1. ECE544: Communication Networks-II, Spring 2018
D. Raychaudhuri
Lecture 8 (QoS)
Includes teaching materials from D. Raychaudhuri, L. Peterson

2. Today’s Lecture Congestion control in best effort networks
Basic principles & mechanisms
FQ, WFQ, congestion feedback, TCP, RED
Quality-of-service (QoS)
Mechanisms (traffic shaping, admission control, reservation, priority queuing)
RSVP Intserv and Diffserv, RIO
Comparison to ATM (CBR, VBR; ABR)

3. Congestion Control & QoS in Packet Networks
Congestion control – reactive methods used in best effort networks
Packet scheduling at network nodes
Feedback congestion control
End-to-end
Hop-by-hop
QoS control – proactive methods used for premium or guaranteed services:
Source traffic shaping & policing at entry points
Priority queuing and packet drop at routers
End-to-end reservation and admission control

4. Network Congestion All networks have saturating throughput
Reduction in performance beyond max capacity
Need to keep input load below G0
Also must avoid unstable equilibrium point in overload region
Unstable network load
Capacity Limit
Smax
Overload
region
Traffic
margin
Thru
Congestion control policies
Normal operating
Point (G0)
Stable network load lines
with congestion control
Offered Traffic (G)

5. Queue Scheduling A queue scheduler employs 2 strategies:
Scheduling discipline: Which packet to serve (transmit) next
Drop policy: Which packet to drop next (when required)

6. FIFO Queuing FIFO:first-in-first-out (or FCFS: first-come-first-serve)
Arriving packets get dropped when queue is full regardless of flow or importance - implies drop-tail
Important distinction:
FIFO: scheduling discipline
Drop-tail: drop policy

7. Fair Queuing Main idea:
maintain a separate queue for each flow currently flowing through router
router services queues in Round-Robin fashion

8. FQ illustration Flow 1 Flow 2 I/P O/P Flow n
Variation: Weighted Fair Queuing (WFQ)

9. Some Complications Packets are of different length
We really need bit-by-bit round-robin (RR)
FQ simulates bit-by-bit RR
Not feasible to interleave bits!

10. Bit-by-bit RR
Single flow: suppose clock ticks when a bit is transmitted. For packet i:
Pi: length, Ai = arrival time, Si: begin transmit time, Fi: finish transmit time. Fi = Si+Pi
Fi = max (Fi-1, Ai) + Pi
Multiple flows: clock ticks when a bit from all active flows is transmitted, that is, the clock advances by one tick when n bits are transmitted (assuming n flows)
calculate Fi for each packet
transmit packet with earliest Fi
RR is only simulated, packet in transmission is not interrupted.
If n flows have data to transmit, each gets 1/nth bandwidth.

11. Bit-by-bit RR Fi = max (Fi-1, Ai) + Pi
Start with A(*,*)=0 (all pkts arrive at T=0)
P(1,1) = 2
P(1,2) = 1
P(1,3) = 1
F(1,1) = 1
F(1,2) = 1.5
F(1,3) = 2
P(2,1) = 3
P(2,2) = 2
F(2,1) = 1.5
F(2,2) = 2.5
Flow 1
Flow 2
Pkt 2-2=2 units
Pkt 1-3=
1 unit
Fi = max (Fi-1, Ai) + Pi
Pkt 1-2=
1 unit
Pkt 2-1=3 units
Pkt 1-1=2 units
Channel clock -
1-1
2-1
1-2
1-3
2-2

12. Weighted Fair Queuing (WFQ)
Weighted Fair Queuing (WFQ): assign a weight to each flow
Assume transmitting wq bits each time the router serves queue q (simulate in packet scheduling)
Control the percentage of the link’s bandwidth that a flow will get
The bandwidth that flow q gets (n active queues sending data):
FQ is a special case of WFQ with a weight of 1 for each queue

