Betterthanbesteffort: QoS, IntServ, DiffServ, RSVP, RTP: BRIEF VERSION

1
Better-than-best-effort QoS, IntServ, DiffServ,
RSVP, RTPBRIEF VERSION

• Shivkumar Kalyanaraman
• Rensselaer Polytechnic Institute
• shivkuma_at_ecse.rpi.edu
• http//www.ecse.rpi.edu/Homepages/shivkuma

Based in part on slides of Ion Stoica, Jim
Kurose, Srini Seshan, Srini Keshav
2
Overview

• Why better-than-best-effort (QoS-enabled)
  Internet ?
• Quality of Service (QoS) building blocks
• End-to-end protocols RTP, H.323
• Network protocols
• Integrated Services(IntServ), RSVP.
• Scalable differentiated services DiffServ
• Control plane QoS routing, traffic engineering,
  policy management, pricing models

3
Why Better-than-Best-Effort (QoS)?

• To support a wider range of applications
• Real-time, Multimedia, etc
• To develop sustainable economic models and new
  private networking services
• Current flat priced models, and best-effort
  services do not cut it for businesses

4
Quality of Service What is it?
Multimedia applications network audio and video
5
What is QoS?

• Better performance as described by a set of
  parameters or measured by a set of metrics.
• Generic parameters
• Bandwidth
• Delay, Delay-jitter
• Packet loss rate (or loss probability)
• Transport/Application-specific parameters
• Timeouts
• Percentage of important packets lost

6
What is QoS (contd) ?

• These parameters can be measured at several
  granularities
• micro flow, aggregate flow, population.
• QoS considered better if
• a) more parameters can be specified
• b) QoS can be specified at a fine-granularity.
• QoS spectrum

Best Effort
Leased Line
7
Fundamental Problems

• In a FIFO service discipline, the performance
  assigned to one flow is convoluted with the
  arrivals of packets from all other flows!
• Cant get QoS with a free-for-all
• Need to use new scheduling disciplines which
  provide isolation of performance from arrival
  rates of background traffic

8
Fundamental Problems

• Conservation Law (Kleinrock) ??(i)Wq(i) K
• Irrespective of scheduling discipline chosen
• Average backlog (delay) is constant
• Average bandwidth is constant
• Zero-sum game gt need to set-aside resources
  for premium services

9
QoS Big Picture Control/Data Planes
10
QoS Components

• QoS gt set aside resources for premium services
• QoS components
• a) Specification of premium services
  (service/service level agreement design)
• b) How much resources to set aside? (admission
  control/provisioning)
• c) How to ensure network resource utilization, do
  load balancing, flexibly manage traffic
  aggregates and paths ?
• (QoS routing, traffic engineering)

11
QoS Components (Continued)

• d) How to actually set aside these resources in a
  distributed manner ?
• (signaling, provisioning, policy)
• e) How to deliver the service when the traffic
  actually comes in (claim/police resources)?
• (traffic shaping, classification, scheduling)
• f) How to monitor quality, account and price
  these services?
• (network mgmt, accounting, billing, pricing)

12
How to upgrade the Internet for QoS?

• Approach de-couple end-system evolution from
  network evolution
• End-to-end protocols RTP, H.323 etc to spur the
  growth of adaptive multimedia applications
• Assume best-effort or better-than-best-effort
  clouds
• Network protocols IntServ, DiffServ, RSVP, MPLS,
  COPS
• To support better-than-best-effort capabilities
  at the network (IP) level

13
QOS SPECIFICATION, TRAFFIC, SERVICE
CHARACTERIZATION, BASIC MECHANISMS
14
Service Specification

• Loss probability that a flows packet is lost
• Delay time it takes a packets flow to get from
  source to destination
• Delay jitter maximum difference between the
  delays experienced by two packets of the flow
• Bandwidth maximum rate at which the soource can
  send traffic
• QoS spectrum

Best Effort
Leased Line
15
Traffic and Service Characterization

• To quantify a service one has two know
• Flows traffic arrival
• Service provided by the router, i.e., resources
  reserved at each router
• Examples
• Traffic characterization token bucket
• Service provided by router fix rate and fix
  buffer space
• Characterized by a service model (service curve
  framework)

