Hello,
I need to measure round trip times among some hosts
in the same LAN (10BaseT), a few meters away from each
other. This is part of a packet video application
running on top of IP/UDP/pseudo_RTP (rfc1889). So, I
just:
a) Tried with the control packets from the video
session:
-A--B--C------------------------J--K--L> S
\ /
\ /
\ /
-----------D--E--F----G--H--I---------> R
Host S builds a UDP packet with a 32 bits NTP
timestamp
(orig_time) at time A and makes a sendto() at time B,
just aftert building the whole control packet. This is
finally dropped onto the net at time C.
Host R receives the packet at time D, the
application is notified (via select()) at time E and
the packet is parsed at time F.
Host R build a UDP packet at time G, containing the 32
bit NTP timestap just received (orig_time), and of the
delay (G-F). This packet is sentto() host S at time H,
and is finally dropped onto the net at time I.
Host S receives the packet at time J, the application
is notified (via select()) at time K and the packet is
parsed at time L. Host S reads a 32 bit NTP timestamp
value into variable "now" and computes:
rtt = now - delay_at_host_R - orig_time;
rtt should be an approximation to the round trip time
but I get values in the range of [40ms to 210ms].
b) Then I tried my own ping: Same as method "a", but
no video data is transmited. Just the control packets
to measure time delays.
c) Finally, I used /bin/ping.
Methods "b" and "c" resulted in reasonable round trip
times (0.4ms to 3.2ms), but method "a" shows
unreasonably high values (40ms, 70ms, 90ms, 200ms,
55ms, 45ms, 96ms), even though they are all running at
the same time.
If I reduce frame rate to a minimum (e.g. 2 frames per
second) things start to look better. Methods "b" and
"c" keep the same values, but method "a" reports
better results: 1.5ms to 5ms with some sporadic high
peaks at 40ms~65ms.
I have double-checked time periods B-A, F-E, H-G, L-K
and they just fine: 0.1ms to 0.3ms. I know nothing
about time periods C-B, E-D, I-H, and K-J.
Could these _large_ delays be at the IP module?
Shouldn't this affect all of the three applications
(packet_video, myping, and /bin/ping) ?
Thank you,
AB.
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