Transcript 8-2_diffserv07

Differentiated Services
2007
Two Approaches for Providing
QoS on the Internet
“Freeway model” -- integrated services Internet
(intserv)
– Build a dedicated highway or “circuit” between
communicating points (VIP)
 “Doctor’s model” -- differentiated services
(diffserv)
– Mark a doctor’s vehicle (e. g.,ambulance) or
“packet” to get priority the road and limit the
percentage of such high- priority vehicles in the
total traffic mix (fire-engine, policeman)

Difficulties with Intserv and RSVP
• Intsev
– A connection-oriented solution
– QoS on a per-flow basis
– Depends on resource reservation (RSVP) - signaling
• Scalability
– Keeping a state (and using it!) for each flow
overloads the routers
– Periodic messages to refresh the states create
more traffic
• Router complexity increases
• How to satisfy heterogeneous QoS requirements
for different receivers
• Ambitious signaling is not practical
Differentiated Services (Diff-Serv)
• Motivations
– Don’t want end-to-end signaling and per-flow state, for
scalability, complexity and quick-to-deploy reasons
• Connectionless mode
• QoS for aggregates of traffic, such as premium, assured
and best- effort
– Avoid strong assumptions about traffic types
• Keep the forwarding path simple
– Push complexity to network edge -> Differentiated
Services Domain (DS Domain)
• Per-Hop-Behavior (PHB)
– Define behavior of individual routers rather than endto-end services
– forwarding behavior
Differentiated Service (DS) Field
• Diff-Serv Approach: Use the TOS field to sort packets into
classes and routers treat them differently
– e.g., delay requirement, drop precedence etc.
• DS filed reuse the first 6 bits from the TOS byte
– Diff- Service Code Point (DSCP)
• The other two bits are proposed to be used by ECN
0
5 6 7
DS Filed
0
4
Version HLen
8
19
TOS
Identification
TTL
16
31
Length
Flags
Fragment offset
Protocol
Header checksum
Source address
Destination address
Data
IP
header
Differentiated Services (Diffserv)
• Build around the concept of domain
• Domain – a contiguous region of network under
the same administrative ownership
• Differentiate between edge and core routers
• Edge routers
– Perform per aggregate shaping or policing
– Mark packets with a small number of bits; each
bit encoding represents a class (subclass)
• Core routers
– Process packets based on packet marking
• Far more scalable than Intserv, but provides
weaker services
Diffserv Architecture
• Ingress routers
– Police/shape traffic
– Set Diff- Service Code Point (DSCP) in Diffserv (DS) field
• Core routers
– Implement Per Hop Behavior (PHB) for each DSCP
– Process packets based on DSCP
DS-2
DS-1
Ingress
Ingress
Egress
Edge router
Core router
Egress
Differentiated Services
• Two types of QoS service
– Two PHBs defined so far
Premium service
• EF - Expedited forwarding (type P)
Assured service
• AF - Assured forwarding (type A)
– 4 classes, each guaranteed a minimum
amount of bandwidth and buffering
– each with three drop preference
partitions
• Plus, best-effort service
Premium Service
• EF - Expedited Forwarding
• Provides guaranteed peak bandwidth service with
negligible delay / jitter
– Provides the abstraction of a virtual pipe
between an ingress and an egress router
• Network: guarantees that premium packets are
not dropped and they experience low delay with
requested profile
• User: sends within profile. User does not send
more than the size of the pipe
– If it sends more, excess traffic is delayed, and
dropped when buffer overflows
• Rate limiting of EF packets at edges only, using
token bucket to shape transmission
Assured Service (1)
• AF -- Assured Forwarding
• Provides an expected level of bandwidth with delay
• Permits flows to use any additional available
bandwidth
• Network: provides lower loss rate than best-effort
– In case of congestion best-effort packets are
dropped first
• User: sends no more assured traffic than its profile
– If it sends more, the excess traffic is converted to
best-effort
Assured Service (2)
• User and network agree to some traffic profile
(how much traffic is a user allowed to send)
– Edges mark packets up to allowed rate as “inprofile” or low drop precedence
– Other packets are marked with higher drop
precedence
• A congested DS node tries to protect packets with a
lower drop precedence value from being lost by
preferably discarding packets with a higher drop
precedence value
– Implemented using buffer management, e. g., RIO -RED (random early detection) with In and Out
Assured Service (3)
• Large spatial granularity service
• Theoretically, user profile is defined irrespective of
destination
– All other services we learnt are end-to-end, i.e.,
we know destination(s) apriority
• This makes service very useful, but hard to
provision (why ?)
Traffic profile
Ingress
Components of Diff-Serv
(1) Edge node algorithms
– Classification, policing, shaping,
metering, marking, etc.
