Differentiated Services 2007
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Transcript of Differentiated Services 2007
Differentiated Services
2007
Two Approaches for ProvidingQoS on the Internet
“Freeway model” -- integrated services Internet (intserv)– Build a dedicated highway or “circuit” between
communicating points (VIP) “Doctor’s model” -- differentiated services (diffser
v)– Mark a doctor’s vehicle (e. g.,ambulance) or “p
acket” 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 ov
erloads the routers– Periodic messages to refresh the states create
more traffic• Router complexity increases • How to satisfy heterogeneous QoS requirements f
or 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 a
nd 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 end-to-end services
– forwarding behavior
Differentiated Service (DS) Field• Diff-Serv Approach: Use the TOS field to sort packets into cl
asses 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
Version HLen TOS Length
Identification Fragment offsetFlags
Source address
Destination address
TTL Protocol Header checksum
0 4 8 16 19 31
Data
IPheader
DS Filed0 5 6 7
Differentiated Services (Diffserv)• Build around the concept of domain• Domain – a contiguous region of network under th
e 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 weak
er 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
IngressEgressEgress
IngressEgressEgress
DS-1 DS-2
Edge router Core router
Differentiated Services• Two types of QoS service
– Two PHBs defined so farPremium 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 partit
ions• 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 “in-
profile” 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 ?)
Ingress
Traffic profile
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-of-profile traffic
– Priority, low-latency queueing
Ingress Edge Router
Classifier
Traffic conditioner
Traffic conditioner
Scheduler
Class 1
Class 2
Best-effort
Marked traffic
Ingress
Per aggregateClassification (e.g., user)
Data traffic
Traffic Conditioning
Wait fortoken
Set EF bitPacketinput
Packetoutput
Test iftoken
Set AF “in” bit
token
No token
Packetinput
Packetoutput
Drop on overflow
• Traffic conditioner (TC) implement– Metering– Marking– Shaping
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
Minin MaxinMaxoutMinout
AvgLen
Scheduler Example
• Premium traffic sent at high priority
• Assured and best-effort traffic pass through RIO and then sent at low priority
P-bit set?
A-bit set? RIO
yes
no
yes
no
high priority
low priority
Core Router Output Processing
What DSCP?
If “in” setincr in_cnt
High-priority Q
Low-priority Q
If “in” setdecr in_cnt
RIO queuemanagement
Packets out
EF
AF
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
BBBB BBBB BBBB
1
2 3
579
sender
receiver
8 profile 6profile
4 profile
Comparison
Service
Service Scope
Complexity
Scalability
• Connectivity• No isolation• No guarantees
• End-to-end
• No set-up
• Highly scalable• (nodes maintain
only routing state)
Best-Efforts
• Per aggregation isolation
• Per aggregation guarantee
• Domain
• Long term setup
• Scalable (edge routers maintains per aggregate state; core routers per class state)
Diffserv
• Per flow isolation• Per flow guarantee
• End-to-end
• Per flow setup
• Not scalable (each router maintains per flow state)
Intserv
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 se
rvice) 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, quantifi
able 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 Diffse
rv Networks
Network Configuration _______ ___________ ______
/ \ / \ / \
/ \ / \ / \
|----| | |-----| |-----| |------| |-----| | |-----|
|Tx |-| |ER1|---|BR1| |BR2|---|ER2| |-|Rx |
|----| | |-----| |-----| |------| |-----| | |-----|
\ / \ / \ /
\________/ \____________/ \_______/
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
• 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 i
ncluding PSTN• Impact of media compression in QoS networks• QoS over Mobile IP• QoS enabled MAC design