Question: In GTP-U protocol, when is the notification of the sequence number mandatory for delivery to the user plane entity?
Option 1: Only at the GGSN
Option 2: At the SGSN and RNC, but optional at the GGSN
Option 3: For all GTP-U Tunnel (TEID) bases without exception
Option 4: Never mandatory, always optional across all network elements
Explanation: According to the handling of sequence numbers in GTP-U protocol, notification is mandatory at the Serving GPRS Support Node (SGSN) and Radio Network Controller (RNC), but not necessary (thus optional or not required) at the Gateway GPRS Support Node (GGSN), reflecting different roles and requirements for in-sequence T-PDU delivery.