Maximum Reconstructed Reception Unit

Description

The Maximum Reconstructed Reception Unit (MRRU) is a parameter defined in the 3GPP technical specification TS 44.065 for the Radio Link Control (RLC) layer in GPRS and EDGE networks. It operates within the context of the Packet Data Convergence Protocol (PDCP) sublayer, specifically for header and data compression. The MRRU does not define a physical unit but a logical limit. It specifies the maximum allowable size, in octets, of a reconstructed upper-layer Protocol Data Unit (PDU) after the receiving entity has performed decompression operations on both header and data parts of the transmitted blocks.

The MRRU's role is critical in the segmentation and reassembly process. When an upper-layer PDU (e.g., an IP packet) is presented to the RLC layer for transmission, it may be larger than the maximum size allowed for a single RLC data block over the radio interface. The RLC layer segments this large PDU into smaller RLC data blocks. If compression is active, these blocks contain compressed headers and possibly compressed payload. Upon reception, the peer RLC entity reassembles these blocks and passes them to the decompression entity. The decompression entity outputs the reconstructed PDU. The MRRU is the agreed-upon limit between the transmitting and receiving ends for the size of this final, reconstructed PDU. It ensures that the receiver has sufficient buffer resources to handle the largest possible PDU after decompression, which can be significantly larger than the compressed blocks received over the air.

Architecturally, the MRRU is negotiated during the establishment of a Packet Data Protocol (PDP) context or a radio bearer, as part of the layer-2 protocol parameters. It is communicated between the Mobile Station (MS) and the Serving GPRS Support Node (SGSN). The value is chosen based on the capabilities of the MS and the network's configuration, balancing the efficiency of sending large packets (minimizing segmentation overhead) against the memory and processing constraints of the device. The MRRU works in conjunction with other parameters like the Maximum Receive Unit (MRU) for the logical link control (LLC) layer and the maximum RLC block size.

In operation, if the decompression process results in a PDU exceeding the MRRU, it is considered an error condition, typically leading to the discard of the PDU. Therefore, the transmitting side must ensure that any PDU it submits for transmission, after accounting for potential expansion during compression (some compression algorithms have a small expansion factor), will not exceed the negotiated MRRU at the receiver. This parameter is a key element in managing radio resource efficiency and ensuring reliable data transfer, especially for services that use robust header compression (ROHC) or other compression techniques to save bandwidth but which require careful management of receiver buffer sizes.

Purpose & Motivation

The MRRU was introduced to address a specific technical challenge in GPRS/EDGE networks related to the interaction between compression and segmentation. Compression algorithms, particularly header compression, are employed to reduce the overhead of protocol headers (like TCP/IP) over bandwidth-constrained radio links. However, compression is not always deterministic in size reduction; in some cases, or during certain states of the compression algorithm, the decompressed data might be larger than the compressed data received. Without a defined maximum for the reconstructed entity, a receiver could be forced to allocate excessively large or unbounded buffers, leading to memory exhaustion and device instability.

Prior to its formal definition, handling large packets after reassembly and decompression was less controlled. The MRRU provides a clear, negotiated contract between the network and the mobile device. It solves the problem of resource management at the receiver by setting a known upper bound. This allows device manufacturers to design their RLC and decompression buffers with a known worst-case size, improving reliability and performance predictability.

The creation of the MRRU was motivated by the need to support efficient Internet access over GPRS, where IP packets of variable and potentially large size are common. It enables the effective use of compression to save radio resources while protecting the receiver from buffer overflow attacks or simply from malformed packets that decompress to an enormous size. It is a fundamental part of the QoS and radio bearer management, ensuring that data flows with different characteristics (e.g., web browsing vs. email) can be handled appropriately by the device's limited resources.

Key Features

• Defines the maximum allowed size of a reassembled PDU after full decompression at the receiver.
• Negotiated between the mobile station and the network during PDP context activation.
• Prevents receiver buffer overflow by providing a known upper bound for memory allocation.
• Works in conjunction with segmentation/reassembly functions at the RLC layer.
• Essential for the safe operation of header and data compression algorithms.
• Ensures reliable data transfer by defining a clear error condition for oversized reconstructed PDUs.

Evolution Across Releases

Defining Specifications