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ONT Notes - Intro to QoS

I’ll try to keep it a little shorter this time.

Major issues for converged enterprise networks

Available bandwidth: competition among applications
- Fixes
  - Increase bandwidth: More power!
  - Properly queue based on classification and marking: QoS
  - Compress: cRTP, TCP header compression, etc.
Delay: Lead time to get a packet to the destination
- Types of delay
  - Processing delay: routing, switch delay
  - Queuing delay: how long a frame stays in an output queue
  - Serialization delay: how long to put the frame on the wire
  - Propagation delay: the time to cross the physical medium
Jitter (delay variation): Variation is the delay
- Different delays mean different arrival times
- De-jitter buffers save up packets to reduce jitter (like the old CD writers)
- Fixes
  - More bandwidth
  - Prioritize sensitive data and forward first
  - Remark (reclassify) packets based on sensitivity
  - Enable L2 payload compression: make sure compression delay isn’t worse than the jitter
  - Use header compression
Packet loss: Packets are lost in the network somewhere
- Fixes
  - More bandwidth
  - Increase buffers space: more room for the queue on the interface
  - Provide guaranteed bandwidth: Queuing and QoS
  - Congestion avoidance
    - Random Early Detection (RED) and weighted RED (WRED) drop packets before the queue is full
    - Selective dropping is better than FIFO or LIFO dropping

QoS History

Priority queuing: gives certain data the right-of-way for transmission
Weighted Fair Queuing (WFQ): prevents small packets from waiting too long for big packets
RTP priority queuing: Gives voice packets the right-of-way
CAC: Makes sure we don’t fill up the queue or pipe with voice traffic

Implementing QoS

Step 1: Identify traffic types and requirements
- Network audit
- Business audit
- Define bandwidth requirements for each class found
Step 2: Classify the traffic
- Common classes
  - VOIP
  - Mission-critical
  - Signal traffic: for VOIP
  - Transactional application: SAP, ERP
  - Best-effort: Everything else
  - Scavenger: Crap you don’t care about like P2P and your boss’s email
Step 3: Define policies for each class
- Tasks for each class
  - Set max bandwidth
  - Set min bandwidth
  - Assign relative priorities
  - Apply congestion avoidance, congestion management, etc.

QoS Models

Best-effort: no QoS
- Scalable
- Easy
- No service guarantee: doesn’t care what you’re trying to do
- No service differentiation: all traffic is equal
Integrated Service (IntServ)
- Hard-QoS
- Uses RSVP to guarantee bandwidth through the entire path
- Requires
  - Admission control
  - Classification
  - Polices the traffic (ceiling)
  - Queuing
  - Scheduling
- Advantages
  - End-to-end resource admission control
  - Per-request policy admission control
  - Signaling of dynamic ports
- Disadvantages
  - Continuous signaling
  - Not scalable
Differentiated Services (DiffServ)
- Soft-QoS
- Configured on each hop
- Traffic is classified
- Enforces different treatment on different classes
- Defined based on business requirements
- Benefits
  - Scalable
  - Supports lots of service levels
- Drawbacks
  - No absolute guarantee of service
  - Complex configuration throughout network

QoS Implementation Methods

CLI
- Old school
- Not used any more
Modules QoS CLI (MQC)
- Step 1: class-map
- Step 2: policy-map
- Step 3: service-policy
AutoQoS
- Automatically generates classes and policies based on traffic it sees
- Super-simple
- Requires CEF, NBAR, and correct bandwidth statements
SDM QoS Wizard
- Next, next, next
- Can be used to implement, monitor, or troubleshoot QoS