16
Token Bucket

• Characterized by three parameters (b, r, R)
• b token depth
• r average arrival rate
• R maximum arrival rate (e.g., R link capacity)
• A bit is transmitted only when there is an
  available token
• When a bit is transmitted exactly one token is
  consumed

r tokens per second
bits
slope r
bR/(R-r)
b tokens
slope R
lt R bps
time
regulator
17
Characterizing a Source by Token Bucket

• Arrival curve maximum amount of bits
  transmitted by time t
• Use token bucket to bound the arrival curve

bits
bps
Arrival curve
time
time
18
Example

• Arrival curve maximum amount of bits
  transmitted by time t
• Use token bucket to bound the arrival curve

Arrival curve
bits
4
bps
3
2
2
1
1
0
1
2
3
4
5
1
2
3
4
5
size of time interval
time
19
Per-hop Reservation

• Given b,r,R and per-hop delay d
• Allocate bandwidth ra and buffer space Ba such
  that to guarantee d

slope ra
slope r
bits
Arrival curve
b
Ba
20
What is a Service Model?
Network element
delivered traffic
offered traffic
(connection oriented)

• The QoS measures (delay,throughput, loss, cost)
  depend on offered traffic, and possibly other
  external processes.
• A service model attempts to characterize the
  relationship between offered traffic, delivered
  traffic, and possibly other external processes.

21
Arrival and Departure Process
bits
Rin(t) arrival process amount of
data arriving up to time t
delay
buffer
Rout(t) departure process amount
of data departing up to time t
t
22
Traffic Envelope (Arrival Curve)

• Maximum amount of service that a flow can send
  during an interval of time t

b(t) Envelope
slope max average rate
Burstiness Constraint
slope peak rate
t
23
Service Curve

• Assume a flow that is idle at time s and it is
  backlogged during the interval (s, t)
• Service curve the minimum service received by
  the flow during the interval (s, t)

24
Big Picture
Service curve
bits
bits
Rin(t)
slope C
t
t
bits
t
25
Delay and Buffer Bounds
bits
E(t) Envelope
Maximum delay
Maximum buffer
S (t) service curve
t
26
Mechanisms Traffic Shaping/Policing

• Token bucket limits input to specified Burst
  Size (b) and Average Rate (r).
• Traffic sent over any time T lt rT b
• a.k.a Linear bounded arrival process (LBAP)
• Excess traffic may be queued, marked BLUE, or
  simply dropped

27
Mechanisms Queuing/Scheduling
Traffic Sources
Traffic Classes

Class A

Class B
Class C

• Use a few bits in header to indicate which queue
  (class) a packet goes into (also branded as CoS)
• High users classified into high priority
  queues, which also may be less populated
• gt lower delay and low likelihood of packet drop
• Ideas priority, round-robin, classification,
  aggregation, ...

28
Mechanisms Buffer Mgmt/Priority Drop
Drop RED and BLUE packets
Drop only BLUE packets

• Ideas packet marking, queue thresholds,
  differential dropping, buffer assignments

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SCHEDULING
30
Packet Scheduling

• Decide when and what packet to send on output
  link
• Usually implemented at output interface

flow 1
flow 2
Classifier
Scheduler
1
2
flow n
Buffer management
31
Focus Scheduling Policies

• Priority Queuing classes have different
  priorities class may depend on explicit marking
  or other header info, eg IP source or
  destination, TCP Port numbers, etc.
• Transmit a packet from the highest priority class
  with a non-empty queue
• Preemptive and non-preemptive versions

32
Scheduling Policies (more)

• Round Robin scan class queues serving one from
  each class that has a non-empty queue

33
Round-Robin Discussion

• Advantages protection among flows
• Misbehaving flows will not affect the performance
  of well-behaving flows
• Misbehaving flow a flow that does not implement
  any congestion control
• FIFO does not have such a property
• Disadvantages
• More complex than FIFO per flow queue/state
• Biased toward large packets a flow receives
  service proportional to the number of packets

