Chap 14 - QOS (part 3) Flashcards
What are the 4 AF classes?
AF1, AF2, AF3, AF4
Assured Forwarding which AF class has priority - AF1 or AF4
Neither, all AF classes are treated independently and should be placed in separate queues.
In Assured Forwarding how many Drop Priorities are there?
3 three drop probabilities: low, medium, and high.
What 5 adjectives can be used to describe what EF builds?
The EF PHB can be used to build
- Low-loss
- Low-latency
- Low-jitter
- Assured bandwidth
- End-to-end service
What is the scavenger class, and what 3 types of traffic should fall into this category?
- Provides less than best-effort services. It is for apps that:
- have little or no contribution to the business objectives
- are typically entertainment-related applications
- are usually heavily rate limited or blocked entirely
What is the Class Selector marking for scavenger class?
CS1
When a PC is attached to a VOIP phone what does the phone do by default if the PC has marked its traffic?
By default it will clear the markings before sending it to the network
What CoS and DSCP markings does an IP Phone apply to its own voice traffic?
IP phones by default mark voice traffic with a CoS value of 5 and a DSCP value of 46 (EF),
Where are optimal trust boundaries?
At the IP phone or at the switch.
What are the 4 levels of QOS that a wireless controller can apply to wireless traffic?
- Platinum
- Gold
- Silver
- Bronze
When a wireless LAN is created what is the default QOS level?
Silver
On a wireless controller which QOS level should be assigned to phones?
Platinum
What are 2 traffic-conditioning mechanisms?
- Traffic Policers
- Shaping
What do Traffic Policers and Shapers do?
They classify traffic and enforce other QOS mechanisms such as rate-limiting
What do Policers do?
Drop or re-mark incoming or outgoing traffic that goes beyond a desired traffic rate.
What do Shapers do?
Buffer and delay egress traffic rates that momentarily peak above the desired rate until the egress traffic rate drops below the defined traffic rate.
What does a Shaper do if the egress traffic is below the desired rate?
Traffic is sent immediately
For incoming traffic where are Policers optimally placed?
At the edge of the network
For outgoing traffic where are Policers optimally placed?
At the edge of the network on core-facing interfaces
Where are Shapers optimally placed?
Shapers are used for egress traffic and typically deployed by enterprise networks on service provider (SP)–facing interfaces.
When is shaping useful?
Shaping is useful in cases where SPs:
- are policing incoming traffic
- have a maximum traffic rate SLA, which, if violated, could incur monetary penalties.
What 2 actions can a Policer take ?
- Drop the traffic
- Re-mark the traffic with a lower priority
If a Policer is marking down traffic what other steps should be taken?
Congestion-avoidance mechanisms, such as DSCP-based Weighted RED should be configured throughout the network to drop AFx3 more aggressively than AFx2 and drop AFx2 more aggressively than AFx1.
What is Committed Information Rate (CIR)?
The policed traffic rate, in bits per second (bps), defined in the traffic contract.
What is Committed Time Interval (Tc), and what is it measured as?
The time interval, in milliseconds (ms), over which the committed burst (Bc) is sent.
What is Committed Burst (Bc), how is it measured, and what is it also known as?
- The maximum amount of traffic that can be sent within a Tc.
- Measured in bytes
- aka the maximum size of the CIR token bucket
What is a Token?
A single token represents 1 byte or 8 bits.
What 3 actions can a traffic conditioning mechanism take if there aren’t enough tokens in the bucket?
- Drop the traffic
- Remark the traffic
- Buffer the traffic
Why is the Bc value recommended to be larger than or equal to the largest possible packet?
Otherwise, there will never be enough tokens in the token bucket for larger packets, and they will always exceed the defined rate.
For a single-bucket algorithm what happens to the accumulating tokens if the Bucket is filled to capacity?
They are dropped.
In a single token bucket algorithm what does conforming traffic mean?
When there are enough tokens in the bucket to send the traffic onto the network.
What is the recommended range for interpacket delay for non-VOIP traffic?
8 msec to 125 msec
What is the recommended range for interpacket delay for voice traffic?
8 msec to 10 msec
What are 3 different policing algorithms?
- Single-rate two-color marker/policer
- Single-rate three-color marker/policer (srTCM)
- Two-rate three-color marker/policer (trTCM)
How many buckets does the Single-rate Two-color Marker/Policer have, what are the 2 colors, and what is done with packets?
- Single token bucket
- Traffic either Conforms to or Exceeds CIR
- Dropping or Marking Down can be done in either state
For Marker/Policers what are the 2 options for rate and 3 options for color?
- Single rate means a single CIR
- Two-rate means a CIR and a Peak Information Rate (PIR)
- Three Color means green for conforming traffic, yellow for exceeding but transmitted and red is exceeding and dropped
For the srTCM how is traffic classified, and what action can be taken?
- Two token buckets
- Traffic classified as conforming, exceeding, or violating the CIR
- Dropping or marking down can be done in each of the 3 states
For the Single-rate Three-color Marker/Policer (srTCM) where does the second bucket get its tokens and what are they used for?
During slow traffic times excess tokens drop to the second bucket and can be used for temporary bursts that exceed CIR
In the Single-rate Three-color Marker/Policer (srTCM) what is the variable assigned to the tokens in the second bucket and what value is assigned to them?
- Te
- The number of tokens in the Be bucket
What are the three colors and what do they mean?
- Conforming or Green, traffic under the Bc
- Exceed or Yellow, over Bc but under Be, can be dropped or marked down
- Violate or Red, usually dropped but optionally could be marked down
In the Single-rate Three-color Marker/Policer (srTCM) why do the exceeding and violating rates vary?
Because they rely on random tokens spilling over from the Bc bucket into the Be.
What are 3 advantages of the Single-rate Three-color Marker/Policer (srTCM)?
- Fewer TCP retransmissions
- More efficient for bandwidth utilization
- Perfect policer for the AF classes AFx1, AFx2, and AFx3
On a wireless network what are the DSCP values and 802.1P values for each of the 4 levels of QOS?
- Platinum - DSCP 46 (EF) (44) and cs5
- Gold - DSCP AF41 (34) and cs4
- Silver - DSCP 0 and cs0
- Bronze - DSCP AF11 (10) and cs1
Formula to calculate Tc
Tc = (Bc / CIR ) × 1000.
Formula to calculate Bc
Bc = (Tc / 1000) x CIR
What is the srTCM?
of buckets:
Name of buckets:
6 variables:
Recommended for:
- # of buckets: 2
- Name of buckets: Bc bucket, Be bucket
- 6 variables
- CIR - the policed rate
- Bc - max size of the CIR token bucket in bytes
- Be - Excess burst size, max size of the excess token bucket in bytes
- Tc - number of tokens in the Bc bucket
- Te - number of tokens in the Be bucket
- B - incoming packet length in bits
- Recommended for AF classes: AFx1, AFx2, and AFx3
