1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 38

3. Real Time Applications
Require “deliver on time” assurances
Must come from inside the network
Example application (audio)
Sample voice once every 125µs
Each sample has a playback time
Packets experience variable delay in network
Add constant factor to playback time: playback point
Sampler
Microphone
Buffer
A D
D A
converter
Speaker

4. Real Time Application Example: Live-Source Audio
One byte per 125 microseconds
Delay between speaker & listener should be fixed
Each packet has “playback time” based on delay
But network delay is variable

5. Real Time Application Example: Live-Source Audio
Solution
Fix delay at 95-99% mark on distribution
i.e. delay by which 95-99% of packets arrive
Some packets (1-5%) still lost, but tolerable
Early packets are buffered until playback time

6. Playback Buffer Packet arrival Packet generation Playback
Sequence number
Buffer
Network
delay T
ime

7. Example Distribution of Delays
90%
97%
98%
99% 3 2
Packets (%) 1 50
100
150
200
Delay (milliseconds)

8. Taxonomy of Applications

9. Taxonomy Applications Real time Elastic T olerant Intolerant
Interactive
Interactive
Asynchronous
bulk
Adaptive
Nonadaptive
Rate-adaptive
Nonadaptive
Delay-
Rate-
adaptive
adaptive

10. Old data are useful, just not as good
Elastic Applications
Old data are useful, just not as good
Interactive applications (e.g. ssh, rlogin, telnet)
Bulk transfers (e.g. http, ftp)
Asynchronous interaction (e.g. )

11. Real-time Applications
Intolerant to loss
Tolerant to loss
Non-adaptive: quality fixed (sample delay and throughput, then assume worst-case variation)
Adaptive: adjust quality to current parameters
Delay adaptive: adjust to achieved delay
Rate adaptive: adjust to achieved throughput

12. Integrated Services : Resource Reservation Protocol (RSVP)

13. Integrated Services Service Classes Guaranteed Controlled-load
Mechanisms
Signaling protocol
Admission control
Policing
Packet scheduling

14. Three Classes of Service
Guaranteed: specified maximum delay
Controlled load
For loss-tolerant, adaptive applications
Network appears lightly loaded (via weighted fair queuing)
Best effort

15. Mechanisms to Support Integrated Services
Flow specification: delay requirements for flow
Admission control: network decision to support flow
Resource reservation: protocol for exchanging flowspecs, performing admission control, etc.
Packet classification: mapping packets to flows
Packet scheduling: forwarding policy

16. Integrated Services Example
Flowspec: 100 ms guaranteed to
Reservation: spec travels down path for approval
Delay guarantee approved by all routers, so admitted
My packets marked as guaranteed
EXAMPLE policy: guaranteed packets sent first

17. Flow Specification Components
RSpec: describes service requested from network
Controlled-delay: level of delay required
Guaranteed/predictive: delay target
TSpec: describe flow traffic characterization
Characterized by token bucket filter
Average throughput r
Maximum buffering requirement B

18. Flowspec Rspec: describes service requested from network
Controlled-load: none
Guaranteed: delay target
Tspec: describes flow’s traffic characteristics
Average bandwidth + burstness: token bucket filter
Token rate r
Bucket depth B
Must have a token to send a byte
Must have n tokens to send n bytes
Start with no tokens
Accumulate tokens at rate of r per second
Can accumulate no more than B tokens

19. Flows with Equal Average Rates but Different Token Bucket Descriptions

20. Unrealistic Expectations …
Be explicit about requirements
“In general, it is good to be as explicit about the bandwidth needs of an application as possible...”
How much bandwidth does your browser need? What about buffering? Please be explicit!

21. Each byte needs a token in order to pass
Token Bucket Filters
r tokens/sec
Token Bucket, Capacity B
Each byte needs a token in order to pass
Data
Dropping filter: drops packets if token is not available
Buffered filter: buffers data until tokens become available

22. Token Bucket
Given a rate r and a finite data trace, what is minimum bucket capacity B such that the filter has no effect?
Simply observe the maximum buffer size
Does this approach work? Why?
For buffer of size b, the number of empty buffer positions is equal to the number of tokens in an (r,b) token bucket filter

23. Token Bucket

24. Per-Router (Router-Centric) Mechanisms
Admission Control
Decide if a new flow can be supported
Answer depends on service class
Not the same as policing
Packet Processing
Classification: associate each packet with the appropriate reservation
Scheduling: manage queues so each packet receives the requested service

25. Reservation Protocol Called signaling in ATM
Proposed Internet standard: RSVP
Consistent with robustness of today’s connectionless model
Uses soft state (refresh periodically)
Designed to support multicast (can specify number of speakers)
Receiver-oriented
RSVP uses two messages
PATH transmitted by source every 30 sec
Destination responds with RESV message
Requirements must be merged for multicast

27. RSVP – Receiver Oriented Layered Multicast
RSVP addresses disparate delay requirements
Can different rates be supported?

28. RSVP - Solution Break data into ordered layers
Deliver subset of layers to each receiver

29. Integrated Services – Scalability Issue
Per-flow monitoring at routers requires per-flow state information at routers
An OC-48 link can carry 39,000 audio streams at 64 Kbps each

30. Differentiated Services
Problem with IntServ: scalability
Goal: use small number of classes to provide scalable solution
Idea: support two classes of packets
Premium (like first-class)
Best-effort, regular (like bulk mail)
Diffserv proposes 6 bits of IP ToS field (64 classes)

31. Differentiated Services Questions
Who is allowed to set the premium bit?
Typically an ISP
What about an individual customer or application?
How do routers react to such a classification?
IETF has specified per-hop behavior

32. Differentiated Services
Mechanisms
Packets: ‘in’ and ‘out’ bit
Edge routers: tag packets
Core routers: RIO (RED with in and out)
P(drop)
1.0
MaxP
Min in
Max
out A
vgLen

33. Differentiated Services
Expedited forwarding
Per-hop behavior
Must strictly limit use
Mechanisms
Strict priority
Weighted fair queuing (WFQ) with large weights for expedited forwarding

34. Differentiated Services
Assured forwarding
Per-hop behavior
Like RED but with “in” and “out” packets (RIO)
Does not reorder packets
Weighted RED generalizes to greater than two (2) classes
Edge routes can mark packets as “in” or “out”

35. QoS in ATM Similar to RSVP Five service classes
Constant bit rate (CBR)
Variable bit rate (VBR) – real-time
Variable bit rate (VBR) – non-real-time
Unspecified bit rate (UBR) – like best effort
Available bit rate (ABR) – like best effort + ATM’s congestion control

36. RSVP versus ATM (Q.2931) RSVP Receiver generates reservation
Soft state (refresh/timeout)
Separate from route establishment
QoS can change dynamically
Supports receiver heterogeneity

37. RSVP versus ATM (Q.2931) ATM Sender generates connection request
Hard state (explicit delete)
Concurrent with route establishment
QoS is static for life of connection (except ABR)
Uniform QoS to all receivers