Quality of Service
Outline
Realtime Applications
Integrated Services
Differentiated Services
MPLS
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Realtime Applications
• Realtime applications raise new challenges for Internet, which is
originally designed for data applications.
• Require “deliver on time” assurances
– must come from inside the network
Microphone
Sampler,
A
D
converter
Buffer,
D
A
Speaker
• 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
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Playback Buffer
Sequence number
Packet
arrival
Packet
generation
Playback
Network
delay
Buffer
Time
3
Example Distribution of Delays
90% 97% 98%
3
99%
2
1
50
100
150
200
Delay (milliseconds)
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Taxonomy
Applications
Elastic
Real time
Intolerant
T olerant
Nonadaptive
Adaptive
Rate adaptive
Delay adaptive
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Type of QoS
• ATM classify the QoS services provided to the
user into five categories:
– CBR: constant bit rate service, e.g., telephony.
– RT-VBR: real-time variable bit rate service, e.g.,
compressed videoconferencing.
– NR-VBR: non real-time variable bit rate service, e.g.,
watch a movie on internet.
– ABR: available bit rate service, e.g., file transfer.
– UBR: unspecified bit rate service, e.g., web browsing.
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Two Approaches to QoS
• Fine-grained:
– Integrated Service (RSVP), ATM
• Coarse-grained:
– Differentiated Service (Class-Based)
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Integrated Services
• Service Classes
– Guaranteed
• Provides hard bound for delay, bandwidth, etc
– controlled-load
• Emulate lightly loaded network
• Mechanisms
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–
–
–
signaling protocol (request reservation)
admission control (accept reservation)
packet scheduling (implement reservation)
policing (enforce reservation)
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Flowspec
• Provides information about traffic to the network
• Rspec: describes service requested from network
– controlled-load: no parameter
– guaranteed: delay target
• Tspec: describes flow’s traffic characteristics
– average bandwidth + burstiness
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Token Bucket
• Used for charactering user traffic, two parameters
– token rate r
– bucket depth B
• Mechanism
– must have a token to send a byte, must have n tokens to send n
bytes
– accumulate tokens at rate of r per second
– can accumulate no more than B tokens
– maximum burst duration (D) is governed by the following
equality: B+ D*r = D*L, where L is the link transmission speed.
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Per-Router Mechanisms
• Admission Control
– decide if a new flow can be supported
• to make such decision, a traffic source offers a flow specification,
which routers translate into resource usage and make the admission
decision
– 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
– policing: handling traffic out of flow spec.
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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
Receiver-oriented
Two messages: PATH and RESV
Source transmits PATH messages every 30 seconds
Destination responds with RESV message
Merge requirements in case of multicast
Can specify number of speakers for reserving resources
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RSVP Example
Sender 1
PATH
R
Sender 2
R
PATH
RESV
(merged)
R
RESV
R
R
Receiver A
RESV
Receiver B
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RSVP versus ATM (Q.2931)
• RSVP
–
–
–
–
–
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
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Differentiated Services
• Problem with IntServ: scalability
• Idea: segregate packets into a small number of classes
– e.g., premium vs best-effort
• Packets marked according to class at edge of network
• Core routers implement some per-hop-behavior (PHB)
• The classes defined in DiffServ include
–
–
–
–
expedited forwarding
assured forwarding
best effort service
classes are indicated in the 8-bit IP TOS field
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Expedited Forwarding (EF)
– EF is for applications such as internet telephony. It
provides low delay or CBR service
– Rate-limit EF packets at the edges
– PHB implemented with class-based priority queues or
WFQ
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Diffserv (con’t)
• Assured forwarding uses RED drop probabilities
to differentiate service qualities, and in or out of
profile traffic. The aim is to simulate a lighted
loaded network even if the network is congested.
• Best effort service does not guarantee any quality
of service, packets in this category get whatever
bandwidth that is left over after expedited and
assured services.
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DiffServ (cont)
• Assured Forwarding (AF)
– customers sign service
agreements with ISPs
– edge routers mark packets
as being “in” or “out” of
profile
– core routers run RIO: RED
with in/out
P(drop)
1.0
MaxP
AvgLen
Min out
Min in Max out
Max in
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Label Switching and MPLS
• Multiprotocol label switching (MPLS) has the following
advantages:
– Fast forwarding by using a short fixed label that facilitates fast
table lookup in hardware.
– Traffic engineering by pinning down the route, which is different
from packet switching.
– Multi-protocol capable, can be used to forward IP packets as well
as ATM cells.
• The label is inserted between L3 and L2 headers.
– It has four fields: a 20 bit label field, a 3 bit QoS field, a label
stacking indicator bit, and a 8 bit TTL.
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MPLS Header
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MPLS
• The label plays the same role as ATM VCI/VPI.
• The virtual circuit (label switched path) in MPLS
defines a forwarding equivalent class (FEC)
– shares the same path through the network and QoS
treatment,
– is typically a trunk.
• Labels can be stacked
– a backbone network can stack a label to facilitate trunk
switching
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MPLS
• Two approaches to distribute labels
– Data driven approach
• when a packet hits a router, the router asks the downstream
router to generate a label, which is attached to the packet. So
the label is generated hop-by-hop in a way similar to datagram
routing.
– Control driven approach
• the source sets up a label-switched path to the destination
before traffic arrives, which is a kind of source routing and
similar to signaling.
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MPLS
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