QOS - Policers and Shapers Flashcards
What do Policers do?
They drop or re-mark incoming or outgoing traffic that goes beyond a desired traffic rate.
What do Shapers do?
They buffer and delay egress traffic rates that momentarily peak above the desired rate until the egress traffic rate drops below the defined traffic rate.
Where is the best place to put Policers for incoming traffic?
At the edge of the network to keep traffic from wasting valuable bandwidth in the core of the network.
Where is the best place to put Policers for outgoing traffic?
On core-facing interfaces on network edge devices.
What is a downside to using a Policer?
It causes TCP retransmissions when it drops traffic.
Where is a typical place to put a Shaper?
Typically deployed by enterprise networks on service provider– facing interfaces if the service provider is policing or if there are monetary penalties when a certain rate is exceeded
What 2 actions does a Policer take?
- Drops traffic
- Marks down traffic with a lower priority
What congestion avoidance method should be used when a Policer marks down traffic?
WRED
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)
The time interval, in milliseconds (ms), over which the committed burst (Bc) is sent.
What is Committed Burst Size (Bc)?
The maximum size of the CIR token bucket, measured in bytes, and the maximum amount of traffic that can be sent within a Tc.
What is a Token?
A single token represents 1 byte or 8 bits.
What is a Token Bucket?
A bucket that accumulates tokens until a maximum predefined number of tokens is reached.
What rate are Tokens added to the Token Bucket?
The CIR
What 3 actions can be taken if there aren’t enough tokens in the token bucket?
- Buffer the packets while waiting for more tokens to be added to the bucket (shaping)
- Drop the packet (policing)
- Mark down the packet (policing)
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)
When defining Policers what are the 3 colors?
- Conform
- Exceed
- Violate
When defining Policers what are the rates?
- Single-rate is CIR
- Two-rate is CIR and PIR (Peak Information Rate)
What is a Single-rate Two Color Policer
A single bucket policer algorithm with a single rate (CIR) and 2 states (colors) of Conform or Exceed.
What is a Single-rate Three Color Policer?
A two token bucket policer algorithm with a single rate (CIR) and 3 states (colors) of Conform, Exceed, or Violate
In a Two-bucket algorithm what is the 2nd bucket for?
- During quiet times any left over tokens in the first bucket are moved to the second bucket
- Tokens in the second bucket are used for temporary bursts that may exceed the CIR
- Tokens in the second bucket are referred to as Burst Exceeded (Be)
What is Burst Exceeded (Be)?
The maximum number of bits that can exceed the Bc burst size.
What are the 6 parameters used to meter traffic by Single-rate Three-color Marker/Policer?
- CIR
- Bc
- Be
- Bc Bucket Token Count (Tc)
- Be Bucket Token Count (Te)
- Incoming packet length (B)
What policer should be used with AF classes?
Single-rate Three-color policer but only if the actions for each color differ.
What makes the srTCM variable and somewhat unpredictable?
It relies on tokens spilling over into the second bucket which may or may not happen.
What is the Two-rate Three Color Marker/Policer?
A two token bucket policer algorithm with two rates (CIR and PIR) and three colors (Conform, Exceed, Violate)
How is the two bucket algorithm in trTCM different than in srTCM?
- Each bucket has its own tokens - Bc bucket filled with CIR tokens and Be bucket filled with PIR tokens
- Logic differs - PIR tokens checked first for Violate, then Bc Bucket for Exceed, lastly Conform