13. Congestion Control and Congestion Avoidance
TCP’s “blind” approach:
Detect congestion (loss) after it happens and back off on offered rate
Increase load trying to maximize utilization until loss occurs
Source
Rate
(bps)
Congestion detected
(via packet loss)
Time-out
Alternatively:
We can try to predict congestion and reduce rate before packets start being discarded
This is called congestion avoidance

14. Congestion Control via Router Feedback
Router has unified view of queuing behavior
Routers can distinguish between propagation and persistent queuing delays
Routers can decide on transient congestion, based on workload

15. Solving the Full Queues Problem
Router monitors the load
Drop (or mark) packets before queue becomes full (early drop)
Intuition: notify senders of incipient congestion
Simple example:
If qlen > drop level, drop (or mark) each new packet with a fixed probability p
Does not control misbehaving users

16. Random Early Detection (RED)
Motivation:
High bw-delay flows have large queues to accommodate transient congestion
TCP detects congestion from loss - after queues have built up and increase delay
Aim:
Keep throughput high and delay low
Accommodate bursts

17. Random Early Detection (RED)
Detect incipient congestion, allow bursts
Keep power (throughput/delay) high
keep average queue size low
assume hosts respond to lost packets
Avoid window synchronization
randomly mark packets, instead of dropping
Avoid bias against bursty traffic
Some protection against ill-behaved users

18. RED Algorithm Maintain running average of queue length
If avg < minth do nothing
Low queuing, send packets through
If avg > maxth, drop all packets
Protection from misbehaving sources
Else randomly drop (or mark) some packets in a manner proportional to queue length
Notify sources of incipient congestion

19. RED Operation If AvgLen <= MinThreshold
Queue the packet
If MinThreshold < Avglen < MaxThreshold
Calculate probability P
Drop the arriving packets with probability P
If Avglen >= MaxThreshold
Drop the arriving packet
AveLen = (1-W) x AveLen + W x SampleLen
Max threshold
Min threshold
Average queue
length
minthresh
maxthresh
MaxP
1.0
Avg length
P(drop)
Count:# of newly arriving packets that have been queued (not dropped) while AvgLen has been between the two thresholds
Count increases, P increases
Make drop more evenly distributed (Avoid bias against bursty traffic)

20. Explicit Congestion ControlV
ersion
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable) 4 8 16 19 31
Data
Router can signal the congestion by marking packets instead of dropping using RED
Set the ECN bit (bit 7 of the IP TOS field) in IP header
The destination copies the ECN bit into the ACK sent back to the source
The source TCP responds to the ECN bit set in the same way as a packet drop
ToS field => now used for DiffServ and ECN
Bits 0-5: Differentiated Services Code Point (DSCP)
Bit 6: ECN-capable
Bit 7: ECN

21. Quality of Service Outline Realtime Applications Integrated Services
Differentiated Services

22. Realtime Applications
Microphone
Speaker
Sampler ,
A D
converter
Buffer
D A
Require “deliver on time” assurances
must come from inside the network
Example application (audio)
sample voice once every 125us
each sample has a playback time
packets experience variable delay in network
add constant factor to playback time: playback point
Use initial buffering delay to compensate jitter, but result in longer end-to-end delay

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

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

25. Application requirements & Services Classes
Different application requirements
Elastic: no restrict delay requirements, traditional data
Real-time: delay bound, jitter, loss
Loss: intolerant or tolerant to some loss
Delay: adaptive (e.g. lengthening/shortening the silence between words, playing back video a little slower, etc) or not adaptive
Data rate: adaptive (e.g. reduce video quality by compressing video more) or not adaptive
Different application requirements=>different service classes (not only best effort anymore)
A network that can provide these different levels of service is said to support QoS
Integrated service: fine-grained approach, provide QoS to individual applications or flows
Allow individual application flows to specify their needs to the routers using an explicit signaling mechanism (RSVP)
Scalability is an issue
Differentiated Service: coarse-grained approach, provide QoS to several classes of data or aggregated traffic
Assign packets into a small number of classes that receive differentiated treatment in the routers

