I concur with other comments in the thread - your hex dump is consistent with the output (and, FWIW, your unpack looks consistent with the RFC definition).

Logic suggests therefore that the problem is arising before the data arrives in your function. I have two suggestions there:

• Post the code that captures the packet, in case there's a problem or invalid assumption in it
• You say in a response that "The routers belong in private test networks, hence the use of private address space" - that's true of 10.x.x.x, but 172.50.x.x is not a private address as defined in rfc1918, so if (although it seems unlikely) some special processing for private addresses is happening, it won't affect addresses in that range.

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