The ToS/DiffServ field
The ToS (type of service) or DiffServ (differentiated services) field in the IPv4 header, and the Traffic Class field in the IPv6 header are used to classify IPInternet protocol packets so that routers can make QoSQuality of Service (quality of service) decisions about what path packets should traverse across the network.
For example, users may want to ensure that VoIP utilizes high quality, low latency (and expensive) links, or, they might want to ensure email or recreational traffic uses cheaper (but less reliable) links.
Previously, there were 5 different categories that users could classify their traffic with using the IP ToS field (see RFC 791).
- Normal Service
- Minimize Cost
- Maximize Reliability
- Maximize Throughput
- Minimize Delay
These have since been replaced by a new set of values called DSCPDifferentiated Services Code Point (DiffServ Code Points, see RFC 2474). A DSCP is a 6-bit number. This provides 64 possible DSCP combinations, of which, only a portion have been standardized and are listed below.
IPv6 contains an 8 bit Traffic Class field. The 6 most significant bits are treated the same as IPv4 DSCP. The least 2 significant bits are not modified by the appliance.
| DSCP Class Name | Binary Value | Decimal Value |
|---|---|---|
|
BE (best effort, default) |
000000 |
0 |
|
AF11 (assured forwarding, see RFC 2597) |
001010 |
10 |
|
AF12 |
001100 |
12 |
|
AF13 |
001110 |
14 |
|
AF21 |
010010 |
18 |
|
AF22 |
010100 |
20 |
|
AF23 |
010110 |
22 |
|
AF31 |
011010 |
26 |
|
AF32 |
011100 |
28 |
|
AF33 |
011110 |
30 |
|
AF41 |
100010 |
34 |
|
AF42 |
100100 |
36 |
|
AF43 |
100110 |
38 |
|
CS1 (class selector) |
001000 |
8 |
|
CS2 |
010000 |
16 |
|
CS3 |
011000 |
24 |
|
CS4 |
100000 |
32 |
|
CS5 |
101000 |
40 |
|
CS6 |
110000 |
48 |
|
CS7 |
111000 |
56 |
|
EF (expedited forwarding, see RFC 2598) |
101110 |
46 |