Chapter 9 - QoS Concepts Flashcards
how does congestion of packets occur?
- multiple communicate lines aggregate onto a single device and data is placed on few/slow outbound interfaces
- when large data packets prevent smaller packets from being transmitted in a timely manner
a technique for QoS
classifying data into multiple queues (high, medium, low priority)
code delay
- source of delay
- fixed amount of time it takes to compress data at the source before transmitting to the first internetworking device, usually a switch
packetization delay
- source of delay
- fixed time it takes to encapsulate a packet with all the necessary header info
queuing delay
- source of delay
- variable amount of time a frame or packet waits to be transmitted on the link
serialization delay
- source of delay
- fixed amount of time it takes to transmit a frame onto the wire
propagation delay
- source of delay
- variable amount of time it takes for the frame to travel btwn src and dst
de-jitter delay
- source of delay
- fixed amount of time it takes to buffer a flow of packets and then send them out in evenly spaced intervals
jitter
variance in time delays btwn packets
what is playout delay buffer
helps compensate for jitter by buffering packets and then plays them out in a steady stream on the outbound interface
voice traffic characteristics
smooth
benign
drop and delay sensitive
UDP priority
voice traffic minimum requirements
latency ≤ 150ms
jitter ≤ 30 ms
loss ≤ 1% bandwidth (30-128 Kbps)
video traffic characteristics
bursty greedy drop and delay sensitive UDP priority (unpredictable, inconsistent and bursty)
video minimum requirements
latency ≤ 200-400 ms
jitter ≤ 30-50 ms
loss ≤ 0.1-1% bandwidth (384 Kbps - >20Mbps)
what port does Real-Time Streaming Protocol use?
UDP 554
what ports does Cisco products use for voice traffic
UDP 16384-32767
data traffic characteristics
smooth/bursty
benign/greedy
drop and delay insensitive
TCP retransmits
what is FIFO
First-In, First-Out Queuing Algorithm buffers and forwards packets in the order of their arrival all packets are treated equally fastest method of queuing
what type of queuing algorithm is used by default by all interfaces except serial interfaces at E1 and below
FIFO
what is WFQ
Weighted Fair Queuing
Queuing Algorithm
applies priority to identified traffic and classifies it into flows then determines how much bandwidth each flow is allowed relative to other flows
- low bandwidth traffic flows receive preferential service while high-volume traffic flows share remaining capacity equally among themselves
what does WFQ use in the packet header to classify traffic
- source and dst IP add
- MAC add
- port number
- protocol
- ToS
limitations of WFQ
- not supported with tunneling and encryption
- no precise control over bandwidth allocation
what is CBWFQ
Class-Based Weighted Fair Queuing
- allows users to define traffic classes based on match criteria like protocols, ACLs and input interfaces
characteristics you can assign after defining a class to its match criteria
- bandwidth
- weight
- maximum packet limit
when using CBWFQ, what happens when maximum packet limit is reached?
it causes packet drop or tail drop to take effect
what is a tail drop
a router discards any packets that arrives at the tail end of a queue that has completely used up its packet-holding resources (default mode)
what is LLQ
Low Latency Queuing
Combines CBWFQ and strict PQ (priority queuing)
Allows delay-sensitive packets like voice to be sent before packets in other queues
what queuing algorithm simultaneously schedules interactive traffic to the front of a queue to reduce response times?
WFQ
what queuing algorithm is effective for large links that have little delay and minimal congestion?
FIFO
what queuing algorithm classifies traffic into different flows based on packet header addressing?
WFQ
what queuing algorithm provides support for user-defined traffic classes?
CBWFQ
what queuing algorithm applies priority, or weight, to identify traffic and classify it?
WFQ
benefits of the QoS Best-Effort Model
most scalable and only limited by available bandwidth
no special QoS mechanism required
easiest and quickest model to deploy
which QoS model provides no guarantees
Best-Effort model
which QoS model provides per-request policy admission control
IntServ
which QoS model requires no special QoS mechanisms
Best Effort
which QoS model provides many different levels of quality
DiffServ
which QoS model uses explicit end-to-end resource admission control
IntServ
Which QoS model is the most scalable?
DiffServ
what is WRED
weighted random early detection, Cisco IOS QoS mechanism
- used as part of congestion avoidance tools
- buffer management and allows TCP traffic to decrease or throttle back before buffers are exhausted
- can drop lower-priority packets before congestion occurs
what are the 3 categories of QoS Tools
- classification and marking tools
- congestion avoidance tools
- congestion management tools
In Classification and Marking Tools, what method is used to classify packets at Layer 2 and 3?
Interfaces, ACLs and class maps
In Classification and Marking Tools, what method is used to classify packets at Layer 4-7
NBAR - Network Based Application Recognition
marking a packet should be done as close to the _____ device as possible
source
In Layer 2 traffic marking, where is the CoS marking located?
In the first 3-bits of the TCI (Tag Control Information) field - called the Priority (PRI) field
In Layer 3 traffic marking, where are marks specified?
IPv4 - ToS (Type of Service)
IPv6 - Traffic Class
what is the DSCP field
Differentiated Services Code Point
6-bit field in ToS for IPv4 that allows packet marking
what is ECN field
Explicit Congestion Notification
2-bits in the ToS field for IPv4 that informs downstream routers that there is a congestion in the packet flow
DSCP values are organized into three categories
- Best-Effort (BE)
- Expedited Forwarding (EF)
- Assured Forwarding (AF)
which DSCP values category uses the AFxy formula
Assured Forwarding
- first 3-bits are used to designate class
- 4th-5th bits are used to designate drop preference
which DSCP values category has a value of 46
Expedited Forwarding
examples of trusted endpoints
IP phones, wireless APs, videoconferencing gateways and systems, IP conferencing stations and more
where can trust boundaries be established?
- at trusted endpoints
- at Layer 2 switches
- at Layer 3 switches/routers
traffic shaping
- Cisco IOS QoS software to prevent congestion
- retains excess packets in a queue and then schedules excess for later transmission over increments of time
- produces a smooth packet output rate
which detects when traffic rates reach a configured maximum rate and drops excess traffic
traffic policing
