Prioritizing Traffic
- When traffic volume is greater than what can be transported across the network, devices queue (hold) the packets in memory until resources become available to transmit them.
- Queuing packets causes delay because new packets cannot be transmitted until previous packets have been processed.
- If the number of packets to be queued continues to increase, the memory within the device fills up and packets are dropped.
One QoS technique that can help with this problem is to classify data into multiple queues, as shown in the figure
Bandwidth, Congestion, Delay, and Jitter
- Network bandwidth is measured in the number of bits that can be transmitted in a single second, or bits per second (bps).
- Network congestion causes delay. An interface experiences congestion when it is presented with more traffic than it can handle. Network congestion points are ideal candidates for QoS mechanisms.
Bandwidth, Congestion, Delay, and Jitter
- Network bandwidth is measured in the number of bits that can be transmitted in a single second, or bits per second (bps).
- Network congestion causes delay. An interface experiences congestion when it is presented with more traffic than it can handle. Network congestion points are ideal candidates for QoS mechanisms.
Bandwidth, Congestion, Delay, and Jitter
Delay or latency refers to the time it takes for a packet to travel from the source to the destination.
- Fixed delay is the amount of time a specific process takes, such as how long it takes to place a bit on the transmission media.
- Variable delay takes an unspecified amount of time and is affected by factors such as how much traffic is being processed.
Jitter is the variation of delay of received packets
- Network congestion causes delay. An interface experiences congestion when it is presented with more traffic than it can handle. Network congestion points are ideal candidates for QoS mechanisms.
Queuing Algorithms
First In First Out (FIFO)
- First In First Out (FIFO) queuing buffers and forwards packets in the order of their arrival.
- FIFO has no concept of priority or classes of traffic and consequently, makes no decision about packet priority.
- There is only one queue, and all packets are treated equally.
- Packets are sent out an interface in the order in which they arrive.
Queuing Algorithms
Weighted Fair Queuing (WFQ)
Weighted Fair Queuing (WFQ) is an automated scheduling method that provides fair bandwidth allocation to all network traffic.
WFQ applies priority, or weights, to identified traffic, classifies it into conversations or flows, and then determines how much bandwidth each flow is allowed relative to other flows.
WFQ classifies traffic into different flows based on source and destination IP addresses, MAC addresses, port numbers, protocol, and Type of Service (ToS) value.
Queuing Algorithms
Class-Based Weighted Fair Queuing (CBWFQ)
Class-Based Weighted Fair Queuing (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes.
- Traffic classes are defined based on match criteria including protocols, access control lists (ACLs), and input interfaces.
- Packets satisfying the match criteria for a class constitute the traffic for that class.
- A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class.
- A class can be assigned characteristics, such as bandwidth, weight, and maximum packet limit. The bandwidth assigned to a class is the guaranteed bandwidth delivered during congestion.
- Packets belonging to a class are subject to the bandwidth and queue limits, which is the maximum number of packets allowed to accumulate in the queue, that characterize the class.
Queuing Algorithms
Low Latency Queuing (LLQ)
The Low Latency Queuing (LLQ) feature brings strict priority queuing (PQ) to CBWFQ.
- Strict PQ allows delay-sensitive packets such as voice to be sent before packets in other queues.
- LLQ allows delay-sensitive packets such as voice to be sent first (before packets in other queues), giving delay-sensitive packets preferential treatment over other traffic.
- Cisco recommends that only voice traffic be directed to the priority queue.
QOS Tools
Classification
How a packet is classified depends on the QoS implementation.
- Methods of classifying traffic flows at Layer 2 and 3 include using interfaces, ACLs, and class maps.
- Traffic can also be classified at Layers 4 to 7 using Network Based Application Recognition (NBAR).
QOS Tools
Marking (DSCP Value)
- Expedited Forwarding (EF) – RFC 3246 defines EF as the DSCP decimal value 46 (binary 101110). The first 3 bits (101) map directly to the Layer 2 CoS value 5 used for voice traffic. At Layer 3, Cisco recommends that EF only be used to mark voice packets.
- Assured Forwarding (AF) – RFC 2597 defines AF to use the 5 most significant DSCP bits to indicate queues and drop preference.
- The first 3 most significant bits are used to designate the class. Class 4 is the best queue and Class 1 is the worst queue.
- The 4th and 5th most significant bits are used to designate the drop preference.
- The 6th most significant bit is set to zero.
QOS Tools
Marking (DSCP Value) Class Selector Bits
Class Selector (CS) bits:
- The first 3 most significant bits of the DSCP field and indicate the class.
Map directly to the 3 bits of the CoS field and the IPP field to maintain compatibility with 802.1p and RFC 791
QOS Tools
Congestion Avoidance
- Weighted random early detection (WRED) allows for congestion avoidance on network interfaces by providing buffer management and allowing TCP traffic to decrease, or throttle back, before buffers are exhausted.
- WRED helps avoid tail drops and maximizes network use and TCP-based application performance.
R1 Configuration
class-map voice
match protocol rtp
exit
class-map http
match protocol http
exit
class-map icmp
match protocol icmp
exit
policy-map mark
class voice
set ip dscp ef priority 100 —–bandwidth
class http
set ip dscp af31
bandwidth 50
class icmp
set ip dscp af11
bandwidth 25
exit
sh run
interface s0/1/0
service-policy output mark
R2 Configuration
class-map voice
match ip dscp ef
exit
class-map http
match ip dcsp af31
class-map icmp
match ip dscp af11
policy-map remark
class voice
set precedence critical or 5
exit
class http
set precedence 3
class icmp
set precedence routine or 0
interface s0/2/0
service-policy inbound remark
show policy-map
show policy-map interface s0/1/0