1. Internet Quality of Service
Lecture 9

2. Puzzle
Assume A & B
A flips a coin. If the result is heads, A multiplies
2 90 digit prime numbers, and if the result is tails,
A multiplies 3 60 digit prime numbers
A gives B the result of the multiplication
B calls the toss
A gives B the numbers
How can a toss be called over the phone (without requiring trust)?

3. Feedback & Grades Presentations need more technical preparation
More preparation by non-presenting students (for questions & discussions)
Grades will be sent out later this week
First exam soon!

4. Motivation for QoS? Real-time applications Convergence in the Internet
Users willing to pay for better service

5. QoS Parameters
Bandwidth
Delay
Delay jitter
Loss
Goodput

6. Types of QoS Absolute Relative 100Kbps, 5ms delay bound, 2% loss rate
Olympic model (gold, silver, bronze)
Gold better than silver, etc.
Gold gets k times bandwidth as silver

7. Approaches Integrated Services Model Differentiated Services Model
Intserv
Can provide per flow QoS
Problem?
Differentiated Services Model
Diffserv
Can provide aggregate QoS
Newer approaches
Core-stateless schemes

8. Elements of QoS Flow specifications Routing Reservations
Admission control
Packet scheduling

9. Integrated Services Approach
Guaranteed service
Controlled load service
Best effort service

10. Guaranteed Service Similar to a leased line
Hard guarantees on bandwidth and maximum delay
Addressed toward critical applications
Advantages?
Disadvantages?

11. Controlled load service
Service provided equivalent to that of an “unloaded” network
Admission control necessary
No hard guarantees for bandwidth, delay, or loss!
Advantages?
Disadvantages?

12. Best-effort Service Currently supported in the Internet
No guarantees whatsoever
Advantages?
Disadvantages?

13. Specifications and Buckets
Leaky Bucket r
Token Bucket r B

14. RSVP: Resource Reservation Protocol
Signaling protocol used to convey specifications of flow and desired quality of service
Token bucket specification used in both Tspec (flow characteristics) and Rspec (desired QoS characteristics)

15. Scheduling Priority Scheduling Round-robin Weighted round-robin
Weighted fair queuing
WRR with WFQ spread

16. Recap Quality of Service Integrated Services
Guaranteed
Controlled
Best-effort
Traffic shaping/policing with buckets
Scheduling

17. Integrated Services Disadvantages
Per-flow state and processing at every router in the network
Not scalable with increasing number of flows (in transit routers) and line-speeds

18. Differentiated Services Architecture
Goal: To provide quality of service while ensuring scalability with increasing number of flows and line-speeds
Approach: Have aggregate behaviors at the core with any per-flow state maintenance and processing done at the edges

19. Diffserv Approach Other autonomous domains Other autonomous domains
Bandwidth
Broker
Admission control
Edge Router:
Policing, Shaping, & Marking
Per-flow state and processing
Core Router:
Forwarding based on PHBs
No Per-flow state and processing
Other
autonomous
domains
Other
autonomous
domains

20. Diffserv Service Classes
Premium Service
Assured Forwarding Service
Best effort Service

21. Diffserv Classes - Premium
Strict admission control
Can still experience delays if a router has multiple incoming links
Policing and Shaping done at the ingress points
Similar to controlled-load service with no bursts

22. Diffserv Classes – Assured Forwarding
Loose admission control
More efficient
Delays and drops possible during congestion
Policing at the ingress points (no shaping)
Non-conforming packets let through without marking

23. Intserv and Diffserv Intserv at the access and edge networks
Diffserv at the core and transit networks
RSVP can still inter-operate with the diffserv architecture
When request arrives at ingress point, redirected to Bandwidth broker and forwarded to egress router

24. Diffserv Architecture
Disadvantages
No per-flow processing and hence no per-flow fine-grained QoS
Example: no per-flow fairness possible in the diffserv architecture

25. Core-stateless QoS
Goal: To provide per-flow fine grained QoS without maintaining any per-flow state at core-routers
QoS Parameter: Rate fairness (delay fairness, bandwidth guarantees also possible)
Approaches: CSFQ (Core-stateless Fair Queuing), Corelite

26. CSFQ
Edge router labels packets belonging to a flow k with the rate of the flow rk
Core router measures the total incoming traffic A during an epoch (say 1 second)
If A > C (capacity), congestion – need to drop packets
If A < C (capacity), no congestion – no need to ensure fairness

27. CSFQ - continued If A > C,
Drop every incoming packet with a probability of:
1 – fs/rk
Example: Let C = 10, # of flows = 3, rates = (5,5,2)
fs = 4 (how?)
Drop probabilities:
Flow 1: 1/5, Flow 2: 1/5, Flow 3: 0
Achieved rates:
Flow 1: 4, Flow 2: 4, Flow 3: 2

28. CSFQ - continued Challenge: how do you compute fs?
If # of flows known, fs = C/n
CSFQ:
Keep track of “accepted” packets during last epoch F (F <= A)
If F>C
fsnew = fsold * C/F

29. Recap Intserv Diffserv Core-stateless Per-flow QoS
Per-flow state/processing – not scalable
Diffserv
Coarse QoS
No per-flow state/processing at all routers
Core-stateless
Scalable network model
Per-flow QoS achieved

30. Puzzle
Two twins A & B
A always speaks the truth, and believes all true propositions (say 2+2=4) to be true, and all false propositions (say 2+2=3) to be false
B always lies, and believes all true propositions to be false, and all false propositions to be true
You meet one of the twins. How many questions do you need to identify which twin he is?