26. Taxonomy of applications
Real-Time
Elastic
Loss, delay
tolerant
Intolerant
Interactive
Asynchronous
adaptive
Non-adaptive
Rate
adaptive
Non-adaptive
Interactive-bulk
Delay
adaptive
Rate
adaptive

27. Components of Integrated Services architecture
Flowspec: information of the flow traffic characteristics and its service
Reservations (includes reservation signaling protocol)
Admission control based on flow description and current load
Scheduling to meet the reservation
Traffic shaping at edges to fit reservation
Traffic policing to mark or drop non-conforming traffic
Some application adaptation

28. Types of guarantees Absolute bound on delay and jitter
Absolute bound on delay only
Statistical bound on delay
No quantitative delay bound but admission control and preferential treatment
None

29. Internet service classes proposed by IETF
Guaranteed service
firm bounds on e2e delays and bandwidth
Controlled load
“a QoS closely approximating the QoS that same flow would receive from an unloaded network element, but uses capacity (admission) control to assure that this service is received even when the network element is overloaded”
Use a queuing mechanism such as WFQ to isolate the controlled load traffic from other traffic
Admission control to limit the total amount of controlled load traffic
Best effort

30. Overview of mechanisms
Flow specification (flowspec)
type of service we require
Admission control
can the network provide the requested service?
Resource reservation protocol
RSVP
Packet scheduling

31. Flowspecs Tspec: describes the flow’s traffic characteristics
Rspec: describes the service requested from the network

32. Traffic Shaping
Traffic shaping: control traffic in order to conform to the traffic contract by delaying packets to meet certain criteria.
To optimize or guarantee performance (lower latency, higher usable bandwidth)
commonly applied at the network edges to control traffic entering the network, but can also be applied by the traffic source or in the network
Token bucket
tokens are placed in bucket at rate r
if bucket fills, tokens are discarded
sending a packet of size P uses P tokens
if bucket has P tokens, packet sent at max rate,
else must wait for tokens to accumulate

33. Traffic Policing
Traffic policing: router monitors the flow traffic for conformity with a traffic contract
Drop or tag the packets not conforming to the TSpec that used to make the reservation
to enforce compliance with that contract
Token bucket
tokens are placed in bucket at rate r
if bucket fills, tokens are discarded
When a packet of p bytes arrives, p tokens are removed from the bucket, and the packet is sent to the network.
If fewer than p tokens are available, no tokens are removed from the bucket, and the packet is considered to be non-conformant.
Drop or mark the non-conformant packets

34. Token bucket filter Described by 2 parameters: Operation:
token rate r: rate of tokens placed in the bucket
bucket depth B: capacity of the bucket
Operation:
tokens are placed in bucket at rate r
if bucket fills, tokens are discarded
sending a packet of size P uses P tokens
if bucket has P tokens, packet sent at max rate, else must wait for tokens to accumulate

35. Token bucket operation
tokens
tokens
tokens
overflow
Packet
Packet
Not enough
tokens - wait for
tokens to
accumulate
Enough tokens
packet goes through,
tokens removed

36. TB characteristics On the long run, rate is limited to r
On the short run, a burst of size B can be sent at peak data rate
Amount of traffic entering at interval T is bounded by:
traffic = B + r*T
Information useful to admission algorithm

37. Token bucket specs Flow A: r = 1 Mbps, B=1 byte BW Flow B
Flow B: r = 1 Mbps, B=1MB 2 1
Flow A 1 2 3
Time

38. Admission control
When new flow request arrives, look at Rspec and Tspec and decide whether to admit or reject
Can it provide the desired service requested by the flow, given the currently available resources without causing any previously admitted flow to receive worse service that agreed?
Not policing

39. Reservation protocol: RSVP
Upper layer protocols and applications
IP service interface IP
ICMP
IGMP
RSVP
Link layer service interface
Link layer modules

