Add tcpdump for XST.

doc/erko_howto_tools.txt

@@ -487,6 +487,40 @@ then:
    > ifconfig   # to find ethernet port
    > sudo tcpdump -vvXXSnelfi enp5s0 port 5001 # for UDP only
    > sudo tcpdump -vvXXSnelfi enp5s0 port 5001 > tcpdump.txt  (> is new file, >> is append)
+   > 5001 = SST, 5003 = XST
+
+
+   Start tcpdump in one terminal and then the offload test in another terminal
+   > sudo tcpdump -vvXXSnelfi enp5s0 port 5003 > tcpdump.txt
+   > stat_stream_xst.py --host 10.99.0.250 --port 4842 --ip dop386 --mac dop386 --test-data --nof_crosslets 4 -vv
+
+   Stop tcpdump cntr-c
+
+   > more tcpdump.txt
+
+   Typical header values
+   . udp dest port = 5003 = 0x138b
+   . udp lenght = 2344 = 0x928
+   . marker = 0x58
+   . version = 5
+   . observation id = 0x00000000
+   . station id = 0x0000
+   . block period = 5120 = 0x1400, followed by 8 octetB BSN
+
+   Grep all packets for one BSN --> 576 / 144 = 4 nof_crosslets
+
+   > cat tcpdump.txt | grep '1400 0001 27a1 aa03 3599' | wc
+      576    5760   39168
+
+   Grep all received packets --> 8064 / 576 = 14 intervals
+
+   > cat tcpdump.txt | grep '0x0040:  1400' | wc
+      8064   80640  548352
+
+   Try again for nof_crosslets = 7:
+
+   cat tcpdump.txt | grep '0x0040:  1400' | wc
+     11843  118430  805324
 
 5) opcua-client &  # to verify sdp_rw.py
    - opc.tcp://10.99.0.250:4842 RPi4 connect