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What should be at Inner header and Outer header in tunnel packet ?

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And then we have tunnel IP address(for both router), tunnel source IP address, tunnel destination IP address. enter image description here

In my understanding, for packet send from R2 to R1

  • Outer IP header
    • Source : Tunnel Source (202.123.170.1)
    • Destination : Tunnel Destination (210.115.30.10)
  • Inner IP header
    • Source : Actual source from network behind R2
    • Destination : Acutal destination that is from network behind R1

But I am not sure about that, and what would be the use case for Tunnel IP address (192.168.1.1 and 192.168.1.2) ??

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  • The Outer IP header should be Public IP from ISP in real world situation right ?
    – jacky chong
    Commented Jun 12 at 9:26

1 Answer 1

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That is basically correct. As a rule of thumb, the outer packet uses public IP addresses, the inner private ones.

what would be the use case for Tunnel IP address (192.168.1.1 and 192.168.1.2) ?

The tunnel IP addresses serve as numbered endpoints for the (inner) tunnel. Depending on the protocols and devices used, those addresses are not always necessary. Since a tunnel is a point-to-point connection, addressing is basically redundant.

Note that basic GRE does not provide encryption, so your tunnel over the open Internet might be listened in.

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