Type of Service

Description

Within 3GPP specifications, Type of Service (TOS) refers to the 8-bit field in the IPv4 packet header originally defined in IETF RFC 791. Its purpose is to provide hints to routers and network elements about the quality of service desired for that packet. The TOS field is subdivided into two main parts: the Precedence bits (3 bits) and the TOS bits (4 bits), with the final bit historically unused. The Precedence bits indicate the priority of the packet (ranging from routine to network control). The TOS bits allow the sender to request a particular type of service by setting flags for minimizing delay, maximizing throughput, maximizing reliability, or minimizing monetary cost.

In a 3GPP packet-switched network architecture, such as the GPRS core network or the Evolved Packet Core (EPC), user plane traffic is carried over IP. The TOS field (and its evolution, the DiffServ Code Point - DSCP) plays a critical role in QoS enforcement. When a Packet Data Protocol (PDP) context or an EPS bearer is established, QoS parameters like Traffic Class, Allocation/Retention Priority (ARP), and QoS Class Identifier (QCI) are negotiated. For traffic flowing between the User Equipment (UE) and the Packet Data Network (PDN), the Gateway GPRS Support Node (GGSN) or the Packet Data Network Gateway (P-GW) may map these 3GPP-specific QoS parameters to the IP-layer TOS/DSCP field in the outer IP headers (e.g., in GTP tunnels or on the Gi/SGi interface).

This mapping allows the IP transport network between core nodes (e.g., between SGSN and GGSN, or within the evolved packet core) to provide differentiated forwarding treatment without needing to understand 3GPP-specific bearers. Routers in the transport network can examine the TOS/DSCP value and place packets into appropriate forwarding queues, apply priority scheduling, or manage congestion accordingly. While the original TOS field semantics are largely historical, the concept evolved into the Differentiated Services (DiffServ) architecture, where the same header bits are reinterpreted as the 6-bit DSCP. 3GPP systems leverage this IP-layer marking as a tool to realize the end-to-end QoS promised by the bearer service, ensuring that real-time traffic like VoIP receives lower latency treatment than background download traffic.

Purpose & Motivation

The TOS field was created to address the early Internet's lack of built-in quality of service differentiation, where all IP packets were treated as 'best-effort.' This was insufficient for emerging applications with specific needs, such as interactive voice (sensitive to delay) or bulk data transfer (sensitive to throughput). The TOS field provided a simple, packet-level signaling mechanism to express these needs, allowing routers to implement basic priority queuing and routing decisions.

In the context of 3GPP's adoption of IP for its packet core, the TOS field (and later DSCP) provided a vital bridge between the cellular-specific QoS model (built on bearers with guaranteed bit rates, ARP, etc.) and the QoS capabilities of the underlying IP transport infrastructure. It solved the problem of how to convey the complex QoS profile of a 3GPP bearer across routers that are agnostic to 3GPP protocols. By mapping QCI or ARP values to specific TOS/DSCP markings at the gateway, the cellular network could leverage standard IP QoS mechanisms in the backhaul and core transport networks. This was crucial for cost-effective scaling, as it avoided the need for 3GPP-aware routers everywhere in the transport path. The evolution from the original TOS semantics to DiffServ provided a more scalable and flexible framework that 3GPP networks from GPRS (Rel-4) onwards have depended on for scalable QoS delivery.

Key Features

• 8-bit field in IPv4 header for QoS signaling
• Contains Precedence (priority) and TOS (service type) sub-fields
• Basis for the modern DiffServ Code Point (DSCP)
• Used by GGSN/P-GW to mark IP packets based on 3GPP bearer QoS
• Enables QoS differentiation in IP transport networks between core nodes
• Supports mapping to delay, throughput, reliability, and cost optimizations

Evolution Across Releases

Defining Specifications