Chap 14 - QOS (part 1) Flashcards
What are the 4 primary goals for implementing QOS?
- Expediting delivery for real-time applications
- Ensuring business continuance for business-critical applications
- Providing fairness for non-business-critical applications when congestion occurs
- Establishing a trust boundary across the network edge to either accept or reject traffic markings injected by endpoints
What are the 3 tools QOS uses to achieve it goals?
- Classification and marking
- Policing and Shaping
- Congestion management and avoidance
What are the 3 leading causes of quality issues?
- Lack of bandwidth
- Latency and jitter
- Packet loss
What are the 2 categories that network latency can be broken down into?
- Fixed latency
- Variable latency
What are the 3 Fixed latency causes?
- Propogation delay
- Serialization delay
- Processing delay
What is the 1 Variable latency cause?
Delay variation
What is Propogation Delay?
Propagation delay is the time it takes for a packet to travel from the source to a destination at the speed of light over a medium such as fiber-optic cables or copper wires.
What is the Refractive Index and which is worse, higher or lower?
- The ratio arrived at by the lack of vacuum conditions in a fiber-optic cable or a copper wire that slows down the speed of light
- The larger this value is the slower light travels
What is the speed of light?
299 Million meters per second
What is the formula for calculating speed of light through a medium?
- speed of light in a vacuum divided by the refractive index
What is the refractive index value of optical fiber?
1.5
What is the speed of light through a fiber cable with a refractive index of 1.5?
200,000,000 meters per second
What are 2 unknown variables to take into account when calculating average latency of a fiber circuit?
- Fiber does not always take the shortest path to the destination
- Components required for fiber optic cable (repeaters and amplifiers) may introduce additional delay
What is the ITU recommendation for the maximum amount of delay regardless of application type?
Should not exceed 400 ms
What is the ITU recommendation for real time traffic?
Should not exceed 150 ms
What is serialization delay?
the time it takes to place all the bits of a packet onto a link.
What is the formula for calculating serialization delay?
packet size in bits / line speed in bits per second
What is processing delay?
The fixed amount of time it takes for a networking device to take the packet from an input interface and place the packet onto the output queue of the output interface.
What 5 factors affect processing delay?
- CPU speed
- CPU Utilization
- IP Packet switching mode (process switching, software CEF, hardware CEF)
- Router architecture (centralized or distributed)
- Configured features on both input and output interfaces
What is delay variation?
The difference in the latency between packets in a single flow, aka jitter
What are 3 major factors that can help or hurt variable delay?
- Queuing delay
- Dejitter buffers
- Variable packet sizes
When causes jitter (Variable delay) ?
Caused by queueing delay experienced by packets during periods of network congestion.
As it is being experienced what 3 things does Queuing Delay depend on?
- The number and sizes of packets already in the queue
- Link speed
- Queuing mechanism.
Where are de-jitter buffers typically located?
On voice and video endpoints
What are de-jitter buffers?
Dynamic buffers on a voice or video endpoint that can mitigate jitter up to around 30 ms.
What happens if a packet doesn’t arrive in a de-jitter buffer in the 30 ms window?
It gets dropped.
What can be done to prevent jitter?
Use queuing mechanisms such as low-latency queueing (LLQ) that allow matching packets to be forwarded prior to any other low priority traffic during periods of network congestion.
When does packet loss occur?
Usually as a result of congestion on an interface.
What 4 things can be done to prevent Packet Loss?
- Increase bandwidth
- Implement QoS congestion-avoidance and congestion-management mechanism.
- Implement traffic policing to drop low-priority packets and allow high-priority traffic through.
- Implement traffic shaping to delay packets instead of dropping them
Should Traffic Shaping be used on real-time applications?
No, because it relies on queuing that can cause jitter.
What can be used to mitigate micro-bursts?
Use Microsecond or Low-burst shaping
What are the 3 QOS Implementation Models?
- Best effort
- Integrated Services (IntServ)
- Differentiated Services (DiffServ)
What is IntServ?
- Integrated Services
- Applications signal the network to make a bandwidth reservation and to indicate that they require special QoS treatment.
What is DiffServ short for and how does it work?
- Differentiated Services
- The network identifies classes that require special QoS treatment.
What 2 things does IntServ do?
- It uses Resource Reservation Protocol (RSVP) to reserve resources throughout a network for a specific application
- Provides call admission control (CAC) to guarantee that no other IP traffic can use the reserved bandwidth.
With IntServ what happens to bandwith that had been reserved but wasn’t used?
It is wasted
What is the biggest drawback when using IntServ?
All nodes, including the endpoints running the applications, need to support, build, and maintain RSVP path state for every single flow.
What was IntServ created for?
For real-time applications such as voice and video that require bandwidth, delay, and packet-loss guarantees to ensure both predictable and guaranteed service levels.
What was DiffServ designed to do?
To address the limitations of the best-effort and IntServ models.
Is DiffServ considered an end-to-end solution?
No, solution because end-to-end QoS guarantees cannot be enforced.
What are 2 differences between DiffServ as compared to IntServ?
- There is no need for a signaling protocol
- DiffServ is more scalable because there is no RSVP flow state to maintain on every single node.
What QOS model is the most popular and widely deployed?
DiffServ
What is packet classification?
It is a QoS mechanism responsible for distinguishing between different traffic streams. It uses traffic descriptors to categorize an IP packet within a specific class.
Where should packet classification be done?
At the network edge, as close to the source of the traffic as possible.
What are the 7 traffic descriptors typically used for Classification?
- Internal: QoS groups (locally significant to a router)
- Layer 1: Physical interface, subinterface, or port
- Layer 2: MAC address and 802.1Q/p Class of Service (CoS) bits
- Layer 2.5: MPLS Experimental (EXP) bits
- Layer 3: Differentiated Services Code Points (DSCP), IP Precedence (IPP), and source/destination IP address
- Layer 4: TCP or UDP ports
- Layer 7: Next Generation Network-Based Application Recognition (NBAR2)
What are the 4 most frequently used Traffic Descriptors?
Layer 2, Layer 3, Layer 4, and Layer 7 traffic descriptors
What is NBAR2, what does it do, what does it use to do it, and what does it include?
- A deep packet inspection engine
- That can classify and identify a wide variety of protocols and applications
- It uses Layer 3 to Layer 7 data
- This includes applications that dynamically assign TCP or UDP port numbers.
What are NBAR2’s two modes of operation?
- Protocol Discovery
- Modular QOS CLI (MQC)
