly. In the current draft, RTT includes
certain times other than just a propagation time on fiber and processing
delay of a higher layer. If we suppose that the reference point of
TIMESTAMP is the time of transmitting the first bit of a MPCP message
from the MAC layer, RTT would contain below times.
A: Processing time of the lower layer at a sender
B: propagation time on fiber
C: time to receive a whole frame at a receiver
D: processing time of the lower layer at a receiver
E: time to examine the frame validity at a receiver
One big issue is whether or not RTT should indicate exactly the
propagation time on fiber (B). In my opinion, I don't see any problem if
RTT includes all the times shown above. Since the size of MPCPDU is
fixed to 64Byte, the times required A, C, D and E are almost constant.
Thus, as long as these delays are within the delay variation, no more
than 32 bit times, defined in the current draft, considering these
delays into RTT does not cause any problem. All we have to do is
defining a reference point. That's it. We don't have to introduce any
other parameters. I hope this would make our life easier.
Regards,
Hidekazu