Quality of Service

Report
Quality of Service
Outline
Realtime Applications
Integrated Services
Differentiated Services
MPLS
1
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
2
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)
4
Taxonomy
Applications
Elastic
Real time
Intolerant
T olerant
Nonadaptive
Adaptive
Rate adaptive
Delay adaptive
5
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.
6
Two Approaches to QoS
• Fine-grained:
– Integrated Service (RSVP), ATM
• Coarse-grained:
– Differentiated Service (Class-Based)
7
Integrated Services
• Service Classes
– Guaranteed
• Provides hard bound for delay, bandwidth, etc
– controlled-load
• Emulate lightly loaded network
• Mechanisms
–
–
–
–
signaling protocol (request reservation)
admission control (accept reservation)
packet scheduling (implement reservation)
policing (enforce reservation)
8
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
9
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.
10
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.
11
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
12
RSVP Example
Sender 1
PATH
R
Sender 2
R
PATH
RESV
(merged)
R
RESV
R
R
Receiver A
RESV
Receiver B
13
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
14
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
15
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
16
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.
17
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
18
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.
19
MPLS Header
20
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
21
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.
22
MPLS
23

similar documents