Priority Processing Highlighted

Hi, guys!

According to different priority processing policies, the inner and outer VLAN priorities can be set for service-ports, or the user-side priority can be copied for service-ports.

The priority processing feature of the ONU includes remarking the VLAN priority of packets, copying the user-side CoS priority, and copying the user-side ToS priority.

In this way, packets are scheduled according to the priorities when congestion occurs on the device or the upper-layer network.

Priority processing remarks the 802.1p priority according to certain rules. Priority processing is a prerequisite for queue scheduling. In queue scheduling on the MA5616, packets enter queues according to their outer virtual local area network (VLAN) priorities. At the same time, priority processing also makes preparations for the scheduling on the upper-layer network.

Voice calls are characterized by real-time, small delay, and quick connection. Therefore, voice packets are generally forwarded with a high priority. A router forwards packets according to the VLAN priority (complying with the 802.1p), differentiated services code point (DSCP), or type of service (ToS). The MA5616 supports the 802.1p priority setting for the media IP address and signaling IP address separately. By default, the priorities of both the media IP address and signaling IP address are 6.

A packet priority can be 802.1p priority or IP priority.

802.1p Priority

802.1p priority is specified at the link layer and is also called the class of service (CoS) priority. The 802.1p priority occupies three bits in a VLAN tag, as defined in IEEE 802.1Q. Position of the 802.1p priority in an Ethernet frame.

Tag control information (TCI) is divided into the following three fields:

•  Priority field: indicates the 802.1p priority. The field comprises three bits and ranges from 0 to 7. The field indicates the Ethernet frame priority. A maximum of eight priorities are supported. Value 7 indicates the highest priority and value 0 indicates the lowest priority. By using the eight priorities, the field specifies which packet to be transmitted first in the case of congestion on a port.

•  Canonical format indicator (CFI): indicates a one-bit field used to identify different colors.

•  VLAN identification (VLAN ID): indicates the VLAN ID that is a 12-bit field. All the packets transmitted from a host that supports 802.1q contain this field, indicating the VLAN to which the packets belong.

IP Priority

As defined in the IP protocol, DSCP and ToS occupy the same field (one byte) in an IP header. An IP bearer network device identifies whether the field is filled with DSCP or ToS and schedules and forwards packets according to the setting to ensure QoS for various services.

DSCP is used to select a per-hop behavior (PHB) on each node on a network. PHB describes the externally visible behaviors of a DS node when being used for data stream aggregation. Currently, (IETF) defines three standard PHBs: expedited forwarding (EF), assured forwarding (AF), and best-effort (BE).