Question: What is the primary purpose of the MAC-I (Message Authentication Code for Integrity) field in the E-UTRA PDCP protocol, and how is it handled for non-integrity protected control plane data?
Option 1: The MAC-I field is used for encryption key exchange, and for non-integrity protected data, it is omitted to reduce overhead.
Option 2: The MAC-I field carries a sequence number to ensure orderly delivery of packets; for non-integrity protected control plane data, it contains a random value.
Option 3: The MAC-I field is utilized for header compression in the PDCP layer, and for non-integrity protected data, it is filled with the last packet's checksum.
Option 4: The MAC-I field carries a message authentication code for integrity protection; even for control plane data not integrity protected, the MAC-I field is present but padded with zeros.
Explanation: This question assesses understanding of the PDCP protocol's security features in E-UTRA. The correct answer, Option 4, highlights that the MAC-I field serves to ensure data integrity by carrying a message authentication code. Even when control plane data is not subjected to integrity protection, the field remains present to maintain protocol structure consistency but is padded with zeros, indicating no actual integrity check is performed. This demonstrates nuanced understanding of telecom security protocols and their practical application in maintaining network integrity.