I was diffing two PCAPs the other day trying to work out what’s up, and noticed the Instance ID on a GTPv2 IE was different between the working and failing examples.

So what does it denote, well from TS 129.274:

If more than one grouped information elements of the same type, but for a different purpose are sent with a message,
these IEs shall have different Instance values.

So if we’ve got two IEs of the same IE type (As we often do; F-TEIDs with IE Type 87 may have multiple instances in the same message each with different F-TEID interface types), then we differentiate between them by Instance ID.

The only exception to this rule is where we’ve got the same data, so if you’ve got one IE with the exact same values and purpose that exists twice inside the message.