Section 10.3: Differentiated Services QoS
The DiffServ structural design is the basis for QoS performance in Catalyst switches. Packets are classed as they enter the network. Each following router and switch, in a per-hop behaviour, makes use of the specific parameters within the Type of Service and DiffServ fields in the IP header to match the forwarding method to these parameters. Routers that do not have DiffServ enabled will make forwarding decisions based on default queues. Every DiffServe enabled router will have a local configured queuing priority that is used when forwarding classed packets.
At Layer 2 there is no means to point out the significance of a frame's contents. A Layer 2 switch can simply handle forwarding frames as a best-effort delivery. However, there are mechanisms available that allow layer 2 priorities to be mapped to layer 3.
The original IPv4 Type of Service (ToS) fields has 3 bits of Precedence that provides seven levels of Precedence which is configurable on a group basis. A further 4 independently configurable bits can be used to request one of the following four types of service:
• Minimize delay
• Maximize throughput
• Maximize reliability
• Minimize monetary
One bit must be set to 0 (zero) to indicate that it is unused.
At layer 3, the classification and indicators are created by placing bits in ToS field to other values. The DiffServ model uses the existing IP ToS byte, known as the Differentiated Services (DS) field and sets it with a differing value. This 6-bit DS value is used as the Differentiated Service Code Point (DSCP) and is checked by every DiffServ enabled mechanism. A 2-bit currently unused (CU) field is retained.
Traffic type are defined in Table 10.1
Table 10.1: Differentiated Services Types of Traffic
Best Effort Excellent Effort Controlled Load Background Video Voice
Signifies an enormous requirement in order to sustain and support network infrastructure
Normal LAN priority
Entails best efforts meant for key users
Games, bulk transfers etc
Fewer than 100 milliseconds delay
Fewer than 10 milliseconds delay
10.3.1: IEEE 802.1p
IEEE 802.1p provides QoS at MAC Level. It identifies 3 bits in the 802.1Q header that is assigned to the Class of Service (CoS). IEEE 802.1p also specifies vital methods for speeding up traffic class and it enables vigorous multicast filtering. It set ups eight priority levels that are very alike to the 3 bits specified by IP Precedence. Layer 3 switches are able to map the 802.1p Precedence to DiffServ fields inside the IP while
Layer 2 switches are able to prioritize output buffer data in accordance to the priority levels. This ensures end to end QoS.
10.3.2: Using the QoS Model
The classification process is the first step in determining the manner in which switches and routers prioritize traffic. Packets need to be classified by means of an indicator or marking that shows that it should be handled differently. The next step is to traffic policing. This entails the process taken by a switch and router that decides on whether a packet conforms to the preconfigured profiles. Bandwidth limits are set for conforming traffic and non-conforming traffic are dropped. The third step is to mark the packet. Data can be marked in the IEEE 802.1p header at Layer 2. At Layer 3 data can be marked inside the IP header.
When the switch is operating as Layer 3 switch, a packet can be forwarded with QoS. The traffic type is mapped to the DiffServ number. Once the packet has been through the three steps outlined above, it is allocated to the applicable queue, prior to exiting the switch. The process can be automated when a switch receives a packet inside an 802.3 frame with a particular IEEE 802.1p priority. When this does not occur, the process must be manually mapped. Next, a queuing process is established and traffic is placed into various queues with reference to the policies. The packet is then forwarded out of the shared output buffer on the media to the next hop.
10.3.3: Prioritizing the Traffic Classes
Traffic marking is usually done by using the class-maps and policy-maps mapping commands in Cisco IOS. Maps start with a match command that explicitly identifies some traffic type at the packet, frame or application layers. Access list are utilized during this identification process. Class-maps facilitate the matching of an IP address, a protocol, or an incoming interface. After traffic has been matched, the policy-map is used to set the Differentiated Service Code Point (DSCP).
10.3.4: Queuing Methods
There are numerous queuing mechanisms on Cisco Layer 2 and Cisco Layer 3 switches to cater for the need
that different network administrators need different prioritizations for their different networks that run a vast
diversity of applications. Some of these methods are listed below:
• First In, First Out Queuing (FIFO): This is the default method and sends packets and frames with reference to the timed arrival of the initial bits, in the packet or frame, at the input interface.
• Weighed Fair Queuing (WFQ): This method uses the conversation index linked with each packet to put data into various queues. A conversation index is a phrase used for various applications that have their packets marked by means of a number within the switch or router.
• Custom Queuing enables an administrator to establish a maximum to 16 queues that have configurable sizes and forwarding thresholds. Data is put into queues in accordance to access lists. Queues are cleared on a round robin basis
• Weighted Round Robin Queuing: This is an easier form of Custom Queuing whereby a set number of queues are serviced in round-robin fashion. Each queue is configurable just to the size of the queue.
• Priority Queuing: This method enables an administrator to establish queues and to configure the size of each queue. Data is located in queues according to access lists. Packets that are part of the highest priority queue are always sent first, while packets that are part of the lower priority queues are only sent when the higher queues are cleared. QosQue
This is the process used to ease the deployment of QoS features whereby a switch or router can automatically decide on whether a port connection holds any particular QoS condition. The switch is able to prioritize various traffic flows. It uses the output queues specifically as an alternative to using the default QoS behavior of Best Effort Delivery from a single queue. Traffic are automatically classified and put in the suitable output queue. With auto-QoS, the switch is capable of identifying ports that have IP telephones connected to them. It then assigns sufficient buffer space to give the Voice over IP (VoIP) calls the proper QoS. The feature also applies to uplinks that contain the VoIP calls to the next switch. This process is known as trust and is configured across a QoS domain. Trust permits ports that carry VoIP traffic that do not have IP phones immediately connected to them, to conclude that a packet carrying this facility is given the identical QoS, as though it was immediately connected. Packets are marked only at the entrance to the domain and are trusted from that point onwards. This eliminates the need to mark at each switch or router.