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WMM works by dividing traffic into 4 access categories: background, best effort, video, voice. QoS policy (different handling of access categories) is applied on transmitted packets, therefore the transmitting device is treating different packets differently, e.g. AP does not have control over how clients are transmitting packets, and clients do not have control over how AP transmits packets.

Mikrotik AP and client classifies classify packets based on the priority assigned to them, according to the table (as per WMM specification): 1,2 - background 0,3 - best effort 4,5 - video 6,7 - voice.

To be able to use multiple WMM access categories, not just the best effort where all packets with default priority 0 go, priority must be set for those packets. By default, all packets (incoming and locally generated) inside the router have priority 0.

The "Better" access category for a packet does not necessarily mean that it will be sent over the air before all other packets with the "worse" access category. WMM works by executing the DCF method for medium access with different settings for each access category (EDCF), which basically means that the "better" access category has a higher probability of getting access to medium - WMM enabled station can be considered to be 4 stations, one per access category, and the ones with "better" access category use settings that make them more likely to get chance to transmit (by using shorter backoff timeouts) when all are contending for medium. Details can be studied in 802.11e and WMM specifications.

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The VLAN priority is a 3-bit field called Priority Code Point (PCP) within a VLAN-tagged header and values are between 0 and 7. It is used for implementing QoS on bridges and switches. MikroTik devices by default are sending VLAN packets (locally generated or encapsulated) with a priority of 0. The RouterOS bridge forwards VLAN tagged packets unaltered, which means that received VLAN tagged packets with a certain VLAN priority will leave the bridge with the same VLAN priority. The only exception is when the bridge untags the packet, in this situation VLAN priority is not preserved due to the missing VLAN header. 

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Priority of packets can be set using action=set-priority of in IP firewall mangle rules or bridge filter/nat rules. Priority can be set to a specific value or taken from the ingress priority using the from-ingress setting. Ingress priority is the priority value that was detected on the incoming packet, if available. Currently, there are 2 sources of ingress priority - priority in the VLAN header and priority from the WMM packet received over a wireless interface. For all other packets ingress priority is 0.

Note that ingress priority value is not automatically copied to IP mangle priority value, the correct rule needs to be set up to do this.

There are basically 2 ways to control priority - assign priority with rules with particular matchers (protocol, addresses, etc.) or set it from ingress priority. Both options require setting up correct rules.

This essentially means that if it is not possible or wanted to classify packets by rules, the configuration of the network must be such that the router can extract ingress priority from incoming frames. Remember there are currently 2 sources for this - VLAN tag in packets and received WMM packets.

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When a wireless packet is received with an already set WMM priority, the RouterOS bridge does not automatically translate it to a VLAN header. It means, that received wireless packets with WMM priority that gets get VLAN tagged by the bridge will be forwarded with a VLAN priority of 0. However, we can use a bridge filter rule with from-ingress setting to keep the priority in VLAN packets. For example, we would like to forward wireless packets over ether2 with a VLAN 10 header and keep the already set WMM priority (set by the wireless client).

/interface bridge
add name=bridge1 vlan-filtering=yes
/interface bridge port
add bridge=bridge1 interface=ether2
add bridge=bridge1 interface=wlan2 pvid=10
/interface bridge vlan
add bridge=bridge1 tagged=ether2 vlan-ids=10

# translates WMM priority to VLAN priority 
/interface bridge filter
add action=set-priority chain=forward new-priority=from-ingress out-interface=ether2

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Another way of setting VLAN or WMM priority is by using the DSCP field in the IP header, this can only be done by the IP firewall mangle rule with new-priority=from-dscp or new-priority=from-dscp-high-3-bits settings and set-priority action property. Note that DSCP in the IP header can have values 0-63, but priority only 0-7. When using the the new-priority=from-dscp setting, the priority will be 3 low bits of the DSCP value, but when using new-priority=from-dscp-high-3-bits the priority will be 3 high bits of DSCP value.

Remember that DSCP can only be accessed on IP packets and the DSCP value in the IP header should be set somewhere (either by client devices or IP mangle rules).

It is best to set the DSCP value in the IP header of packets on some border router (e.g. main router used for connection to the Internet), based on traffic type e.g. set DSCP value for packets coming from the Internet belonging to SIP connections to 7, and 0 for the rest. This way packets must be marked only in one place. Then all APs on the network can set packet priority from the DSCP value with just one rule.

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DSCP from Priority

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Similarly, the DSCP value can be set if the received packet contains VLAN or WMM priority. This can be achieved with IP mangle rules with new-dscp=from-priority or new-dscp=from-priority-to-high-3-bits settings and change-dscp action property. Note that priority in VLAN or WMM packets can have values 0-7, but DSCP in IP headers are 0-63. When using the new-dscp=from-priority setting, the value of priority will set the 3 low bits of the DSCP, but when using new-dscp=from-priority-to-high-3-bits  the value of priority will set the 3 high bits of the DSCP. 

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Combining priority setting and handling solutions

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