Accommodating Voice Traffic on Campus Switches
QoS traffic classification and marking

Classification and marking is the process of identifying traffic for proper prioritization as that traffic traverses the campus network. Traffic is classified by examining information at various Layers of the OSI model. All traffic classified in a certain manner will receive an associate mark or QoS value. IP Traffic can be classified according to any values configurable in an ACL or any of the following criteria:

  • Layer 2 parameters – MAC address, Multiprotocol Label Switching (MPLS), ATM cell loss priority (CLP) bit, Frame Relay discard eligible (DE) bit, ingress interface
  • Layer 3 parameters – IP precedence, DSCP, QoS group, IP address, ingress interface
  • Layer 4 parameters – TCP or UDP ports, ingress interface
  • Layer 7 parameters – application signatures, ingress interface

All traffic classified or grouped according to the criteria above, will be marked according to that classification. QoS marks or values establish priority levels or priority classes of service for network traffic as it is processed by each switch. Once traffic is marked with a QoS value, then QoS policies on switches and interfaces will handle traffic according to the QoS values contained in individual frames and packets. As a result of classification and marking, traffic will be prioritized accordingly at each switch to ensure that delay sensitive traffic receives priority processing as the switch manages congestion, delay and bandwidth allocation.

Layer 2 QoS Marking
QoS Layer 2 classification occurs by examining information in the Ethernet or 802.1Q header such as destination MAC address or VLAN ID. QoS Layer 2 marking occurs in the Priority field of 802.1Q header. LAN Layer 2 headers have no means of carrying a QoS value so 802.1Q encapsulation is required if Layer 2 QoS marking is to occur. The Priority field is 3 bits in length and is also known as the 8021.p User Priority or Class of Service (CoS) value.

This 3 bit field hosts CoS values ranging from 1-7; 1 being associated with delay tolerant traffic such as TCP/IP. Voice traffic, which by nature is not delay tolerant, receives higher default CoS values such as 3 for Call Signaling. A CoS value of 5 is given to Voice Bearer traffic which is the phone conversation itself where voice quality is impaired if any packets are dropped or delayed.

As a result of Layer 2 classification and marking, the following QoS operations can occur:

  • Input queue scheduling – When a frame enters a port, it can be assigned to one of a number of port-based queues prior to being scheduled for switching to an egress port. Typically, multiple queues are used where traffic requires different service levels.
  • Policing – Policing is the process of inspecting a frame to see if it has exceeded a predefined rate of traffic within a certain time frame that is typically a fixed number internal to the switch. If a frame is determined to be in excess of the predefined rate limit, it can either be dropped or the CoS value can be marked down.
  • Output queue scheduling – The switch will place the frame into an appropriate outbound (egress) queue for switching. The switch will perform buffer management on this queue by ensuring that the buffer does not overflow.

Layer 3 QoS Marking
QoS Layer 3 classification results from the examination of header values such as Destination IP address or Protocol. QoS Layer 3 marking occurs in the Type of Service (ToS) byte in the IP Header. The first 3 bits of the ToS byte are occupied by IP Precedence, which correlates to the 3 CoS Bits carried in the Layer 2 header.

The ToS Byte can also be used for Differentiated Services Code Point (DSCP) marking. DSCP allows prioritization hop by hop as packets are processed on each switch and interface. The ToS bits are used by DSCP values as shown below. The first 3 DSCP bits, correlating to Precedence and CoS, identify the DSCP Class of Service for the packet.

The next three DS bits establish a drop precedence for the packet. Packets with a high DSCP drop precedence value will be dropped before those with a low value if a device or a queue becomes overloaded and must drop packets. Voice traffic will be marked with a low DSCP drop precedence value to minimize voice packet drop.

Each 6 bit DSCP value is also given a DSCP Codepoint name. DSCP classes 1-4 are Assured Forwarding classes (AF). Therefore, if the DSCP class value was 3 and the Drop Precedence was 1, the DSCP Codepoint would be AF31.