34
Generalized Processor Sharing(GPS)

• Assume a fluid model of traffic
• Visit each non-empty queue in turn (RR)
• Serve infinitesimal from each
• Leads to max-min fairness
• GPS is un-implementable!
• We cannot serve infinitesimals, only packets

35
Generalized Processor Sharing

• A work conserving GPS is defined as
• where
• wi weight of flow i
• Wi(t1, t2) total service received by flow i
  during t1, t2)
• W(t1, t2) total service allocated to all flows
  during t1, t2)
• B(t) number of flows backlogged

36
Fair Rate Computation in GPS

• Associate a weight wi with each flow i
• If link congested, compute f such that

f 2 min(8, 23) 6 min(6, 21) 2 min(2,
21) 2
8
(w1 3)
10
4
6
(w2 1)
4
2
2
(w3 1)
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Bit-by-bit Round Robin

• Single flow clock ticks when a bit is
  transmitted. For packet i
• Pi length, Ai arrival time, Si begin
  transmit time, Fi finish transmit time
• Fi SiPi max (Fi-1, Ai) Pi
• Multiple flows clock ticks when a bit from all
  active flows is transmitted ? round number
• Can calculate Fi for each packet if number of
  flows is known at all times
• This can be complicated

38
Packet Approximation of Fluid System

• Standard techniques of approximating fluid GPS
• Select packet that finishes first in GPS assuming
  that there are no future arrivals
• Important properties of GPS
• Finishing order of packets currently in system
  independent of future arrivals
• Implementation based on virtual time
• Assign virtual finish time to each packet upon
  arrival
• Packets served in increasing order of virtual
  times

39
Fair Queuing (FQ)

• Idea serve packets in the order in which they
  would have finished transmission in the fluid
  flow system
• Mapping bit-by-bit schedule onto packet
  transmission schedule
• Transmit packet with the lowest Fi at any given
  time
• Variation Weighted Fair Queuing (WFQ)

40
Approximating GPS with WFQ

• Fluid GPS system service order

0
2
10
4
6
8

• Weighted Fair Queueing
• select the first packet that finishes in GPS

41
FQ Advantages

• FQ protect well-behaved flows from ill-behaved
  flows
• Example 1 UDP (10 Mbps) and 31 TCPs sharing a
  10 Mbps link

42
Modeling System Virtual Time V(t)

• Measure service, instead of time
• V(t) slope rate at which every active flow
  receives service
• C link capacity
• N(t) number of active flows in fluid system at
  time t

V(t)
time
Service in fluid flow system
1
2
3
4
5
6
1
2
3
4
5
time
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Big Picture

• FQ does not eliminate congestion ? it just
  manages the congestion
• You need both end-host congestion control and
  router support for congestion control
• end-host congestion control to adapt
• router congestion control to protect/isolate
• Dont forget buffer management you still need to
  drop in case of congestion. Which packets would
  you drop in FQ?
• one possibility packet from the longest queue

44
QoS ARCHITECTURES
45
Parekh-Gallager theorem

• Let a connection be allocated weights at each WFQ
  scheduler along its path, so that the least
  bandwidth it is allocated is g
• Let it be leaky-bucket regulated such that bits
  sent in time t1, t2 lt g(t2 - t1) ?
• Let the connection pass through K schedulers,
  where the kth scheduler has a rate r(k)
• Let the largest packet size in the network be P

46
Significance

• P-G Theorem shows that WFQ scheduling can provide
  end-to-end delay bounds in a network of
  multiplexed bottlenecks
• WFQ provides both bandwidth and delay guarantees
• Bound holds regardless of cross traffic behavior
  (isolation)
• Needs shapers at the entrance of the network
• Can be generalized for networks where schedulers
  are variants of WFQ, and the link service rate
  changes over time

47
Stateless vs. Stateful QoS Solutions

• Stateless solutions routers maintain no fine
  grained state about traffic
• scalable, robust
• weak services
• Stateful solutions routers maintain per-flow
  state
• powerful services
• guaranteed services high resource utilization
• fine grained differentiation
• protection
• much less scalable and robust