40. RSVP Used on connectionless networks
Relies on soft state: reservations must be refreshed and do not have to be explicitly deleted
Aims to support multicast as effectively as unicast flows - mcast apps good candidates for real-time, and are heterogeneous
Receiver-oriented approach

41. Basic message types PATH message RESV message CONFIRMATION message
generated only upon request
unicast to receiver when RESV reaches node with established state
TEARDOWN message
ERROR message (if path or RESV fails)

42. Making a reservation Receivers make reservation
Before making a reservation, receiver must know:
type of traffic sender will send (Tspec)
path the sender’s packets will follow
Both can be accomplished by sending PATH messages by the sender

43. PATH messages
PATH messages carry sender’s Tspec and sent from the sender to the receiver
Record the path from the sender to the receiver in the PATH message
Receivers send RESV messages that follow reverse path and setup reservations
If reservation cannot be made, user gets an error

44. PATH and RESV messages R R R R Sender 1 Sender 2 PATH RESV (merged)
receiver 1 R
RESV
receiver 2

45. Soft State
Allow increasing or decreasing the level of resource reservation
Adapt to link or router failure and topology changes
Routing protocol makes routing changes, RSVP adjusts reservation state
In absence of route or membership changes, periodic PATH and RESV msgs refresh established reservation state
When change, new PATH msgs follow new path, new RESV msgs set reservation
Non-refreshed state times out automatically

46. Router handling of RESV messages
If new request rejected, send error message
If admitted:
install packet filter into forwarding dbase
pass flow parameters to scheduler
activate packet policing if needed
forward RESV msg upstream

47. Packet classifying and scheduling
Each arriving packet must be:
classified: associated with the application reservation
Examining up to five fields in the packet: source + destination address, protocol number, source + destination port
scheduled: managed in the queue so that it receives the requested service
implementation not specified in the service model

48. RSVP and multicast
Reservations from multiple receivers for a single sender are merged together at branching points
Reservations for multiple senders may not be added up:
audio conference, not many talk at same time
Different reservation styles:
Reserve resources for all speakers
Reserve resources for any n speakers
Reserve resources for speakers A and B only

49. RSVP versus ATM (Q.2931) RSVP ATM receiver generates reservation
soft state (refresh/timeout)
separate from route establishment
QoS can change dynamically
receiver heterogeneity
ATM
sender generates connection request
hard state (explicit delete)
concurrent with route establishment
QoS is static for life of connection
uniform QoS to all receivers

50. ATM Service Categories
CBR
Constant Bit Rate
Continuous flow of data with tight bounds on delay and delay variation
rt-VBR
Real-Time Variable Bit Rate
Variable bandwidth with tight bounds on delay and delay variation
nrt-VBR
Non-Real-Time Variable Bit Rate
Variable bandwidth with tight bound on cell loss
UBR
Unspecified Bit Rate
No guarantees (i.e., best effort delivery)
ABR
Available Bit Rate
Flow control on source with tight bound on cell loss

51. Differentiated Services (DiffServ)
Analogy:
airline service, first class, coach, various restrictions on coach as a function of payment
Best-effort expected to make up bulk of traffic, but revenue from “premium” service important to economic base (will pay for more plentiful bandwidth overall)
Not motivated by real-time, motivated by economics and assurances

52. Differentiated Services (cont)
Divide traffic into a small number of classes and allocates resource on a per-class basis, instead of individual flows
Scalable
No need for reservations: just mark packets
E.g. packet marking can be done at administrative boundaries before injecting packets into network
Significant savings in signaling, much simpler overall

53. IP DiffServ IP packets carry 6-bit service code points (DSCP)
ersion
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable) 4 8 16 19 31
Data
IP packets carry 6-bit service code points (DSCP)
Potentially support 64-different classes of services
In implementation, the number of DSCPs used in a network is much smaller, e.g. simple two-class network
ToS field => now used for DiffServ and ECN
Bits 0-5: Differentiated Services Code Point (DSCP)
Bit 6: ECN-capable
Bit 7: ECN

