Application Note 126. QoS Priority Support KS8993M / KS8995MA / XA. Introduction. Egress Port Priority Mechanism. Strict Priority Queuing

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1 Application Note 126 QoS Priority Support KS8993M / KS8995MA / XA Introduction Latency critical applications such as Voice over IP (VoIP) and video typically need to guarantee a minimum quality of service (QoS) throughout the network. QoS can be supported by various priority schemes offered by the switches and routers. This application note describes the different priority schemes supported by the KS8993M, KS8995MA, and KS8995XA switches and how they are configured. There are three different priority schemes supported by the above switches: Port-based priority, 802.1p Tag-based Priority, and DiffServ-based priority. The mechanisms of each of these priority schemes differ only with respect to the receive (ingress) port. The transmit (egress) port buffer scheme is common to all three priority schemes. Egress Port Priority Mechanism Each egress port buffer can be individually split into two separate queues, one for high priority and the other for low priority packets using the Priority bit. Strict Priority Queuing Priority output queuing allows a switch to define two priorities of traffic (high and low) on a given interface. As traffic comes into the switch, it is assigned to one of the two output queues. Packets on the highest-priority queue are transmitted first. When that queue empties, traffic on the next highest-priority queue is transmitted, and so on. This mechanism assures that during congestion, the highest-priority data does not get delayed by lowerpriority traffic. Some examples of priority queuing are: Voice and video packets are assigned a highpriority queue level. Web traffic and back-up data traffic are assigned a low-priority queue level. There is a potential, undesirable side effect when using strict priority queuing: low-priority traffic can become locked out. In other words, it is possible to starve low-priority traffic over a low-speed link if there is too much high-priority traffic coming in an unending stream. If that low-priority traffic is truly unimportant, then this may be a desirable outcome. Weighted priority queuing (WPQ) tries to ensure that the low-priority packets will not be starved under extreme congestion. WPQ is implemented in the KS8993M and KS8995MA/XA switches by controlling the transmit ratio between the high- and low-priority packets. The transmit ratio is defined using the Priority Scheme Select bits in the switch register map It is also recommended to enable the Priority Buffer Reserve bit. Global Ctrl 2 (0x04) 0 Priority Buffer Reserve Global Ctrl 3 (0x05) 3-2 Priority Scheme Select Port n Ctrl 0 (0xn0) 0 Priority 1= each output queue is pre-allocated 48 buffers exclusively for high priority packets 0 = no reserved buffers for high priority packets 00 = always deliver high priority packets first 01 = deliver high/low packets at ratio 10/1 10 = deliver high/low packets at ratio 5/1 11 = deliver high/low packets at ratio 2/1 1= output port split into high and low priority queues 0 = single output queue on port 95M = M = pin value during reset: P1_TXQ2 (port 1) P2_TXQ2 (port 2) P3_TXQ3 (port 3) September 2004 M

2 Note: If the Priority bit is not enabled, then only a single output queue will be present at the egress port. Hence, the priority scheme selection will have no effect, irrespective of the other settings for the ingress and egress ports. Port Based Priority Port based priority is the simplest form of QoS. Each ingress port can be individually configured as either high priority or low priority. Untagged packets (see 802.1p Tag-based Priority) arriving at the ingress port will be passed either to the high- or low-priority queue at the egress port, depending upon the configuration of the ingress port. Each ingress port can be configured as either high or low priority using the Port Based Priority Classification bit. Global Ctrl 2 (0x04) 0 Priority Buffer Reserve 1= each output queue is pre-allocated 48 buffers exclusively for high priority packets 0 = no reserved buffers for high priority packets Note: Both 802.1p Tag=based priority and DSCP (DiffServ) priority can override the port based priority classification (see below) p Tag Based Priority 802.1p priority can be enabled by the 802.1p Priority Classification bit in the ingress port s Control 0 register. Ethernet packets can have an optional 4 byte 802.1q VLAN tag inserted between the source address (SA) and the length/type fields. Embedded in the 4-byte tag is a 3-bit priority field, as shown below, used for 802.1p priority classification. Bytes Preamble DA SA VPID TCI Length LLC Data FCS Bits q VLAN Tag Tagged Packet Type (8100 for Ethernet) 802.1p CFI VLAN ID Figure 1. Ethernet Packet with 802.1q VLAN Tag September M

3 These three priority bits are used to determine whether the packet should be treated as high priority or low priority. A base value from 0 to 7 is configured in the switch and compared to the binary equivalent of the incoming packet s priority field in the VLAN tag. If the priority field value is greater than the base priority value, then the packet is classified as high priority. If the priority field value is less than the base priority value, then the packet is classified as low priority. The priority base value is set using the 802.1p Base Priority Bits, as shown below. Port n Ctrl 0 (0xn0) p Priority Classification Global Ctrl 0 (0x02) /1p Base Piority 1= enable 802.1p priority classification for ingress packets on port 0 = disable 802.1p priority Used to classify priority for incoming 802.1q packets p priority field is compared against this value. >= : classified as high priority < : classified as low priority 93M = pin value during reset: P1_1PEN (port 1) P2_1PEN (port 2) P3_1PEN (port 3) x4 x4 Note: The OR ed result of 802.1p and DSCP (see below) priority classification overrides any port priority. DSCP (DiffServ) Priority The differentiated service code point (DSCP) priority operates in the Layer 3, IP protocol. IP datagrams are embedded within the Ethernet data field (see Figure 1). The DSCP priority bits are located inside the type of service (TOS) field, within the standard IPv4 header. The IPv4 header is shown below in more detail. The TOS byte is the second byte located after the header length field (HLEN) Version HLEN Type of Service Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source IP Address Destination IP Address IP Options (if any) Figure 2. Format of IPv4 Datagram Header Padding Bits 0 to 5 of the ToS field are then taken and fully decoded in 64 separate QoS service codes DS Field, DSCP DSCP: Differentiated Services Codepoint ECN: Explicit Congestion Notification (Unused) ECN Field Figure 3. Differential Services (DS) Codepoint Within the ToS Field of an IP Datagram September M

