Intermediate iRules: Clarifying IP::tos command usage

The IP::tos command has been around since the v4 days, and there has been some confusion on IP tos versus DSCP. The command takes the supplied value (or returns  a value) and applies to the tos field overall, which is eight bits. So if you take express forwarding, which is DSCP 46 and use IP::tos 46, you'll notice on the wire that something is amiss:

What gives? Well, the DSCP markings are in the first 6 bits of the tos field, so you need to bit-shift to the left a couple places. That results in 46 * 2² = 184. So 184 is the value you would use with the IP::tos command.

00101110 = 46

10111000 = 184

By setting IP::tos 184, you can see on the wire that all is now well in the world:

The below table maps the DSCP values in decimal to the value you'll need to use with the IP::tos command to get expected results.

DSCP Mappings for IP::tos Command

Precedence	Type of Service	DSCP Class	DSCP Value	IP::tos Value
0	0	none	0	0
1	0	cs1	8	32
1	1	af11	10	40
1	10	af12	12	48
1	11	af13	14	56
10	0	cs2	16	64
10	1	af21	18	72
10	10	af22	20	80
10	11	af23	22	88
11	0	cs3	24	96
11	1	af31	26	104
11	10	af32	28	112
11	11	af33	30	120
100	0	cs4	32	128
100	1	af41	34	136
100	10	af42	36	144
100	11	af43	38	152
101	0	cs5	40	160
101	11	ef	46	184
110	0	cs6	48	192
111	0	cs7	56	224

I updated the TCP Profile QoS article I wrote a few years back with this updated table, as well as the IP::tos command page in the iRules documentation. Thanks to F5er Alex for tracking this down!