OsmoSGSN Accepts two MO L3 Messages with N(U) set to zero
- the MS sends its first GMM message "ATTACH REQUEST" in LLC with N(U) set to 0 (expected)
- the SGSN then inquires abou the IMEI using GMM "IDENTITY REQUEST"
- the MS subsequently sends its GMM "IDENTITY RESPONSE" in LLC with N(U) set to 0 again (broken behavior!)
Then OsmoSGSN still accepts that IDENTITY RESPONSE. This is odd. Later on, OsmoSGSN detects duplicate N(U) sequence numbers. But at the initial stage (or maybe when it's 0?) it doesn't detect the duplicate sequence number [which should be dropped].