Normally, ICMP payload will be very small in size. 64 bytes or less. In SRX it is 56 bytes by default. Remaining calcuation is ok. You can find out working packet size (without fragmentation) using "do-not-fragment" option in SRX ping command. In windows, use -f flag. In Linux, it is -M flag. Example are given below. Decrease the packet size gradually to find out a working packet size like below:
SRX:
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root> ping 192.168.1.10 size 1500 do-not-fragment
PING 192.168.1.10 (192.168.1.10): 1500 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
root> ping 192.168.1.10 size 1472 do-not-fragment
PING 192.168.1.10 (192.168.1.10): 1472 data bytes
1480 bytes from 192.168.1.10: icmp_seq=0 ttl=64 time=0.966 ms
1480 bytes from 192.168.1.10: icmp_seq=1 ttl=64 time=0.942 ms
Windows:
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C:\Users\Nellikka>ping 8.8.8.8 -f -l 1490
Pinging 8.8.8.8 with 1490 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Ping statistics for 8.8.8.8:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
C:\Users\Nellikka>ping 8.8.8.8 -f -l 1400
Pinging 8.8.8.8 with 1400 bytes of data:
Reply from 8.8.8.8: bytes=68 (sent 1400) time=21ms TTL=50
Reply from 8.8.8.8: bytes=68 (sent 1400) time=36ms TTL=50
Reply from 8.8.8.8: bytes=68 (sent 1400) time=11ms TTL=50
Reply from 8.8.8.8: bytes=68 (sent 1400) time=11ms TTL=50
Ping statistics for 8.8.8.8:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Linux:-
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Nellikka@linx01:~$ ping 8.8.8.8 -M do -s 1500 -c 2
PING 8.8.8.8 (8.8.8.8) 1500(1528) bytes of data.
From 192.168.1.254 icmp_seq=1 Frag needed and DF set (mtu = 1500)
From 192.168.1.254 icmp_seq=1 Frag needed and DF set (mtu = 1500)
--- 8.8.8.8 ping statistics ---
0 packets transmitted, 0 received, +2 errors
Nellikka@linx01:~$ ping 8.8.8.8 -M do -s 1472 -c 2
PING 8.8.8.8 (8.8.8.8) 1472(1500) bytes of data.
76 bytes from 8.8.8.8: icmp_req=1 ttl=55 (truncated)
76 bytes from 8.8.8.8: icmp_req=2 ttl=55 (truncated)
--- 8.8.8.8 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 1.922/1.929/1.937/0.044 ms