DSCP

Differentiated Services Code Point

QoS →
Introduced in R99

DSCP is a 6-bit field in an IP packet header used within the Differentiated Services architecture to classify traffic, enabling 3GPP systems to map QoS flows to IP service classes for consistent transport network treatment.

Category
QoS
Introduced
R99
Where
Core Network › 5G Core
Specifications
37 specs
DSCP Description Purpose Related Classification Detected Changes Specifications

Description

The Differentiated Services Code Point (DSCP) is a standardized field within the Type of Service (ToS) byte of an IPv4 header or the Traffic Class byte of an IPv6 header. It utilizes six bits, allowing for up to 64 distinct code points (values 0-63). Within the 3GPP architecture, the DSCP is a crucial tool for implementing end-to-end Quality of Service (QoS) across the transport segments that interconnect network functions, such as between the eNodeB and the S-GW/UPF, or within the core network itself. It operates as part of the broader IETF Differentiated Services (DiffServ) model, which provides scalable service differentiation in IP networks without requiring per-flow state in every router.

The mechanism works by marking packets as they enter a DiffServ domain (e.g., the mobile transport network). A 3GPP network function, such as a Packet Data Network Gateway (P-GW) or User Plane Function (UPF), classifies uplink and downlink user traffic based on QoS rules associated with an EPS bearer or 5G QoS Flow. This classification determines the appropriate DSCP value to write into the IP header of the user plane packets. Routers and switches in the transport network are then configured with Per-Hop Behaviors (PHBs) that correspond to specific DSCP values. These PHBs define how packets are queued, scheduled, and potentially dropped, providing behaviors like Expedited Forwarding (EF) for low-latency traffic, Assured Forwarding (AF) for guaranteed throughput, or Default (BE) for best-effort.

In 3GPP systems, the mapping between the QoS parameters of a radio bearer (QCI in 4G, 5QI in 5G) and the DSCP value is a critical configuration item. This ensures that the QoS intended by the radio and core network is appropriately signaled to, and honored by, the underlying IP transport infrastructure. For example, a QoS Flow for Voice over NR (5G VoIP) with a 5QI of 1 (conversational voice) would be marked with a DSCP value corresponding to the EF PHB (often DSCP 46) to prioritize it through the transport network. This creates a cohesive QoS hierarchy from the application layer down through the radio, core, and transport layers.

Purpose & Motivation

DSCP was adopted within 3GPP to solve the problem of maintaining consistent QoS treatment for user traffic as it traverses the IP-based transport networks that interconnect RAN and core nodes. Early mobile networks had more monolithic, circuit-switched transport where QoS was implicit. With the move to all-IP architecture in 3GPP Release 5 and beyond, a standardized, IP-layer marking mechanism was necessary to signal packet priority to routers and switches.

Its use was motivated by the need for scalability. The DiffServ model, using DSCP, does not require transport routers to maintain state for millions of individual user bearers, unlike the earlier Integrated Services (IntServ) model. This makes it perfectly suited for large-scale mobile networks. DSCP allows the mobile network's intricate QoS framework (with QCIs/5QIs, ARP, etc.) to be seamlessly translated into a simple, widely supported IP networking standard, ensuring that latency-sensitive or high-priority traffic receives appropriate treatment on every hop of its journey through the operator's backbone network.

Classification

Specific typesPHB
Related approachesQCI5QI

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (44 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Rel-15 11 changes

In Release 15, specific corrections and clarifications were introduced for the DSCP function, including a correction for the DSCP in the outer IP header carrying uplink user data packets. Furthermore, the release specified the SMF's role in the derivation of DSCP on the N4 interface and provided clarifications on the UE's reflective QoS procedures for DSCP marking in the uplink.

  • Resolving Editor's Note on QCI to DSCP mapping TS 24.302CR0641
  • Support for volume based charging of IMS services TS 29.214CR1614
  • FEC and ROHC for mission critical services over MBMS TS 29.468CR0047
  • Correction for DSCP in outer IP header carrying uplink user data packet TS 24.502CR0031
  • Support priority for MCVideo services TS 29.213CR0726
  • Support priority for MCVideo services TS 29.214CR1618

+ 5 more changes

Rel-16 8 changes

In Release 16, a key enhancement for the DSCP function was the introduction of a dedicated child Security Association (SA) with a specific DSCP value for individual QoS flows. This builds upon the existing framework where DSCP marking is used for QoS treatment and remapping in fixed broadband access networks, such as by a BRAS/BNG based on the outer IP header. The change specifically ties the DSCP marking for a flow to its dedicated child SA within the secure tunneling architecture.

  • N3IWF FQDN configured in a UE to support access to PLMN/SNPN services via SNPN/PLMN TS 24.502CR0079
  • A dedicated child SA and a DSCP value for QoS flows TS 24.502CR0086
  • Support for Restricted Local Operator Services TS 29.213CR0737
  • Support for Restricted Local Operator Services TS 29.214CR1624
  • Annex B, IMS Restricted Local Operator Services TS 29.513CR0136
  • FQDN for N3IWF selection to access PLMN services via an SNPN TS 24.502CR0102

+ 2 more changes

Rel-17 8 changes

In Release 17, the enhancement for the DSCP function was the introduction of an informative guideline for mapping between QoS parameters and DSCP marking. This provides a standardized reference for network elements, such as the BRAS/BNG in a Fixed Broadband Access network, to perform QoS treatment and remapping based on the DSCP value of the packet's outer IP header. The update formalizes the handling of per-flow DSCP marking for both EPC-routed and NS-WLAN offloaded traffic across the interworking architecture.

