Another way of setting priority is by using DSCP field in IP header, this can only be done by firewall mange rule “set priority” action. Note that DSCP in IP header can have values 0-63, but priority only 0-7. Effective priority after set from DSCP value will be 3 low bits of DSCP value which is the same as reminder of division by 8. So for example, priority from DSCP values 0,8,16,etc will be 0, from DSCP values 7,15,…,63 - 7.
Can someone give me a better explanation of this please?
7/8 = 0,875. Why does this translate in priority 7?
63/8 = 7,875. Why does this translate in priority 7?
According some tables DSCP value 0-7 is Best Effort or priority “0” which means “lowest”
So how can it be that DSCP value “0” is priority “0” (= lowest) and DSCP value “7” is priority “7” (= highest) while DSCP (TOS) values is in same group?
I want to setup priority in packets depending on VOIP, P2P and all other traffic. I want to use TOS for this and then wireless routers can use this TOS mark for priority setting to use in WMM.
but I need better understanding of the correlation between DSCP (TOS), WMM and packet priority.
I understand when which methode is used but do not understand how the different values correlate.
Another example: DSCP for Background traffic can be “8-15”. So, do I use “8”, or “15”, or any number in between? Is “15” better then “8” or what? What happens if I use “12”, or “14”??
http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml
IP precedence and DSCP are expressed in the same TOS byte. IP precedence is older, and uses the most significant three bits (hence 2^3 = 8 values). DSCP uses the same byte, but the six most significant bits, which include the three most significant bits also used for preference (2^6 = 64 values). In short, they overlap. Also there are two more bits not used by either.
Look at this conversion table, specifically the binary values for the TOS, IP precedence, and DSCP fields: http://blog.webdir.bg/tos-to-dscp-mapping/. Note how the fragments in decimal map back to the bit sections used for each field in relation to the whole byte
fewi,
Maybe my old brain just can’t handle this kind of stuff but I can’t see the trees through the forest no more…
Too much info.
Basically my questions is:
In my border router I can identify my VOIP traffic by the srce IP addresses. So I want to give these packets a marker that travels in my network (and outside on my ISP’s network). Now further down in my network other routers (AP’s with wireless clients) can filter for these packet marks and set priority that then can be used for WMM.
So, in mangle I choose “change DSCP (TOS)” as my action for the packets classified by the known IP.
Now I can set a number in the "New DSCP (TOS) field.
BWAMM, it hits me now!! What number do I set now for DSCP or TOS? Do I set for DSCP or do I set for TOS??
Looking at the options given by MT I would think DSCP and TOS is just two words for the same thing. Like a “car” and “automobile”.
If I now fill in number “7”, is this now a TOS that equalls to DSCP field 56?
Next router can now either filter for DSCP (TOS) field “7” or “56” and it will catch the same packet?
And if I use the action “set priority” and for “New Priority” I use “from dscp”, should I now have set “7” or “56”?
In other words “from dscp” means the process is to look for a dscp number that can be 0.8,10,… 48,56 and give correlated priority for packet or does it mean that a packet that had been set with TOS “7” somewhere therefore means it also has an DSCP value “56” so it will be catched and converted in proper priority anyway?
Thus I can set TOS “7” somewhere and next router can filter on DSCP “56” to catch the same packet, even if the text in the option of MT says “from dscp”?
Sorry for all this, but I really need understanding, but in ´plain´ language. Thanks