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IP Precedence, TOS & DSCP
The IP Type of Service Header Field
The Type of Service field in the IP header was originally defined in RFC 791. It defined a mechanism for assigning a priority to each IP packet as well as a mechanism to request specific treatment such as high throughput, high reliability or low latency.
In practice, only the IP Precedence part of the field was ever used. At its simplest, the higher the value of the IP Precedence field, the higher the priority of the IP packet. Simple...
In RFC 2474 the definition of this entire field was changed. It is now called the "DS" (Differentiated Services) field and the upper 6 bits contain a value called the "DSCP" (Differentiated Services Code Point). Since RFC 3168, the remaining two bits (the two least siginficant bits) are used for Explicit Congestion Notification.
The diagram below illustrates the relationship between the bits in the Type of Services/Differeniated Services field in the IP header:-
| Type of Service (TOS) | X | X | X | X | X | X | X | 0 |
|---|---|---|---|---|---|---|---|---|
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| Differentiated Services (DS) | X | X | X | X | X | X | X | X |
Interpretation of the TOS/DS Field
RFC791/RFC1349 Interpretation
| Bits | Meaning | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-5 | IP Precedence:
| ||||||||||||||||
| 4 | 1 = Low Delay; 0 = Normal Delay | ||||||||||||||||
| 3 | 1 = High Throughput; 0 = Normal Throughput | ||||||||||||||||
| 2 | 1 = High Reliability; 0 = Normal Reliability | ||||||||||||||||
| 1 | 1 = Minimise monetary cost (RFC 1349) | ||||||||||||||||
| 0 | Must be 0 |
RFC 2474 (Differentiated Services) Interpretation
| Bits | Meaning |
|---|---|
| 7-2 | DSCP |
| 1-0 | ECN (Explicit Congestion Notification) |
The DSCP
The most signficant three bits of the DSCP (corresponding directly to the IP Precedence) are referred to as the Class Selector.
DSCP <=> IP Precedence Conversion Table
| DSCP Name | DS Field Value | IP Precedence | |
|---|---|---|---|
| Binary | Decimal | ||
| CS0 | 000 000 | 0 | 0 |
| CS1 | 001 000 | 8 | 1 |
| AF11 | 001 010 | 10 | 1 |
| AF12 | 001 100 | 12 | 1 |
| AF13 | 001 110 | 14 | 1 |
| CS2 | 010 000 | 16 | 2 |
| AF21 | 010 010 | 18 | 2 |
| AF22 | 010 100 | 20 | 2 |
| AF23 | 010 110 | 22 | 2 |
| CS3 | 011 000 | 24 | 3 |
| AF31 | 011 010 | 26 | 3 |
| AF32 | 011 100 | 28 | 3 |
| AF33 | 011 110 | 30 | 3 |
| CS4 | 100 000 | 32 | 4 |
| AF41 | 100 010 | 34 | 4 |
| AF42 | 100 100 | 36 | 4 |
| AF43 | 100 110 | 38 | 4 |
| CS5 | 101 000 | 40 | 5 |
| EF | 101 110 | 46 | 5 |
| CS6 | 110 000 | 48 | 6 |
| CS7 | 111 000 | 56 | 7 |
| CS | Class Selector (RFC 2474) |
|---|---|
| AFxy | Assured Forwarding (x=class, y=drop precedence) (RFC2597) |
| EF | Expedited Forwarding (RFC 3246) |
References
- RFC 791 - Internet Protocol Protocol Specification
- RFC 2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers
- RFC 2475 - An Architecture for Differentiated Service
- RFC 2597 - Assured Forwarding PHB Group
- RFC 3246 - An Expedited Forwarding PHB (Per-Hop Behavior)
- RFC 3168 - The Addition of Explicit Congestion Notification (ECN) to IP
- Chapter from "Traffic Engineering with MPLS"
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