hernet IPSec Payload IPSec Cipher Payload Payload Payload Payload PDCP Cipher PDCP Protocol Payload Payload RLC Protocol PDCP PDU PDCP PDU MAC Protocol RLC PDU RLC PDU Physical Layer Transport Block FIGURE 6-4 L2 Stack memory domains. Chapter 6 User-plane application components 219 Requirement for fast/optimized implementation of physical to virtual and vice versa address translation as required for interfacing to accelerator hardware (hardware security accelerator if used, Ethernet controller) The postsecurity PDCP memory domain. This memory domain is characterized by: Buffer size defined by maximum transport block size (and/or other L2 defined maximum buffer size) Multiuse buffers as required for optimized implementations of HARQ (Hybrid Automatic Repeat ReQuest) and ARQ (Automatic Repeat ReQuest) Requirement for fast/optimized implementation of physical to virtual and vice versa address translation as required for interfacing to accelerator hardware (hardware security engine, DMA engine) Data copy between the two memory domains is handled by the security component, either implemented in software (effectively becoming a memory copy operation) or by means of a lookaside hardware accelerator which has an integrated Direct Memory Access (DMA) controller. Robust header compression The "Data" field in the PDCP PDU example shown below (Fig. 6-5) contains the (Ethernet or) IP packet that is delivered over the transport interface to the core network. These packets are option- ally compressed with Robust Header Compression (RoHC) which is an Internet Engineering Task Force (IETF) standard (technically, multiple RFCs, starting with RFC3095) that defines a set of header compression schemes for Internet Protocol (IP), Real Time Protocol (RTP), User Datagram Protocol (UDP), IP Security (IPSec) and other headers, allowing for more efficient transmission of small IP packets (e.g., VoIP) over bandwidth sensitive links (e.g., wireless). Although the RoHC specification defines multiple compression schemes, VoIP is the most tar- 