48
Existing Solutions
49
Integrated Services (IntServ)

• An architecture for providing QOS guarantees in
  IP networks for individual application sessions
• Relies on resource reservation, and routers need
  to maintain state information of allocated
  resources (eg g) and respond to new Call setup
  requests

50
Signaling semantics

• Classic scheme sender initiated
• SETUP, SETUP_ACK, SETUP_RESPONSE
• Admission control
• Tentative resource reservation and confirmation
• Simplex and duplex setup no multicast support

51
RSVP Internet Signaling

• Creates and maintains distributed reservation
  state
• De-coupled from routing
• Multicast trees setup by routing protocols, not
  RSVP (unlike ATM or telephony signaling)
• Receiver-initiated scales for multicast
• Soft-state reservation times out unless
  refreshed
• Latest paths discovered through PATH messages
  (forward direction) and used by RESV mesgs
  (reverse direction).

52
Call Admission

• Session must first declare its QOS requirement
  and characterize the traffic it will send through
  the network
• R-spec defines the QOS being requested
• T-spec defines the traffic characteristics
• A signaling protocol is needed to carry the
  R-spec and T-spec to the routers where
  reservation is required RSVP is a leading
  candidate for such signaling protocol

53
Call Admission

• Call Admission routers will admit calls based on
  their R-spec and T-spec and base on the current
  resource allocated at the routers to other calls.

54
Stateful Solution Guaranteed Services

• Achieve per-flow bandwidth and delay guarantees
• Example guarantee 1MBps and lt 100 ms delay to a
  flow

Receiver
Sender







55
Stateful Solution Guaranteed Services

• Allocate resources - perform per-flow admission
  control

Receiver
Sender







56
Stateful Solution Guaranteed Services

• Install per-flow state

Receiver
Sender







57
Stateful Solution Guaranteed Services

• Challenge maintain per-flow state consistent

Receiver
Sender







58
Stateful Solution Guaranteed Services

• Per-flow classification

Receiver
Sender











59
Stateful Solution Guaranteed Services

• Per-flow buffer management

Receiver
Sender











60
Stateful Solution Guaranteed Services

• Per-flow scheduling

Receiver
Sender











61
Stateful Solution Complexity

• Data path
• Per-flow classification
• Per-flow buffer
• management
• Per-flow scheduling
• Control path
• install and maintain
• per-flow state for
• data and control paths

Per-flow State

flow 1
flow 2
Scheduler
Classifier
flow n
Buffer management
output interface
62
Stateless vs. Stateful

• Stateless solutions are more
• scalable
• robust
• Stateful solutions provide more powerful and
  flexible services
• guaranteed services high resource utilization
• fine grained differentiation
• protection

63
Question

• Can we achieve the best of two worlds, i.e.,
  provide services implemented by stateful networks
  while maintaining advantages of stateless
  architectures?
• Yes, in some interesting cases. DPS, CSFQ.
• Can we provide reduced state services, I.e.,
  maintain state only for larger granular flows
  rather than end-to-end flows?
• Yes Diff-serv

64
Differentiated Services (DiffServ)

• Intended to address the following difficulties
  with Intserv and RSVP
• Scalability maintaining states by routers in
  high speed networks is difficult sue to the very
  large number of flows
• Flexible Service Models Intserv has only two
  classes, want to provide more qualitative service
  classes want to provide relative service
  distinction (Platinum, Gold, Silver, )
• Simpler signaling (than RSVP) many applications
  and users may only w ant to specify a more
  qualitative notion of service

65
Differentiated Services Model
Interior Router
Egress Edge Router
Ingress Edge Router

• Edge routers traffic conditioning (policing,
  marking, dropping), SLA negotiation
• Set values in DS-byte in IP header based upon
  negotiated service and observed traffic.
• Interior routers traffic classification and
  forwarding (near stateless core!)
• Use DS-byte as index into forwarding table

