DSCP Table Reference
January 8, 2026 ยท View on GitHub
Differentiated Services Code Point (DSCP) values for Quality of Service (QoS) marking.
Quick Reference
To display the DSCP table from the command line:
twampy dscptable
DSCP Values
DSCP is a 6-bit field in the IP header used for packet classification and QoS.
Class Selector (CS) PHBs
Class Selector PHBs provide backward compatibility with IP Precedence:
| DSCP | Binary | Decimal | Name | Description | Use Case |
|---|---|---|---|---|---|
| CS0 | 000000 | 0 | Default | Best Effort | Regular traffic |
| CS1 | 001000 | 8 | Scavenger | Lower than best effort | Bulk/background |
| CS2 | 010000 | 16 | OAM | Operations, Administration, Management | Network management |
| CS3 | 011000 | 24 | Broadcast Video | Broadcast/streaming | Video streaming |
| CS4 | 100000 | 32 | Real-Time Interactive | Real-time apps | Video conferencing |
| CS5 | 101000 | 40 | Signaling | Signaling traffic | VoIP signaling |
| CS6 | 110000 | 48 | Network Control | Network control traffic | Routing protocols |
| CS7 | 111000 | 56 | Reserved | Reserved for future use | Reserved |
Assured Forwarding (AF) PHBs
AF provides reliable delivery with different drop precedences:
| DSCP | Binary | Decimal | Name | Class | Drop Prob | Use Case |
|---|---|---|---|---|---|---|
| AF11 | 001010 | 10 | AF11 | 1 | Low | High-priority data |
| AF12 | 001100 | 12 | AF12 | 1 | Medium | High-priority data |
| AF13 | 001110 | 14 | AF13 | 1 | High | High-priority data |
| AF21 | 010010 | 18 | AF21 | 2 | Low | Medium-priority data |
| AF22 | 010100 | 20 | AF22 | 2 | Medium | Medium-priority data |
| AF23 | 010110 | 22 | AF23 | 2 | High | Medium-priority data |
| AF31 | 011010 | 26 | AF31 | 3 | Low | Multimedia streaming |
| AF32 | 011100 | 28 | AF32 | 3 | Medium | Multimedia streaming |
| AF33 | 011110 | 30 | AF33 | 3 | High | Multimedia streaming |
| AF41 | 100010 | 34 | AF41 | 4 | Low | Multimedia conferencing |
| AF42 | 100100 | 36 | AF42 | 4 | Medium | Multimedia conferencing |
| AF43 | 100110 | 38 | AF43 | 4 | High | Multimedia conferencing |
Expedited Forwarding (EF) PHB
EF provides low-loss, low-latency, low-jitter service:
| DSCP | Binary | Decimal | Name | Description | Use Case |
|---|---|---|---|---|---|
| EF | 101110 | 46 | Expedited Forwarding | Premium service | VoIP, real-time |
Voice Admit (VA) PHB
| DSCP | Binary | Decimal | Name | Description | Use Case |
|---|---|---|---|---|---|
| VA | 101100 | 44 | Voice Admit | Call admission control | VoIP CAC |
Common DSCP Mappings
By Traffic Type
| Traffic Type | Recommended DSCP | Decimal | Description |
|---|---|---|---|
| Voice (VoIP) | EF | 46 | Low latency, guaranteed |
| Video Conferencing | AF41 | 34 | Interactive video |
| Streaming Video | AF31 | 26 | One-way video |
| Signaling (SIP/H.323) | CS5 | 40 | Call control |
| Network Management | CS2 | 16 | SNMP, NetFlow |
| Routing Protocols | CS6 | 48 | OSPF, BGP |
| Transactional Data | AF21 | 18 | Business-critical |
| Bulk Data | CS1 | 8 | FTP, backup |
| Best Effort | CS0/DF | 0 | Default traffic |
By Priority Level
| Priority | DSCP | Decimal | Queue | Description |
|---|---|---|---|---|
| Highest | EF | 46 | P1 | Real-time voice |
| High | AF41 | 34 | P2 | Video, interactive |
| Medium | AF31 | 26 | P3 | Streaming media |
| Normal | AF21 | 18 | P4 | Business apps |
| Low | CS1 | 8 | P5 | Background/bulk |
| Default | CS0 | 0 | P6 | Best effort |
Usage with twampy
Set DSCP for Test Traffic
# Voice traffic (EF)
twampy sender 192.168.1.100 --dscp ef
# Video conferencing (AF41)
twampy sender 192.168.1.100 --dscp af41
# Best effort (default)
twampy sender 192.168.1.100 --dscp be
Test Different QoS Classes
# Priority 1: Voice
twampy sender 192.168.1.100 --dscp ef --count 1000 --interval 20
# Priority 2: Video
twampy sender 192.168.1.100 --dscp af41 --count 1000 --interval 33
# Priority 3: Data
twampy sender 192.168.1.100 --dscp af21 --count 1000 --interval 100
DSCP to TOS Conversion
DSCP occupies the upper 6 bits of the TOS byte:
TOS = DSCP << 2
| DSCP | DSCP (Dec) | TOS (Dec) | TOS (Hex) |
|---|---|---|---|
| EF | 46 | 184 | 0xB8 |
| AF41 | 34 | 136 | 0x88 |
| AF31 | 26 | 104 | 0x68 |
| AF21 | 18 | 72 | 0x48 |
| CS5 | 40 | 160 | 0xA0 |
| CS0 | 0 | 0 | 0x00 |
Using TOS Value Directly
# Equivalent to --dscp 46
twampy sender 192.168.1.100 --tos 184
# Equivalent to --dscp 34
twampy sender 192.168.1.100 --tos 136
PHB Descriptions
Assured Forwarding (AF)
AF provides assurance of delivery under prescribed conditions. Format: AFxy
- x (1-4): AF class (traffic category)
- y (1-3): Drop precedence (1=low, 2=medium, 3=high)
Example: AF41 = Class 4, Low drop precedence
Expedited Forwarding (EF)
EF provides:
- Low delay
- Low jitter
- Low loss
- Assured bandwidth
Best for: VoIP, real-time gaming
Class Selector (CS)
CS provides 8 classes based on IP Precedence:
- CS0 (0): Default/Best Effort
- CS1 (8): Scavenger/Bulk
- CS2-CS7: Increasing priority
Platform Considerations
Windows
Requires registry modification to enable user-space TOS/DSCP setting:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\TcpIp\Parameters
DisableUserTOSSetting = 0x00000000
See Installation Guide for details.
Linux
Full DSCP/TOS support without special configuration.
macOS / FreeBSD
DSCP/TOS marking supported, but may require elevated privileges.
Testing QoS Configuration
Verify DSCP Preservation
Test that network devices preserve DSCP markings:
# Send with DSCP EF
twampy sender 192.168.1.100 --dscp ef --count 100
# Capture on receiver and verify DSCP value
Compare Treatment Across Classes
Run tests with different DSCP values and compare results:
# EF (should get priority treatment)
twampy sender 192.168.1.100 --dscp ef > ef-results.txt
# Best Effort (normal treatment)
twampy sender 192.168.1.100 --dscp be > be-results.txt
# Compare latency/loss between classes