Defending against lameness since 2008

Out of the three QoS models Differentiated Services (DiffServ) is the most used due to its scalability and ability to classify multiple levels of service.  The only real drawbacks of DiffServ is that it doesn’t have an absolute service guarantee like IntServ and its configuration can get quite complex.

DiffServ uses a 6-bit DSCP (Differentiated Services Code Point) to mark packets so they can be treated with different levels of service throughout the network. Basically DSCP is a code used to mark the traffic using the first six bits of the ToS byte inside of a packet. Most people attribute DSCP to a box of crayons and you can mark or color the packet for classification. DSCP looks at those 6-bits in two different sections, the first three which are the priority bits and the second three bits which are the drop preference.

Priority Bits (bits 1 through 3 of the ToS byte)

1. 000 = Default
2. 001 = Assured Forwarding (AF1)
3. 010 = Assured Forwarding (AF2)
4. 011 = Assured Forwarding (AF3)
5. 100 = Assured Forwarding (AF4)
6. 101 = Expedited Forwarding (EF)

Expedited Forwarding is what you will be using for your voice and some of your video traffic. It ensures a minimum departure rate, guarantees bandwidth and polices the bandwidth (the class is not allowed to exceed the guaranteed amount of bandwidth).

Assured Forwarding guarantees bandwidth and allows access to extra bandwidth if available. Each AFK classes uses three DSCP values and is independently forwarded with its own guaranteed bandwidth. The first two bits of the drop preference determines the drop probability of the packet. The lower the drop probability the better.

Drop Probability (bits 4 and 5 of the ToS byte)

1. 01 = Low (AF11)
2. 10 = Medium (AF12)
3. 11 = High (AF13)

The IntServ, also known as ‘Hard QoS’, model was introduced to guarantee predictable network behavior for applications that require consistent, dedicated bandwidth for acceptable quality. Because IntServ reserves the bandwidth for the applications, no other traffic can use that bandwidth. IntServ guarantees bandwidth, delay and packet-loss rates from end-to-end. This guarantee ensures predictable service levels for mission-critical applications. IntServ uses RSVP (Resource Reservation Protocol) to reserve the required network resources.

Some advantages of IntServ are: Multiple Service Levels, Intelligent queueing. dynamic port signaling and end-to-end guarantees. Some disadvantages of IntServ are continuous signaling and it is not scalable for large networks.

1. EF - Expedited Forwarding
2. AF - Assured Forwarding
3. CQ - Custom Queueing
4. PQ  - Priority Queueing
5. RTP - Real-Time Transport Protocol
6. FIFO - First In First Out
7. CEF - Cisco Express Forwarding
8. CDP - Cisco Discovery Protocol
9. DE - Discard Eligibility (Frame-Relay)
10. CLP - Cell Loss Priority (ATM)
11. EXP  - Experimental Bits (MPLS)
12. PDLM - Packet Description Language Module
13. HSSI - High Speed Serial Interface

1. What is the ’best’ solution for reducing delay on a link? increase the bandwidth of the link
2. What are three characteristics of converged network traffic? constant small-traffic flow, time sensitive packets, and brief outages are unacceptable
3. How much one-way delay can a voice packet tolerate? 150ms
4. Which transport layer protocol is used for voice traffic? UDP
5. What are three advantages of using MQC? reduction in configuration time, ability to apply one policy to multiple interfaces, and seperation of classification from policy definition
6. Which QoS model is least scalable? Integrated Services
7. What three QoS mechanisims work together to provide a set of complete integrated services? Weighted Random Early Detection, Weighted Fair Queueing, Resource Reservation Protocol
8. What is the most important advantage of the DiffServ QoS model? High Scalability
9. Services are provided to which DiffServ entitity? Classes of Traffic
10. How many bits is the DSCP field of the IP Header? 6
11. If DSCP were respectively set to 101000, 011000, 111000 and 001000, which packet would have the greatest probability of timely forwarding? 111000
12. Which QoS mechanism is used on both input and output interfaces? Traffic Policing 

• WFQ - Weighted Fair Queueing
• CBWFQ - Class-Based Weighted Fair Queueing
• LLQ - Low-Latency Queueing
• WRED - Weighted Random Early Detection
• ERP - Enterprise Resource Planning
• MQC - Modular Command-Line Interface
• QPM - CiscoWorks QoS Policy Manager
• NBAR - Network-Based Application Recognition
• Best-Effort - No QoS is applied to packets
• IntServ - Applications signal to the network that they require special QoS
• DiffServ - The network recognizes classes that require special QoS
• CAC - Call Admission Controls
• RSVP - Resource Reservation Protocol
• DSCP - Differentiated Services Code Point
• PHB - Per-Hop Behaviors
• CoS - Class of Service
• ToS - Type of Service

So yesterday I decided to start studying for the Quality of Service (642-642) exam which is in the Cisco Certified Voice Professional (CCVP) certification track. I will be using the Cisco Press books and a Global Knowledge student guide which I believe was provided at a Cisco approved CCVP boot camp. I had firewall experience when I started down the CCSP path last year but I have zero VoIP experience so this test should provide much more of a challenge when it comes to grasping the technology. As with every other certification process there is a plethora of acronyms and definitions that one must stockpile in order to be successful when it comes examination time.  My goal with this post is to document some of the things I learned and in the process help me to remember them by actually typing them out and having a quick reference for later use.

First, the four major problems facing coverged networks:

• Bandwidth Capacity
• End-to-End Delay (both fixed and variable) - Delay is the time it takes for a packed to reach the recipient after being transmitted by the sender.
• Variation of Delay (also known as jitter) - Jitter is the difference in the total end-to-end delay values of two voice packets.
• Packet Loss

The following are types of delay:

• Processing delay - The time it takes for a router to receive a packet on the input interface and place the packet into the output queue.
• Queueing delay - The time a packet stays in the output queue.
• Serialization delay - The time it takes to place a frame on the physical transport medium.
• Propagation delay - The time it takes to transmit a packet.

Things you can do to reduce delay:

• Increase Link Capacity
• Prioritize Delay-Sensitive Packets
• Compress Payload
• Header Compression