4 The service code is then compared against the corresponding bit in the TOS Priority Control Registers 0 to 7 of the KS8995MA/XA or KS8993M (8 bit x 8 registers = 64 bits). If the corresponding register bit is a 1, then the packet is treated as high priority; if it is a 0, it is treated as a low priority. DSCP priority is enabled using the DiffServ Priority Classification bit in the ingress port s Control 0 register. Port n Ctrl 0 (0xn0) 6 Diffserv Priority Classification TOS Priority Ctrl 0 (0x60) TOS Priority Ctrl 1 (0x61) TOS Priority Ctrl 2 (0x62) TOS Priority Ctrl 3 (0x63) TOS Priority Ctrl 4 (0x64) TOS Priority Ctrl 5 (0x65) TOS Priority Ctrl 6 (0x66) TOS Priority Ctrl 7 (0x67) 1= enable diffserv priority classification for ingress packets on port 0 = disable diffserv priority 7-0 DSCP[63:56] Differential Services Codepoint [63:56] x0 7-0 DSCP[55:48] Differential Services Codepoint [55:48] x0 7-0 DSCP[47:40] Differential Services Codepoint [47:40] x0 7-0 DSCP[39:32] Differential Services Codepoint [39:32] x0 7-0 DSCP[31:24] Differential Services Codepoint [31:24] x0 7-0 DSCP[23:16] Differential Services Codepoint [23:16] x0 7-0 DSCP[15:8] Differential Services Codepoint [15:8] x0 7-0 DSCP[7:0] Differential Services Codepoint [7:0] x0 Port n Ctrl 0 (0xn0) 6 Diffserv Priority Classification 1= enable diffserv priority classification for ingress packets on port 0 = disable diffserv priority Note: The OR ed result of 802.1p and DSCP priority classification overrides any port priority. Priority Re-mapping The KS8993M 3-port switch is ideal for Voice-over-IP (VoIP) phone or analogue terminal adapter (ATA) applications. To guarantee a minimum QoS for these voice services, the KS8993M has a unique ability to re-map the ingress packets 802.1p priority field. The importance of priority re-mapping can be shown when looking at the basic block diagram of a VoIP application. September M

5 VoIP CPU / DSP CORRUPT PC CPU sends highpriority 802.1p tagged egress packets PC config modified to send high-priority 802.1p tagged egress packets MII KS8993M High- and Low-Priority 802.1p Packets KS8993M can re-map all ingress packets at the PC port to LOW priority 802.1p tagged packets, irrespective of the ingress tag. LAN Figure 4. KS8993M 802.1p Priority Field Re-Mapping The problem occurs if you have a corrupt PC transmitting data packets containing high priority 802.1p tags. This causes the VoIP and PC (data) packets to both be tagged as high priority and hence, there is no differentiation between them. An acceptable QoS for the voice traffic can no longer be guaranteed. Priority re-mapping is only available on the KS8993M. Each ingress port has a default VLAN tag as specified in the Port Control registers 3 and 4. If the incoming packet s 802.1p priority field is greater than the priority field of the ingress port s default tag, then the packet s priority field is replaced with the default priority field. Priority remapping is enabled using the User Priority Ceiling bit, as shown below. Port n Ctrl 0 (0xn0) 3 User Priority Ceiling Port n Ctrl 3 (0xn3) 7-0 Default tag [15:8] 1= if the packet s 802.1p priority field is greater than the priority field in the ingress port default tag register, replace the packet s priority field with the priority field in the port default tag register. 0 = do no compare and replace the packet s priority field. Ingress port default tag, containing 7-5 : User Priority bits 4 : CFI bit 3-0 : VID[11:8] 0 Port n Ctrl 4 (0xn4) 7-0 Default tag [7:0] Port n Ctrl 0 (0xn0) 3 User Priority Ceiling Ingress port default tag, containing 7-0 : VID[7:0] 1= if the packet s 802.1p priority field is greater than the priority field in the ingress port default tag register, replace the packet s priority field with the priority field in the port default tag register. 0 = do no compare and replace the packet s priority field. 1 September M

6 Conclusion With emerging applications such VoIP, Video Broadcasting, the network must be ready to handle different type of service in a cost effective manner. The network should be designed with end-to-end QoS capabilities for the future or risk redeployment of the entire network. The Micrel family of switches provides a rich set of QoS function for the end nodes and DTE equipment. It can be used in a variety of applications; unmanaged (no CPU assist), lightly managed, or full managed. For additional information, contact your local Micrel Field Application Engineer or salesperson. MICREL, INC FORTUNE DRIVE SAN JOSE, CA USA TEL +1 (408) FAX +1 (408) WEB The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale Micrel, Incorporated. September M