Transcript 投影片 1 - National Tsing Hua University

Internet Quality of Service
Quality of Service (QoS)
• The best-effort model , in which the network tries
to deliver data from source to destination but
makes no promises about end-to-end delay, is
not sufficient for real-time applications.
• The primary goal of Quality of Service (QoS) is
to support different levels of services by provide
priorities including dedicated bandwidth,
controlled jitter and latency (required by some
real-time and interactive traffic), and improved
loss characteristics.
Real-time Applications
• Require “deliver on time” assurances
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
Playback Buffer
Sequence number
Packet
arrival
Packet
generation
Playback
Network
delay
Time
Buffer
Application Requirements
Data loss
Bandwidth
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
loss-tolerant
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
financial apps
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
same as above
few Kbps up
elastic
Application
yes, few secs
yes, 100’s msec
yes and no
Taxonomy
Applications
Real time
Tolerant
Adaptive
Delayadaptive
Nonadaptive
Rateadaptive
Elastic
Intolerant
Rate-adaptive
Interactive
Nonadaptive
Interactive
bulk
Asynchronous
QoS Approaches
• IPv4 Type of Service (TOS)
– IPv6 Traffic class + Flow label
• Integrated Services (Intserv)
• Differentiated Services (Deffserv)
• ATM service classes
– CBR, VBR-rt, VBR-nrt, ABR, UBR
IPv4
1
2
3
Precedence
4
5
TOS
6
7
8
MBZ
• RFC1349
• Precedence: the importance or priority of the datagram
• TOS:
–
–
–
–
–
1000
0100
0010
0001
0000
------
minimize delay
maximize throughput
maximize reliability
minimize monetary cost
normal service
• MBZ: Must Be Zero (Unused)
– Suggested to be used as ECN field in RFC3168
Integrated Services
• Intserv provides individualized quality-of-service
guarantees to individual application sessions by
per-flow resource reservation.
• IETF intserv working group (concluded)
• Resource ReSerVation Protocol (RSVP, RFC
2205)
• Subnet Bandwidth Manager (SBM, RFC 2814)
Service Classes
• Guaranteed service (RFC2212)
– Provides firm bounds on the queueing delays
that a packet will experience in a network
element
• Controlled-load service (RFC 2211)
– provides the flow with a quality of service
closely approximating the QoS that same flow
would receive from an unloaded network
element
Mechanisms
• Traffic characterization
– Tspec (RFC2210)
– Rspec (RFC2215)
• Admission control
• Reservation protocol
– RSVP
• Packet processing
– Weighted Fair Queuing (WFQ)
Flowspecs
• Rspec: describes service requested from network
– controlled-load: none
– guaranteed: delay target
• Tspec: describes flow’s traffic characteristics
–
–
–
–
–
–
–
–
average bandwidth + burstiness: token bucket filter
token rate r
bucket depth B
must have a token to send a byte
must have n tokens to send n bytes
start with no tokens
accumulate tokens at rate of r per second
can accumulate no more than B tokens
Token Bucket
• If the service rate at a network element is R,
then the queuing delay is bounded by b/R.
Admission Control
• A router decides whether to admit a flow based on the Rspec and T-spec of the flow and currently available
resources on the router.
Reservation Protocol
• RSVP is employed in Intserv to setup path
and reserve resources.
RSVP
•
•
•
•
Receiver-oriented reservation
Receiver heterogeneity
Designed to support multicast
Merge requirements in case of multicast
RSVP
• Support multiple senders
• Use soft state (refresh periodically)
– Source transmits PATH messages every 30
seconds
– Destination responds with RESV message
• Separate from route establishment
• QoS can change dynamically
RSVP
Sender 1
PATH
R
Sender 2
R
PATH
RESV
(merged)
R
RESV
R
R
RESV
Receiver B
Receiver A
RSVP
• RSVP does not specify how the network
provides the reserved bandwidth to the
data flows.
• RSVP is not a routing protocol.
• RSVP is a signaling protocol that allows
host to establish and tear down
reservations for data flows.
Packet Processing
• Packet classification associates each packet
with the appropriate reservation class.
– IPv4: SA, DA, SP, DP, Protocol
– IPv6: Flow label
• Packet scheduling manages queues so that
each packet receives the requested service.
– Guaranteed: calculate end-to-end delay
– Controlled load: assign the aggregate flow with a
weight based on the amount of traffic admitted
WFQ
• WFQ provides different amount of service
among queues according to their weights.
