Linux Shell Scripting Cookbook - Third Edition

Sequence number

Each packet that ping sends is assigned a number, sequentially from 1 until ping stops. If a network is near saturation, packets may be returned out of order because of collisions and retries, or may be completely dropped:

$> ping example.com 
64 bytes from example.com (1.2.3.4): icmp_seq=1 ttl=37 time=127.2 ms 
64 bytes from example.com (1.2.3.4): icmp_seq=3 ttl=37 time=150.2 ms 
64 bytes from example.com (1.2.3.4): icmp_seq=2 ttl=30 time=1500.3 ms 

In this example, the second packet was dropped and then retried after a timeout, causing it to be returned out of order and with a longer Round Trip Time.