Quality of Service: Layer 2 Packet Marking

Packets can be marked with quality of service relevant information in the 802.1Q standard.

802.1Q standard specifies the implementation of VLANs in packet switched networks with two fields, each two bytes in size.

The first is a Tag Protocol Identifier, and the second is Tag Control Information. The fields are inserted after the Source MAC address field in an Ethernet frame.

Tag Control Information (TCI)

The tag control information field consists of 2 bytes, or 16 bits. There are three fields inside: Priority Code Point which is 3 bits in size, Drop eligible Indicator is 1 bit in size, and the VLAN Identifier field being the largest at 12 bits.

Priority Code Point

The PCP field is 3 bits in size and is used for assigning class of service to an Ethernet frame. There are eight different class of service priority levels ranging from 0 to 7. The higher the number the higher the priority of that network packet. Each class of service has a named definition under the IEEE 802.1p specification.

PriorityAcronymTraffic Type
1BEBest Effort, Default Setting.
2EEExcellent Effort
3CACritical Application
4VIVideo With < 100ms Latency
5VOVoice With < 10ms Latency
6ICInternetwork Control
7NCNetwork Control
802.1p Class of Service Definitions

The priority code point is preserved whilst the packets move across a 802.1Q VLAN trunk. If the packet travels across a non-802.1Q link, the class of service information that prioritises the packet is lost. Packets should utilise quality of service information in higher layers, such as the the Type of Service (ToS) field in IP.

Drop Eligible Indicator

A single bit field, the drop eligible indicator can be used to indicate that the frame is eligible to be dropped when a link is congested. It can be used independent of the Priority Code Point field or with it. The default value of the field is 0, indicating that the frame is not eligible to be dropped. A value of 1 indicates the frame can be dropped.

VLAN Identifier

The VLAN identifier is a 12 bit field that defines the VLAN used by the packet for 802.1Q

Being 12 bits in length, the number of VLANs are restricted to 4096



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