DSCP TOS CoS Precedence Conversion Chart

In networking, DSCP, TOS, CoS, and Precedence are terms related to Quality of Service (QoS) which is used to prioritize certain types of traffic to ensure the performance of critical applications over a network.

Don’t mix up TOS (Type of Service) and COS (Class of Service). TOS is found in the header of an IP packet (layer 3) and COS is found in the header of 802.1Q (layer 2) and is used for Quality of Service on layer 2 switching.

• DSCP (Differentiated Services Code Point): This is part of the IP header and uses 6 bits to classify traffic. DSCP values can be used to provide different levels of service to network traffic¹.
• TOS (Type of Service): Originally defined in the IP header to specify the priority and request a route for high throughput, low delay, and high reliability. It has evolved over time and is now largely replaced by DSCP¹.
• CoS (Class of Service): This is a 3-bit field found in an Ethernet frame’s VLAN tag (802.1Q). It’s used to prioritize traffic on LANs, especially for trunk links between switches¹.
• Precedence: This is part of the older TOS field in the IP header and uses the first 3 bits of the TOS byte. It was used to define the priority of the packet, with higher values indicating more important packets¹.

Here’s a bit more detail on how they work:

• IP Precedence: The first 3 bits of the TOS byte were used to define precedence. The higher the value, the more important the IP packet is. In case of congestion, routers would drop low-priority packets first¹.
• Type of Service Bits: The next bits (after the precedence bits) in the TOS byte were intended to specify the type of service, such as delay, throughput, and reliability. However, these bits were not widely used and have been superseded by DSCP¹.
• DSCP Values: DSCP replaced the TOS byte with a more granular and flexible system. It allows for 64 different traffic classes, which can be used to ensure that critical traffic gets the bandwidth and low-latency treatment it needs².

In summary, these terms are all about prioritizing traffic to ensure that the most important data gets through the network efficiently, even when the network is congested. They are essential for networks that carry a mix of voice, video, and data traffic, where some types of traffic need to be prioritized over others to maintain quality.

DSCP to ToS conversion table

DSCP Class	DSCP (bin)	DSCP (hex)	DSCP (dec)	ToS (dec)	ToS (hex)	ToS (bin)	ToS Prec. (bin)	ToS Prec. (dec)	ToS Delay Flag	ToS Throghput Flag	ToS Reliability Flag	TOS String Format
be	000000	0x00	0	0	0x00	00000000	000	0	0	0	0
cs1	001000	0×08	8	32	0×20	00100000	001	1	0	0	0	Priority
af11	001010	0×0A	10	40	0×28	00101000	001	1	0	1	0	Priority
af12	001100	0×0C	12	48	0×30	00110000	001	1	1	0	0	Priority
af13	001110	0×0E	14	56	0×38	00111000	001	1	1	1	0	Priority
cs2	010000	0×10	16	64	0×40	01000000	010	2	0	0	0	Immediate
af21	010010	0×12	18	72	0×48	01001000	010	2	0	1	0	Immediate
af22	010100	0×14	20	80	0×50	01010000	010	2	1	0	0	Immediate
af23	010110	0×16	22	88	0×58	01011000	010	2	1	1	0	Immediate
cs3	011000	0×18	24	96	0×60	01100000	011	3	0	0	0	Flash
af31	011010	0×1A	26	104	0×68	01101000	011	3	0	1	0	Flash
af32	011100	0×1C	28	112	0×70	01110000	011	3	1	0	0	Flash
af33	011110	0×1E	30	120	0×78	01111000	011	3	1	1	0	Flash
cs4	100000	0×20	32	128	0×80	10000000	100	4	0	0	0	FlashOverride
af41	100010	0×22	34	136	0×88	10001000	100	4	0	1	0	FlashOverride
af42	100100	0×34	36	144	0×90	10010000	100	4	1	0	0	FlashOverride
af43	100110	0×26	38	152	0×98	10011000	100	4	1	1	0	FlashOverride
cs5	101000	0×28	40	160	0xA0	10100000	101	5	0	0	0	Critical
ef	101110	0×2E	46	184	0xB8	10111000	101	5	1	1	0	Critical
cs6	110000	0×30	48	192	0xC0	11000000	110	6	0	0	0	Internetworkcontrol
cs7	111000	0×38	56	224	0xE0	11100000	111	7	0	0	0	Networkcontrol

DSCP Usage best practices

DSCP	Binary	Class	Usage
0	0	Best Effort	Default traffic, no special QoS treatment.
8	1000	Class Selector 1 (CS1)	Background traffic, less priority than best effort.
16	10000	Assured Forwarding 1 (AF11)	Low priority data, less sensitive to delay.
24	11000	Assured Forwarding 2 (AF21)	Medium priority data, moderate sensitivity to delay.
32	100000	Assured Forwarding 3 (AF31)	High priority data, sensitive to delay.
40	101000	Assured Forwarding 4 (AF41)	Critical data, very sensitive to delay.
46	101110	Expedited Forwarding (EF)	Real-time traffic like voice and video, requires low latency and jitter.
48	11000000	Network Control	Reserved for network control traffic.
56	11100000	Network Control	Network control with the highest priority

Here are some commonly used DSCP values and their typical applications:

1. Best Effort (0): This is the default DSCP value and is used for standard, non-critical traffic that doesn’t require special handling.
2. Expedited Forwarding (EF – 46): This value is used for low-latency, low-loss, and low-jitter traffic, such as voice over IP (VoIP) and video conferencing.
3. Assured Forwarding (AF): These values provide different levels of assurance for delivery:
   • AF11 (10): Low priority with low drop probability.
   • AF12 (12): Low priority with medium drop probability.
   • AF13 (14): Low priority with high drop probability.
   • AF21 (18): Medium priority with low drop probability.
   • AF22 (20): Medium priority with medium drop probability.
   • AF23 (22): Medium priority with high drop probability.
   • AF31 (26): High priority with low drop probability.
   • AF32 (28): High priority with medium drop probability.
   • AF33 (30): High priority with high drop probability.
   • AF41 (34): Highest priority with low drop probability.
   • AF42 (36): Highest priority with medium drop probability.
   • AF43 (38): Highest priority with high drop probability.
4. Class Selector (CS): These values are used for backward compatibility with IP precedence:
   • CS1 (8): Typically used for background traffic.
   • CS2 (16): Used for standard traffic.
   • CS3 (24): Used for critical applications.
   • CS4 (32): Used for real-time applications.
   • CS5 (40): Used for signaling.
   • CS6 (48): Used for network control.
   • CS7 (56): Used for network control with the highest priority.