  • IMEI over Rx reference point TS 29.214CR1653
  • 5QI value for services carried over satellite access/backhaul TS 29.513CR0284
  • Informative guideline for mapping between QoS parameters and DSCP marking TS 29.513CR0325
  • N3IWF selection for emergency services TS 24.502CR0194
  • Incorrect references to N19mb and Nmb9 reference points TS 29.244CR0616
  • Updates on DCAMP related BSF services TS 29.513CR0296

+ 2 more changes

Rel-18 12 changes

In Release 18, the DSCP function was enhanced with clarifications to UE handling of the DSCP header field and a specification that the DSCP field of the outer IP datagram is set to the DSCP of the IP payload. These changes provide clearer guidance on DSCP marking, particularly for ensuring consistent QoS treatment across network segments by aligning the outer header marking with the payload's original DSCP value.

  • SNPN services for N5CW devices TS 24.302CR0750
  • N3IWF selection for non-IMS services supporting extended home N3IWF identifier configuration and slice-specific N3IWF prefix configuration TS 24.502CR0248
  • N3IWF selection for IMS services supporting extended home N3IWF identifier configuration and slice-specific N3IWF prefix configuration TS 24.502CR0236
  • Differentiated QoS for devices behind 5G-RG TS 24.502CR0249
  • Support of high data rate low latency services, XR and interactive media services TS 29.244CR0696
  • N3IWF selection for onboarding services in SNPN in a visited country TS 24.502CR0250

+ 6 more changes

Rel-19 5 changes

In Release 19, the enhancements for the DSCP function specifically introduced support for differentiated QoS handling for multiplexed media flows. Furthermore, the release added the capability for leveraging PDU Set QoS information to perform DSCP marking over the N3 and N9 interfaces within the transport network.

  • Support of differentiated QoS handling for multiplexed media flows TS 29.244CR0893
  • Leveraging PDU Set QoS information for DSCP marking over N3/N9 in the transport network TS 29.244CR0912
  • Support of differentiated QoS handling for multiplexed media flows TS 29.244CR0930
  • Clarification of emergency services in FPLMN TS 24.502CR0309
  • Correction on the descripton of Supported-Services AVP and RIR command TS 29.336CR0192

Explore further

Broader topics and technologies where DSCP plays a role.

Topics
MEC (Edge Computing)5G NR (New Radio)Lawful InterceptServices & ApplicationsRadio Access NetworkCore Network
Technologies
LTE5G

Defining Specifications

3GPP specifications that define or reference DSCP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.

SpecificationTitleRelease
TS 23.139 vj00 3GPP-Fixed Broadband Interworking Stage 2 Rel-19
TS 23.207 vj00 End-to-End QoS Framework for GPRS Rel-19
TS 23.234 vd10 3GPP-WLAN Interworking Index Rel-13
TS 23.802 v1700 Enhanced End-to-End QoS Architecture Rel-7
TS 24.139 vj00 UE-EPC Procedures for Fixed Broadband Access Rel-19
TS 24.302 vj00 Access to EPC via non-3GPP networks; Stage 3 Rel-19
TS 24.502 vj20 5G Core Access via Non-3GPP Networks; Stage 3 Rel-19
TS 24.820 vb00 3GPP-Fixed Broadband Interworking Procedures Rel-11
TS 26.510 vj10 Media Delivery APIs for 5GMS and RTC Systems Rel-19
TS 26.804 vj10 5G Media Streaming Extensions Study Rel-19
TS 29.128 vj10 MME/SGSN-SCEF Diameter Interfaces for PDN Interworking Rel-19
TS 29.139 vj00 H(e)NB - SeGW Interface Specification Rel-19
TS 29.163 vj00 Interworking between 3GPP IM CN and CS networks Rel-19
TS 29.201 vj00 RESTful Rx Interface for AF-PC Communication Rel-19
TS 29.213 vj20 PCC Signalling Flows and QoS Mapping Rel-19
TS 29.214 vj20 Policy and Charging Control over Rx Rel-19
TS 29.228 vj20 Cx and Dx Interface Signaling Flows Rel-19
TS 29.229 vj10 Diameter Protocol for Cx/Dx Interfaces Rel-19
TS 29.238 vj00 H.248 Profile for IBCF-TrGW Interface Rel-19
TS 29.244 vj40 PFCP Specification for Control/User Plane Separation Rel-19
TS 29.272 vj40 Diameter Interfaces for MME/SGSN Rel-19
TS 29.273 vj10 AAA Protocols for Non-3GPP Access in EPS & 5GS NSWO Rel-19
TS 29.328 vj20 Sh and Dh Interfaces: HSS-AS Interactions Rel-19
TS 29.329 vj10 Diameter Protocol for Sh Interface Rel-19
TS 29.334 vj00 IMS-ALG to IMS-AGW Interface Protocol Rel-19
TS 29.336 vj10 HSS Diameter Interfaces for PDN Interworking Rel-19
TS 29.337 vj00 Diameter T4 Interface for MTC Device Triggering Rel-19
TS 29.338 vj30 Diameter protocols for SMS in MME/5GS Rel-19
TS 29.344 vj00 Diameter-based PC4a interface specification Rel-19
TS 29.345 vj00 Diameter-based PC6/PC7 interfaces for ProSe Rel-19
TS 29.468 vj00 MB2 Reference Point Protocol Definition Rel-19
TS 29.513 vj40 5G PCC Signalling Flows & QoS Mapping Rel-19
TS 29.804 v801 CT3 Aspects of System Architecture Evolution Rel-8
TS 29.839 vb00 H(e)NB - SeGW Interface Specification Rel-11
TS 33.102 vj10 3G Security Architecture Specification Rel-19
TS 33.401 vj10 EPS Security Architecture Rel-19
TS 38.340 vj00 Backhaul Adaptation Protocol (BAP) Specification Rel-19