NTH in RouterOS before and after 3.x

Nth in RouterOS 2.9 and below.

example: nth=Every,Counter,Packet nth=2,3,0. 2,3,1 2,3,2
divide all packets into groups of three (2+1). The packets will be numbered from 0 to 2. So, a sequence of packets the rule matches looks like: (0 1 2)(0 1 2)(0 1 2)(0 1 2)(0 1 2)...
the first rule will match the first packet in each group ("Packet"=0). The second rule will match the second packet in each group ("Packet"=1) and so on. Each successful match increments the counter. When a value of "Every" is reached, the counter is reset to 0. For this to work, the "Counter" should be the same for all rules (you can pick any value from 0 to 15, IIRC).


In v3.0 it is a little different implementation of NTH. It has only two parameters 'every' and 'packet'.

How it works in v3.0 and above

Every rule has its own counter. When rule receives packet counter for current rule is increased by one. If counter matches value of 'every' packet will be matched and counter will be set to zero.
If passthrough is not set then packets will be marked as follows:

• first rule nth=2,1 rule will match every first packet of 2, hence, 50% of all the traffic that is matched by the rules
• second rule if passthrough=no will match ONLY 25% of traffic because in 3.0 you need only one rule to catch traffic not like 2.9

Example

Now it is possible to match 50% of all traffic only with one rule:

/ip firewall mangle
add action=mark-packet chain=prerouting new-packet-mark=AAA nth=2,1;

If more than one rule is needed, then there are two ways to match packets:

• first rule sees all packets and matches 1/3 of all, second rule sees 2/3 of packets and matches 1/2, third rule sees and matches all packets that passed through first two rules ( 1/3 of all packets ).

/ip firewall mangle 
add action=mark-packet chain=prerouting new-packet-mark=AAA nth=3,1 passthrough=no;
add action=mark-packet chain=prerouting new-packet-mark=BBB nth=2,1 passthrough=no;
add action=mark-packet chain=prerouting new-packet-mark=CCC ;

• all rules can see all packets and each rule matches every 3-rd packet.

/ip firewall mangle 
add action=mark-packet chain=prerouting new-packet-mark=AAA nth=3,1 passthrough=yes;
add action=mark-packet chain=prerouting new-packet-mark=BBB nth=3,2 passthrough=yes;
add action=mark-packet chain=prerouting new-packet-mark=CCC nth=3,3 passthrough=yes; 

source: http://wiki.mikrotik.com/wiki/Manual:NTH_in_RouterOS_3.x