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Title: 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

29
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)
37
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
43
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
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