(2) Router algorithms
– Packet discard algorithms: RED-like
methods for in-profile and out-ofprofile traffic
– Priority, low-latency queueing
Ingress Edge Router
Ingress
Traffic conditioner
Class 1
Marked traffic
Traffic conditioner
Class 2
Data traffic
Classifier
Per aggregate
Classification
(e.g., user)
Best-effort
Scheduler
Traffic Conditioning
• Traffic conditioner (TC) implement
– Metering
– Marking
– Shaping
Drop on overflow
Packet
input
Wait for
token
Set EF bit
Packet
output
No token
Packet
input
Test if
token
token
Set AF
“in” bit
Packet
output
Scheduler
• Employed by both edge and core routers
• For premium service – use strict priority,
or weighted fair queuing (WFQ)
• For assured service – use RIO (RED with
In and Out)
– Has two classes, “In” and “Out” (of profile)
– Always drop OUT packets first
– For OUT measure entire queue
– For IN measure only in-profile queue
RIO (RED with In and Out)
Drop Probability
P(drop)
1.0
MaxP
AvgLen
Min out
Minin Maxout
Maxin
Scheduler Example
• Premium traffic sent at high priority
• Assured and best-effort traffic pass through
RIO and then sent at low priority
yes
high priority
P-bit set?
no
yes
A-bit set? no
RIO
low priority
Core Router Output Processing
What DSCP?
EF
High-priority Q
Packets out
AF
If “in” set
incr in_cnt
Low-priority Q
RIO queue
management
If “in” set
decr in_cnt
Control Path
• Each domain is assigned a Bandwidth Broker (BB)
– Usually, used to perform ingress-egress
bandwidth allocation
• BB is responsible to perform admission control in
the entire domain
• BB not easy to implement
– Require complete knowledge about domain
– Single point of failure, may be performance
bottleneck
– Designing BB still a research problem
Example
• Achieve end-to-end bandwidth guarantee
3
2
BB
7
BB
1 9
8 profile
6
5
BB
4
profile
profile
receiver
sender
Comparison
Best-Efforts
Diffserv
Intserv
Service
• Connectivity
• No isolation
• No guarantees
• Per aggregation
isolation
• Per aggregation
guarantee
• Per flow isolation
• Per flow guarantee
Service Scope
• End-to-end
• Domain
• End-to-end
Complexity
• No set-up
• Long term setup
• Per flow setup
Scalability
• Highly scalable
• (nodes maintain
only routing state)
• Scalable (edge
• Not scalable
routers maintains
(each router
per aggregate state; maintains per
core routers per
flow state)
class state)
Summary
• Diffserv more scalable than Intserv
– Edge routers maintain per aggregate state
– Core routers maintain state only for a few traffic
classes
• But, provides weaker services than Intserv
– Per aggregate bandwidth guarantee (premium
service) vs. per flow bandwidth and delay
guarantee
• BB is not an entirely solved problem
– Single point of failure
– Handle only long term reservations (hours, days)
Intserv over Diffserv
-- Integrated Services Operation over Diffserv Networks
• Intserv enables per-flow end-to-end QoS
– enables hosts to request per-flow,
quantifiable resources, along end-to-end
data paths and to obtain feedback
regarding admissibility of these requests
• Diffserv enables scalability across large
networks
• => Intserv over Diffserv
– Integrated Services Operation over
Diffserv Networks
Network Configuration
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|------| |-----|
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|Tx |-|
|ER1|---|BR1|
|BR2|---|ER2|
|-|Rx |
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|-----| |-----|
|------| |-----|
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\________/
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Non-Diffserv region Diffserv region Non-Diffserv region
• ER -- edge routers which are adjacent to the Diffserv region
• BR -- border routers within the Diffserv region
QoS over Next Generation Networks
• Concluding Remark by H. T. Kung 1998:
– In 1993 ATM was in, but it was out of favor in 1997
– In 1995 IP switching was in, but it was out of favor
in 1997
– In 1996 RSVP was in, but it is out of favor in 1998
– In 1998 Diff-Serv and MPLS are in, but will they
also be out of favor soon?
• IWQoS2000 panel discussion:
– ATM is on its way out.
– Intserv is dead.
– Diffserv is dying.
– MPLS is in misguiding.
• Growing use of Multi-Protocol Label Switching now
Topics
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Integrated and Differentiated Services
Active Buffer Management techniques, ex. RED.
Multiprotocol Label Switching
QoS in 3G wireless multimedia networks
QoS based routing
QoS management in multi-bearer IP networks
including PSTN
• Impact of media compression in QoS networks
• QoS over Mobile IP
• QoS enabled MAC design