66
Diffserv Architecture
Edge router - per-flow traffic management -
marks packets as in-profile and out-profile
Core router - per class TM - buffering and
scheduling based on marking at edge - preference
given to in-profile packets - Assured Forwarding
67
Packet format support

• Packet is marked in the Type of Service (TOS) in
  IPv4, and Traffic Class in IPv6 renamed as DS
• 6 bits used for Differentiated Service Code Point
  (DSCP) and determine PHB that the packet will
  receive
• 2 bits are currently unused

68
Traffic Conditioning

• It may be desirable to limit traffic injection
  rate of some class user declares traffic profile
  (eg, rate and burst size) traffic is metered and
  shaped if non-conforming

69
Per-hop Behavior (PHB)

• PHB name for interior router data-plane
  functions
• Includes scheduling, buff. mgmt, shaping etc
• Logical spec PHB does not specify mechanisms to
  use to ensure performance behavior
• Examples
• Class A gets x of outgoing link bandwidth over
  time intervals of a specified length
• Class A packets leave first before packets from
  class B

70
PHB (contd)

• PHBs under consideration
• Expedited Forwarding departure rate of packets
  from a class equals or exceeds a specified rate
  (logical link with a minimum guaranteed rate)
• Emulates leased-line behavior
• Assured Forwarding 4 classes, each guaranteed a
  minimum amount of bandwidth and buffering each
  with three drop preference partitions
• Emulates frame-relay behavior

71
Question

• Can we achieve the best of two worlds, i.e.,
  provide services implemented by stateful networks
  while maintaining advantages of stateless
  architectures?
• Yes, in some interesting cases. DPS, CSFQ.
• Can we provide reduced state services, I.e.,
  maintain state only for larger granular flows
  rather than end-to-end flows?
• Yes Diff-serv

72
Scalable Core (SCORE)

• A trusted and contiguous region of network in
  which
• edge nodes perform per flow management
• core nodes do not perform per flow management

73
The DPS Approach

• Define a reference stateful network that
  implements the desired service

Reference Stateful Network
74
The DPS Idea

• Instead of having core routers maintaining
  per-flow state have packets carry per-flow state

Reference Stateful Network
SCORE Network
75
Dynamic Packet State (DPS)

• Ingress node compute and insert flow state in
  packets header

76
Dynamic Packet State (DPS)

• Ingress node compute and insert flow state in
  packets header

77
Dynamic Packet State (DPS)

• Core node
• process packet based on state it carries and
  nodes state
• update both packet and nodes state

78
Dynamic Packet State (DPS)

• Egress node remove state from packets header

79
Example DPS-Guaranteed Services

• Goal provide per-flow delay and bandwidth
  guarantees
• How emulate ideal model in which each flow
  traverses dedicated links of capacity r
• Per-hop packet service time (packet length) / r

r
r
r
flow (reservation r )
80
Reality End-to-end Adaptive Applications
Video Coding, Error Concealment, Unequal Error
Protection (UEP)
Video Coding, Error Concealment, Unequal Error
Protection (UEP)
Packetization, Marking, playout Buffer
Management
Packetization, Marking, Source Buffer Management
Congestion control
Congestion control
Internet
End-to-end Closed-loop control
81
End-to-end Real-Time Protocol (RTP)

• Provides standard packet format for real-time
  application
• Typically runs over UDP
• Specifies header fields below
• Payload Type 7 bits, providing 128 possible
  different types of encoding eg PCM, MPEG2 video,
  etc.
• Sequence Number 16 bits used to detect packet
  loss

82
Real-Time Protocol (RTP)

• Timestamp 32 bytes gives the sampling instant
  of the first audio/video byte in the packet
  used to remove jitter introduced by the network
• Synchronization Source identifier (SSRC) 32
  bits an id for the source of a stream assigned
  randomly by the source

83
RTP Control Protocol (RTCP)

• Protocol specifies report packets exchanged
  between sources and destinations of multimedia
  information
• Three reports are defined Receiver reception,
  Sender, and Source description
• Reports contain statistics such as the number of
  packets sent, number of packets lost,
  inter-arrival jitter
• Used to modify sender transmission rates and
  for diagnostics purposes

