Question: In IPv4 multi-field classification for IPSec traffic in GPRS networks, what element is used in the packet filter instead of port numbers due to encryption?
Option 1: Destination IP Address
Option 2: Source Port Number (mirrored)
Option 3: Security Parameter Index (SPI)
Option 4: Protocol Identifier for TCP
Explanation: In the context of IPSec traffic, encryption renders traditional port numbers unusable for classification. The Security Parameter Index (SPI), a unique identifier for each security association, is utilized in packet filters to circumvent this issue, enabling effective multi-field classification. This approach ensures that encrypted packets can be accurately identified and managed within GPRS networks.