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1lect12.ppt S-38.145 - Introduction to Teletraffic Theory - Fall 2001
12. Traffic management in Internet
2
12. Traffic management in Internet
Contents
• Introduction
• The Integrated Services architecture• The Differentiated Services architecture
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12. Traffic management in Internet
Why not ATM?
• ATM offers a lot ...– traffic management
– voice/data integration: CBR, VBR, ABR, UBR– signaling, QoS routing
• ... but to reach the desktop requires new interfaces (HW+SW) at theend-systems!
– ATM signaling (UNI) considered complex, thus HW + SW for it has beenconsidered expensive for end-systems
– Difficult to use the QoS capabilities of ATM: standards do not consider, howapplications should choose all the traffic contract parameters
– Design favors voice (short cells)– Too much, too soon ...
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12. Traffic management in Internet
Why not ATM?
• More importantly, while the HW+SW for ATM devices were beingdeveloped, the Internet exploded (WWW)!
– the web browser created an easy access to the Internet
– exponential growth in the number of users/traffic amounts
• Internet won the race to the desktop!– Internet is now ubiquitous (everywhere)
• With new requirements from the applications demanding also QoSsupport, the approach is to try to enhance Internet’s capabilities, ratherthan build a new network (based e.g. on ATM)
– Cheaper to fix the old house than to build a new one!
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12. Traffic management in Internet
Why did the web explode?
• The user side– The web browser created an easy access to the Internet (accessible even
to your grand mother!)
– Allows the integration of new types of applications through the browserinterface (streaming media, interactive chats, games etc.)
– Number of users and the amount of traffic (still) increases exponentially
• Internet technology is very scalable– The networking paradigm is based on best-effort service (no guarantees are
made about the service quality) and the network is connectionless
– The nodes of the network do not store any state information of theusers/connections
– New nodes and users can be added to the network (almost) without anycomplexity increases
– Only the routing is affected by the increase in the number of nodes (routecomputation complexity grows with the number of nodes)
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12. Traffic management in Internet
Current trends driving the evolution [2]
• Decreasing HW costs (CPU, memory)
• Increasing computing power– Moore’s law:nof transistors per integrated circuit doubles every 18 months
– More powerful machines � more bandwidth hogging applications
• The link speeds increase dramatically– 1993: 100 Mbit/s (FDDI), 2000: 1 Tbit/s (dense WDM technologies)
• Traffic > capacity vs. capacity > traffic– If traffic is much greater than capacity, engineering and traffic control
mechanisms are required. If the opposite holds, just add more capacity ...– Capacity likely to be a problem in WANs.
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12. Traffic management in Internet
Current trends driving the evolution [2]
• Data traffic > voice traffic– 1998-99 amount of data traffic exceeded voice traffic
• ATM used in the core network
• Everything over IP– Data over IP (requires traffic engineering)– Voice (real time) over IP (requires QoS support)
– IETF is the center of all standardization work.
• From integrated services (IntServ) to differentiated services (DiffServ)– more on this in the next slides ...
• Traffic engineering– Traffic engineering methods needed to manage the complex new Internet
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12. Traffic management in Internet
Overview of QoS architectures in Internet
• Integrated services (IntServ)– defined in IETF in 1995-97
– fine grained QoS approach: provides QoS on a per connection (flow) basis– similar to ATM: resource reservation through signaling
– scalability problems ...
• Differentiated services (DiffServ)– Tries to solve the scalability problems of IntServ
– coarse grained approach: QoS provided for large aggregated traffic streams(nothing is guaranteed for individual flows)
• ATM can be thought of as both fine or coarse grained– QoS per each VC fine grained
– QoS per VPs (aggregation of VCs) coarse grained
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12. Traffic management in Internet
Contents
• Introduction
• The Integrated Services architecture• The Differentiated Services architecture
10
12. Traffic management in Internet
Integrated Services
• Specifications of service classes and mechanisms to implement them
• Service classes (= Service Category in ATM)– Guaranteed service: for intolerant applications that need a bound on the
maximum delay that any packet will experiance
– Controlled service: emulates a lightly loaded network– Controlled service traffic is isolated from guaranteed service traffic
• Mechanisms– Flow specification (flowspec): information characterizing the flow (traffic
contract in ATM)
– Admission control
– Flow reservation protocol RSVP (signaling protocol, e.g. Q.2931, in ATM)– Packet scheduling for implementing the given QoS guarantees
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12. Traffic management in Internet
Flow specification
• flowspec consists of 2 parts: RSpec and TSpec
• RSpec (Request Specification): requested service type– controlled service: no parameters– guaranteed service: delay target
• TSpec (traffic cpecification): bandwidth requirements– used as a basis for admission control– source behavior described by a token bucket
• chracterizes source’s mean rate + burstiness
• token rate (r), bucket depth (B) (+ some other parameters)
– Token bucket description:• To send n bytes, must have n tokens
• Tokens accumulate at rate r
• At most B tokens can be stored– Token bucket determines if a packet is conformant or not
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12. Traffic management in Internet
Router functionality
• Connection admission control– Input: the flow’s RSpec and TSpec
– Decision: The router determines if the connection can be admitted withoutdegrading the QoS of other already admitted connections, or not.
• Policing may be used to enforce each flow’s flowspec (cf. UPC in ATM)– The answer depends heavily on the used scheduling mechanisms implemented
by the router.
• Packet classification and scheduling– Packet classification: associate each packet with the appropriate reservation
• Based on source/destination address, protocol number, source/destinationport
– Different combinations of scheduling mechanisms (processor sharing, priorities,round-robin, ...) can be used to realize the guaranteed and conrolled loadservices
• Vendors can try to create as efficient implementations as possible.
