Essentially, it depends what you want to achieve. Most systems do not honor TOS as part of their forwarding/processing decision. If you do have a system that does honor it, then it is probably documented there.
However, RFC 1349 originally specified the use of the TOS field, which was not really used. It was later obsoleted by RFC 2474, which defines the top 6 bits of that byte to include a Differentiated Services Code Point (the lower two bits are used by ECN). A good overview over the typical values can be found in Wikipedia. So typical values for the higher 6 bits are:
- 0x00 (Default)
- 0x0a, 0x0c, 0x0e; 0x12, 0x14, 0x16; 0x1a, 0x1c, 0x1e; 0x22, 0x24, 0x26 (the twelve different Assured Forwarding behaviors)
- 0x2c (Voice Admit)
- 0x2e (Expedited Forwarding)
To use these values, multiply them by 4 and add the appropriate ECN value to it or just use the
--set-dscp option to the
Setting TOS/DSCP does not directly affect your traffic's treatment, unless the routers are configured to act on these bits, which mostly means you need to talk to your ISP (and probably pay some money).