1. Internet Quality of Service

2. Feedback & Grades
More preparation by both presenting and non-presenting students (for questions & discussions)
More examples to explain concepts
Grades for presentation will be sent out by Friday

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

4. QoS Parameters
Bandwidth
Delay
Delay jitter
Loss
Goodput

5. 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

6. 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

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

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

9. 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?

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

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

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

13. QoS Routing
Determine a path through a network that can satisfy requirements
Bandwidth a min-parameter => determining any one path simple
Shortest widest vs. Widest shortest
Delay an additive-parameter => cost based routing

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. Admission Control & Scheduling
Can a router accommodate the new incoming request without violating service to existing flows it serves?
If yes, admit and appropriately configure scheduling algorithm
If not, reject

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

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. 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

27. Introduction Main Idea: Goals:
- Achieve fair bandwidth allocations at the router without the implementation complexity usually associated with it.
Goals:
- Achieve fair allocation close to Fair Queueing and comparable or better than RED and FRED under most scenarios.
- Reduce complexity by not having the core node maintain per flow state.
- Approximate weighted FQ.

28. FIFO queueing with Drop Tail
SERVER
Disadvantages:
Pushes congestion control out to end hosts (TCP)
Introduces global synchronization when packets are dropped from several connections

29. Fair Queueing Disadvantage:
Need to perform packet classification and maintain state and buffers on per-flow basis and perform operations on per-flow basis

30. Definitions
Island of routers – a contiguous portion of the network with well defined interior and edges.
Edge Router – computes per-flow rate estimates and labels the packets with these estimates.
Core Router – uses FIFO queueing and keeps no per-flow state, employs a probabilistic dropping algorithm that uses the packet label and its own measurement of aggregate traffic.
Stateless – absence of per-flow state at the core routers.

31. Island of Routers

32. CSFQ
In an island of routers, edge routers compute per-flow rate estimates and label the packets with these estimates.
Core routers use FIFO queueing and keep no per-flow state, they employ a probabilistic dropping algorithm based on packet labels and own aggregate traffic estimates.
Bandwidth allocations using this method are approximately fair.
Core routers keep no per-flow state and avoid using complicated packet scheduling and buffering algorithms, hence are easier to adopt.

33. CSFQ
Assume that flow i has arrival rate ri(t) and the fair rate is a(t).
If ri(t) < a(t), all of its traffic is forwarded.
If ri(t) > a(t), then a fraction (ri(t) - a(t))/ ri(t) will be dropped; each packet of the flow is dropped with probability (1-a(t)/ri(t)). Thus the output rate of any flow i will be max(ri(t) ,a(t)).

34. CSFQ
The problem now becomes how to calculate the flow rate ri(t) values and the fair rate a(t), without keeping per flow state in the core routers.
Flow rates ri(t), are calculated at edge routers which keep per flow state and then insert the rate value inside the packet header of packets belonging to that flow.

35. CSFQ
To estimate the fair rate a(t), an iterative procedure is used: core routers estimate aggregate arrival rate A and the aggregate rate of accepted traffic F (arrival rate – dropped packets).
Based on these, the fair rate a is computed periodically as:
- if there is no congestion (A<=C where C is the link’s capacity), then a is set to the maximum ri(t)
- if the links are congested, then anew = aold*C/F

36. Source: Network Reading Group, Stoica
CSFQ - Example
Assume we have two flows f1 and f2, with rates r1 = 20 and r2 = 30
and the link’s capacity is C = 30. Initially let’s say that only r1 is active
and the link is not congested, so a1 = 20. Then r2 becomes active.
Since no packets were dropped, F = 50.
Since A = 50>C, a2 = a1* C/F = 20 * 30/50 = 12
Therefore, for f1 (1-12/20 = 40%) of its packets are dropped while for f2
(1-12/30 = 60%) of its packets are dropped and F = = 24
Since A>C, a3 = a2* C/F = 12 * 30/24 = 15
Now F = 30, and a4 = a3* C/F = 15 * 30/30 = 15. Therefore, a has
converged to the right fair rate.
Source: Network Reading Group, Stoica

37. 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

38. MPLS CoS (class of service) mechanism for the Internet
Addresses speed, scalability, QoS (quality of service), and traffic engineering
IETF defined framework that provides for efficient designation, routing, forwarding, and switching of traffic flows

39. MPLS – key functions
mechanisms to manage traffic flows of various granularities – machines, flows, AS
independent of layer 2 and layer 3 protocols
means to map IP addresses to simple fixed length labels
interfaces to existing protocols like RSVP and OSPF
supports IP, ATM, and frame-relay

40. MPLS LSPs LSP – label switched path
In MPLS, data transmission occurs on label switched paths
LSP – a sequence of labels at each and every node along the path from the source to the destination
LSPs established either before data transmission begins (control-driven) or upon detection of data (data-driven)

41. MPLS LSPs (contd.)
Labels are distributed using LDP (label distribution protocol)
Each data packet in an MPLS network carries a label
High speed switching of data becomes possible as hardware can switch based on labels

42. Routers in MPLS LERs – label edge routers
Operates at the edge of the network
Responsible for establishing LSPs
Adds/removes labels as traffic enters/leaves network
LSRs – label switching routers
High speed core router device
Participates in establishing LSPs
High speed switching of data using labels

43. Forward Equivalence Class (FEC)
Class of packets that share same transport requirements
All packets in a class treated the same
Each LSR maintains a label information base (LIB)
LIB comprises of FEC-to-label bindings

44. Labels
A label identifies the path a packet should traverse within an MPLS network
The label is typically carried in the layer 2 header or in a layer 2.5 header
Label values have local significance only
FR DLCIs (data link channel identifiers) or ATM VPIs/VCIs can be used as labels directly

45. Label Bindings Labels are bound to FECs based on some policy like
destination unicast routing
traffic engineering (RSVP-TE, CR-LDP)
multicast
virtual private network (VPN)
QoS

46. Label Creation & Signaling
[iec.org]

47. 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?