54. DiffServ Where and how to set the DSCP value
Host set DSCP based on knowledge of application requirements
Router at an administrative boundary set DSCP according to a certain policy
What does a router do when it receives a packet marked with a certain DSCP
Routers map DSCP to per-hop-behavior (PHB)
PHBs can be standard or local
Standard PHBs include
Default: No special treatment or best effort
Expedited forwarding (EF): should be forwarded with low delay and loss
Assured forwarding (AF): Multiple classes, each class with multiple drop preference

55. Expedited Forwarding (EF)
A router should forward the EF packets with minimal delay and low loss
To implement, put all EF packets in a dedicated EF queue and ensure that the arrival rate of packets to the queue is less than the service rate
Configure the router at the edge of an administrative domain to allow a certain max rate of EF packets arrivals into the domain
Limit the EF packet rate < the bandwidth of the slowest link in the domain
Give EF packet queue high priority, e.g. WFQ with a high weight to EF queue

56. Assured Forwarding (AF)
A set of AF PHBs is defined
AFxy:
x: class, typically determine a queue for the packet
y:drop preference
For example: AF11, AF12 and AF13 packets are put into the same queue, but AF13 would have a greater chance to be dropped due to congestion. AF11 and AF2y would go to different queues
12 AF PHBs are recommended by IETF, 4 AF classes with three drop preference levels each

57. An Example DiffServ Network Implementation
Support Two-type DiffServ
Premium (expedited) service: (type P)
User sends within profile, network commits to delivery with the agreed profile
Classify packet in one of two queues, use priority
Shaping at trust boundaries only, using token bucket
Allocate resource based on the agreed peak rate
conservative, virtual wire services
unused premium goes to best effort (subsidy!)
Assured service: (type A)
based on expected capacity usage profiles
traffic unlikely to be dropped if user maintains profile. Out-of-profile traffic marked

58. Premium traffic flow Company A ISP Packets in premium
flows have bit set
Premium packet flow
restricted to R bytes/sec
internal
router
ISP
host
border
router
first hop
router
border
router
Unmarked
packet flow

59. 2-bit differentiated service
Use precedence field to encode P & A type packets
P packets are queued at higher priority than ordinary best effort
A packets treated preferentially wrt dropping probability in the normal queue
Leaf and border routers have input and output tasks - other routers just output

60. Leaf router input functionality
Clear
A & P
bits
Packet
classifier
Marker 1
Marker N
Forwarding
engine
Arriving
packet
Best effort
Flow 1
Flow N
Marker: Leaf routers have traffic profiles - they classify packets based on packet header (destination addr & port, source addr & port, protocol ID, etc)
If no profile present, pass as best effort
If profile is for A:
mark in-profile packets with A, forward others unmarked
If profile is for P:
delay out-of -profile packets to shape into profile

61. Markers to implement two different services
P service
Wait for
token
Set P bit
Packet
input
output
Drop on overflow
Test if
token
Set A bit
No token
Packet
input
output
A service

62. Router output interface for two-bit architecture
2 queues: P packets on higher priority queue
Lower priority queue implements RED “In or Out” scheme (RIO)
P-bit set?
If A-bit set
incr A_cnt
High-priority Q
Low-priority Q
decr A_cnt
RIO queue
management
Packets out
yes no

63. Border router input interface Profile Meters
At border routers, profile meters test marked flows:
drop P packets out of profile
unmark A packets
Arriving
packet
Is packet
marked?
Token
available?
Clear A-bit
Drop packet
Forwarding
engine
A set
P set
token
Not marked no

64. Red with In or Out (RIO)
Similar to RED, but with two separate probability curves
Has two classes, “In” and “Out” (of profile)
“Out” class has lower Minthresh, so packets are dropped from this class first
As avg queue length increases, “in” packets are dropped
MaxP
1.0
Minout
Minin
Maxin
Maxout
P(drop)
AvgLen

65. Today’s Homework
Chapter 6
th ed
-6.32
-6.43
-6.44
Due 3/30/15