GE ML1200 instruction manual IP Precedence ToS Field in an IP Packet Header

CHAPTER 14: QUALITY OF SERVICE QUALITY OF SERVICE

Data +FCS

ToS byte

3 bits

IP precedence 754726A1.CDR

FIGURE 14–2: IP Precedence ToS Field in an IP Packet Header

The three most significant bits (correlating to binary settings 32, 64, and 128) of the Type of Service (ToS) field in the IP header constitute the bits used for IP precedence. These bits are used to provide a priority from 0 to 7 for the IP packet.

Because only three bits of the ToS byte are used for IP precedence, you need to differentiate these bits from the rest of the ToS byte.

The MultiLink ML1200 Managed Field Switch has the capability to provide QoS at Layer 2. At Layer 2, the frame uses Type of Service (ToS) as specified in IEEE 802.1p. ToS uses 3 bits, just like IP precedence, and maps well from Layer 2 to layer 3, and vice versa.

The switches have the capability to differentiate frames based on ToS settings. With two queues present - high or low priority queues or buffers in MultiLink ML1200 Managed Field Switch, frames can be placed in either queue and serviced via the weight set on all ports. This placement of queues, added to the weight set plus the particular tag setting on a packet allows each queue to have different service levels.

MultiLink ML1200 Managed Field Switch QoS implementations provide mapping of ToS (or IP precedence) to Class of Service (CoS). A CoS setting in an Ethernet Frame is mapped to the ToS byte of the IP packet, and vice versa. A ToS level of 1 equals a CoS level of 1. This provides end-to-end priority for the traffic flow when MultiLink ML1200 Managed Field Switchs are deployed in the network.

Not all packets received on a port have high priority. IGMP and BPDU packets have high priority by default.

The MultiLink ML1200 Managed Field Switch has the capability to set the priorities based on three different functions. They are

•Port QoS: assigns a high priority to all packets received on a port, regardless of the type of packet.

•TAG QoS: if a packet contains a tag, the port on which the packet was received then looks to see at which level that tag value is set. Regardless of the tag value, if there is a tag, that packet is automatically assigned high priority (sent to the high priority queue)

•ToS QoS: (Layer 3) when a port is set to ToS QoS, the most significant 6-bits of the IPv4 packet (which has 64 bits) are used. If the 6 bits are set to ToS QoS for the specific port number the packet went to, that packet is assigned high priority by that port

MULTILINK ML1200 MANAGED FIELD SWITCH – INSTRUCTION MANUAL 14–3