84
Eg Streaming RTSP

• User interactive control is provided, e.g. the
  public protocol Real Time Streaming Protocol
  (RTSP)
• Helper Application displays content, which is
  typically requested via a Web browser e.g.
  RealPlayer typical functions
• Decompression
• Jitter removal
• Error correction use redundant packets to be
  used for reconstruction of original stream
• GUI for user control

85
Using a Streaming Server

• Web browser requests and receives a Meta File (a
  file describing the object)
• Browser launches the appropriate Player and
  passes it the Meta File
• Player contacts a streaming server, may use a
  choice of UDP vs. TCP to get the stream

86
Receiver Adaptation Options

• If UDP Server sends at a rate appropriate for
  client to reduce jitter, Player buffers
  initially for 2-5 seconds, then starts display
• If TCP sender sends at maximum possible rate
  retransmit when error is encountered Player uses
  a much large buffer to smooth delivery rate of TCP

87
H.323

• H.323 is an ITU standard for multimedia
  communications over best-effort LANs.
• Part of larger set of standards (H.32X) for
  videoconferencing over data networks.
• H.323 includes both stand-alone devices and
  embedded personal computer technology as well as
  point-to-point and multipoint conferences.
• H.323 addresses call control, multimedia
  management, and bandwidth management as well as
  interfaces between LANs and other networks.

88
H.323 Architecture
89
Inter-domain QoS Challenge

• Provide high quality service across ISPs
• Problem intermediate ISPs dont have incentive
  to provide good service
• e.g., hot-potato policy

ISP 2
?
ISP 3
ISP 1
90
Approach Virtual ISP (V-ISP)

• Avoid crossing ISP boundaries
• Each ISP will provide good service V-ISP can
  easily verify it
• Allocate/buy service across each ISP and compose
  them

GPoP (core)
GPoP (core)
ISP 2
Proxy (edge)
Proxy (edge)
ISP 3
ISP 1
91
Composition Tools for QoS

• Dynamic Packet State (DPS)
• Proxy (edge) maintain per flow state label
  packets
• Giga PoPs (core) maintain no per flow state
  process packets based on their labels

92
Closed-Loop Building Blocks for QoS
International Link or
International Link or
93
Closed-loop QoS Building Blocks
Priority/WFQ
FIFO
B
?
B

• Scheduler differentiates service on a
  packet-by-packet basis

• Loops differentiate service on an RTT-by-RTT
  basis using purely edge-based policy
  configuration.

94
Recall Accln-based Control Policy

• control objective keep
• if , no way to probe increase of
  available bandwidth

• control algorithm

94
95
Closed-Loop Service Differentiation Loss-based
or Accumulation-based ?
95
96
Closed-Loop QoS Bandwidth Assurances
Flow 1 with 4 Mbps assured 3 Mbps best effort
Flow 2 with 3 Mbps best effort
97
QoS an application-level approach

• sophisticated services in application
• architecturally above network core

simple, fast, diffserv network
98
QoS an application-level approach

• Application-level infrastructure
• accommodate network-level service
• additional tailoring of user services

99
Content Delivery Motivation congestion
Networks
Browsers
Routers
Web Servers
100
Content Delivery idea

• Reduces load on server
• Avoids network congestion

Browsers
Replicatedcontent
Content Sink
Router
Content Source
Web Server
101
CDN Architectural Layout
Request Routing(RR)
4
1
Client
5
Distribution System
Origin
2
6
3
Surrogate

• Publisher informs RR of Content Availability.
• Content Pushed to Distribution System.
• Client Requests Content, Requested redirected to
  RR.
• RR finds the most suitable Surrogate
• Surrogate services client request.

102
Summary

• QoS big picture, building blocks
• Integrated services RSVP, 2 services,
  scheduling, admission control etc
• Diff-serv edge-routers, core routers DS byte
  marking and PHBs
• Real-time transport/middleware RTP, H.323
• New problems inter-domain QoS, Application-level
  QoS, Content delivery/web caching