Challenges
• Scalability
– Per-flow states
• Overhead
– Signaling messages
– Refresh messages
– Packet processing
• Security
– RSVP deny of service
Differentiated Services
• Diffserv provides scalable and flexible
service differentiation to handle different
classes of traffic in different ways within
the Internet.
• IETF diffserv working group
• DS field (RFC2474, 2475)
• Per-Hop Behaviors (PHB, RFC2474, 2597,
2598)
Architecture
• “Complex at edge, Simple at core.”
– At edge:
•
•
•
•
Classifying
Metering
Marking
Conditioning
– At core
• BA classifying
• Queuing and scheduling
Mechanisms
• Diffserv Code Point (DSCP)
• Behavior Aggregate (BA)
• Per-Hop Behaviors (PHB)
– Expedited Forwarding (EF, RFC3246)
– Assured Forwarding (AF, RFC2597)
DSCP
• Edge routers mark packets of different
classes with different DSCP.
• Core routers treat packets with different
level of services according to its DSCP.
• DS field: TOS(IPv4), Traffic class(IPv6)
• CU: currently unused
Edge Functions
• Meter monitors whether the incoming packet
flow conforms to the negotiated traffic profile.
• Shaper spaces the incoming packets to the
negotiated traffic rate.
PHB
• Per-Hop Behavior is a description of the externally
observable forwarding treatment applied at a
differentiated services-compliant node to a behavior
aggregate.
• EF is intended to provide a building block for low delay,
low jitter and low loss services by ensuring that the EF
aggregate is served at a certain configured rate.
• The AF PHB group provides delivery of IP packets in
four independently forwarded AF classes. Within each
AF class, an IP packet can be assigned one of three
different levels of drop precedence.
backward compatibility
Best-Effort traffic
TOS (RFC 791)
IP precedence (RFC 1349)
0
1
2
3
4
5
6
DSCP
Pool 1
IP precedence
Best-Effort
Default PHB
Expedited
Forwarding
PHB
7
0
CU
1
2
3
4
Precedence
5
6
TOS
7
0
X
X
X
X
X
0
1
1
1
0
0
0
111
Network control
1
1
0
0
0
0
110
Internetwork control
1
0
1
0
0
0
101
Critical
1
0
0
0
0
0
100
Flash override
0
1
1
0
0
0
011
Flash
0
1
0
0
0
0
010
Immediate
0
0
1
0
0
0
001
Priority
0
0
0
0
0
0
000
Routine
High Priority
Class Selector Codepoint
1
0
Assured
Forwarding
PHB
1
1
0
Low Priority
0
Class 1
Class 2
Low Drop
0
1
0
0
0
0
0
1
1
0
0
0
Medium Drop
0
1
0
0
1
0
0
1
1
0
1
0
High Drop
0
1
0
1
0
0
0
1
1
1
0
0
Low Drop
1
0
0
0
0
0
1
0
1
0
0
0
Medium Drop
1
0
0
0
1
0
1
0
1
0
1
0
High Drop
1
0
0
1
0
0
1
0
1
1
0
0
Class 3
Class 4
PHB Implementations
•
•
•
•
Priority queue
WFQ
RED with In and Out (RIO)
Weighted RED (WRED)
P(drop)
1.0
MaxP
AvgLen
Min out
Min in Max out
Max in
E2E DiffServ
• DS Domain
• Bandwidth Broker (BB)
• Service Level Agreement (SLA)
BB
Adjacent BB
Inter-Domain
Interface
Application
Server
User/Host
Network
Operator
Adjacent BB
User/App
Interface
Data
Repository
Edge
Routers
Policy Manager
Interface
Network Management
Interface
Intra-Domain
Interface
Routing
Information
Edge
Routers
ATM QoS
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
Service Classes
ABR
• ABR Resource Management (RM) cells
• Virtual Source/Virtual Destination
H1
Source
RM cells
S1
S2
RM cells
Virtual Virtual
destination source
S3
H2
Destination
RSVP v.s. Q.2931
• RSVP
– receiver generates
reservation
– soft state
(refresh/timeout)
– separate from route
establishment
– QoS can change
dynamically
– receiver heterogeneity
• ATM Q.2931
– sender generates
connection request
– hard state (explicit
delete)
– concurrent with route
establishment
– QoS is static for life of
connection
– uniform QoS to all
receivers