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tcpicmp

How to calculate Frame size for PING request?

In a Standard ethernet TCP/IP, Segment size is 1480 bytes. And the packet size is 1480 + 20 bytes (IP) = 1500 bytes. In the frame, 1500 bytes is MTU size with frame header 26 bytes (Preamble + SOF + Dest. MAC + ORIG. MAC + CRC).

Now, on internet, I found MTU size of ICMP frame is 1500 - 20 bytes (IP header) - 8 bytes (ICMP header) = 1472 bytes. I failed to understand this calculation if the ethernet frame MTU is 1500 bytes, then why are we sending 1472 bytes for ICMP frames ?

Or to put simply, what 1472 bytes of ICMP MTU consists of ?

Solution

  • 1500 bytes is MTU size without Ethernet frame header -- it's the transmission size of Ethernet data, not the whole Ethernet frame. More specifically, it's the whole size of IP packet.

    Similarly, the MTU of ICMP is the transmission size of ICMP data, not the whole ICMP message. Thus, the number would be 1500 (max data size of Ethernet) - 20 (IP header) - 8 (ICMP header) = 1472