This means that you have to take care that the OS will always add a 28 bytes to the size you specify to it (20 for the IP header and 8 for the ICMP header). Generally operating systems (this even applies to Windows) by default excludes any headers when you ping while specifying the size (the size is considered to be the actual application data, payload or ICMP data bytes), this means that in case you ordered the OS to ping using a size of 100 bytes, the OS will actually create a packet of 128 bytes then encapsulate it with the 14 bytes Ethernet header and then throw the packet over the wire. Digesting the details of the fundamentals always makes a difference.Īfter all JUNOS is built over FreeBSD (to be more technically accurate, JUNOS Control plane is based on the FreeBSD kernel), and thus it seems that (it is still our personal speculations) it inherits some of the FreeBSD behaviors and seems like the ping operation is one of them. In the past we used to conduct the tests with Cisco's IOS extended ping, but now we have IOS XR and JUNOS in addition, and we were hit by the fact of the difference in behavior. Actually we were very busy the last few months evaluating, designing and preparing for our company's backbone migration, a little C Vs J with all its fun )Īnyway, while going through the low level design we faced a little confusion when evaluating the MTU issues with MPLS running over. I am very glade to return back after pausing posting for a while.
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