13
12. Traffic management in Internet
RSVP
• Reservation protocol RSVP (corresponds to signaling in ATM)
• Features– Connectionless networks are robust (user state is not stored by the
network)
– RSVP tries to achieve robustness by using soft state information• Connections live as long as they are periodically refreshed (every 30 s)
– RSVP tries to support multicast as effectively as unicast
– Receiver oriented approach:
• Receiver decides how much resources he needs (in ATM andconnection oriented protocols in general, the sender decides theresource requirements)
• Receiver can change resource requirements dynamically with therefresh messages
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12. Traffic management in Internet
RSVP exampleSource: White paper - QoS protocols & architectures, http://www.qosforum.com/tech_resources.htm
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12. Traffic management in Internet
Problems with RSVP
• Router complexity– Admission control
– Classification and scheduling
• Not scalable with the number of flows– Soft state & admission control for every flow
– Route and allocation refreshment messages every 30s for each flow!
• Not suitable for core network (# of flows is too large in the core)– A method for dealing with traffic aggregates is needed � DiffServ
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12. Traffic management in Internet
Contents
• Introduction
• The Integrated Services architecture• The Differentiated Services architecture
17
12. Traffic management in Internet
Differentiated Services - Basic philosophy
• No signaling (instead service agreements)
• Network nodes implement defined per hop behaviors (PHBs) (notservices)
– Expedited Forwarding (EF), Assured Forwarding (AF):
• QoS is differential– No guaranteed QoS
– “Higher priority traffic gets better QoS than low priority traffic”
• Complex (per flow) traffic management functions only at the edge of thenetwork
– Classification and conditioning ...
– For each flow the traffic is classified as belonging to some traffic class
• Core functionality as simple as possible– Traffic management per traffic class (= aggregated traffic)
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12. Traffic management in Internet
Per Hop Behavior (PHB)
• Traffic class = PHB
• Different PHBs provide different QoS– However, no quantifiable guarantees are given!
• Operators configure PHB such that it meets desired (offered) servicequality in the DiffServ domain
• Traffic class of a packet marked in the packet header (Type of Servicefield in IPv4, Traffic Class in IPv6)
– 6 bits reserved for this
– Each bit combination is called a Differentiated Service Code Point (DSCP)– DSCP identifies the PHB of a packet
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12. Traffic management in Internet
Per Hop Behavior (PHB)
• So far, 2 PHBs have been defined
• Expedited Forwarding (EF) PHB– the packet service rate equals or exceeds a specified rate– “premium service”
– not affected by other PHBs (has presedence)
• Assured Forwarding (AF) PHB Group– 4 traffic classes
• Each class has a guaranteed minimum rate
• Within a class, 3 drop precedence priorities
• Total of 12 DSCP values
• PHB definitions do not specify how the PHB should be realized
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12. Traffic management in Internet
SLA (Service Level Agreement)
• DiffServ assumes that a Service Level Agreement (SLA) existsbetween networks (or users) sharing a border with a DiffServ network
• SLA defines the traffic profile of the “customer”– SLA is done off-line, i.e., no signaling to set set up dynamic SLAs!
SLA
SLASLA
SLA
DiffServ network 1
DiffServ network 2
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12. Traffic management in Internet
DiffServ - Functional Overview
SLA
SLASLA
SLA
DiffServ network 1
DiffServ network 2
Edge router
• performs per flow traffic management• marks packets as in-profile or out-profile
• assigns DSCP to the packets
Core router
• performs per class traffic management
• scheduling based on DSCP of packets• “preference given to in-profile packets”
• implements PHBs (EF, AF)
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12. Traffic management in Internet
DiffServ - Edge Router Functions
• Packet classification– Packet classifiers select packets in a traffic stream based on the content of
some portion of the packet header.
– BA (Behavior Aggregate) classifier classifies packets based on DSCP only
– MF (Multi-Field) classifier selects packets based on the value of acombination of several header fields (addresses, DS field, protocol id, ...)
Classifier Shaper/DropperMarkerpackets
discard
enque
Metermeasurements
affects affects
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12. Traffic management in Internet
DiffServ - Edge Router Functions
• Packet conditioning: based on the traffic profile obtained from classification– Meter: used to measure the traffic stream against a traffic profile
– Marker: based on information from the meter, packets are marked as in-profileor out-profile, out-profile packets may be demoted to a lower priority traffic class
– Shaper/Dropper: may delay out-profile packets (shaping) or drop them
Classifier Shaper/DropperMarkerpackets
discard
enque
Metermeasurements
affects affects
24
12. Traffic management in Internet
DiffServ - Core Router Functions
• Performs only packet forwarding– Inspects the DSCP of each packet
– Forwards the packet to the appropriate queue
• Efficient implementation of the queing disciplines required by differentPHBs not trivial!
ClassifierPS
processor sharing or weightedround robin scheduler
EF queue
AF group
2
1
1>2
packets
class 1 has priorityover class 2
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12. Traffic management in Internet
Problems with DiffServ
• End-to-end QoS difficult to realize just based on per-hop QoS
• SLAs are static– Network conditions and traffic needs change highly dynamically
• Network dimensioning inside the DiffServ network is difficult
• QoS provided to traffic aggregates, not individual flows– Long lasting and high bandwidth (e.g. video) flows need per flow
guarantees
• Currently some proposals consider having IntServ (RSVP) on theedges and DiffServ in the core.
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12. Traffic management in Internet
Literature
1 L. L. Peterson and B. S. Davie (2nd ed., 2000)– “Computer Networks: A Systems Approach”
– Morgan Kaufmann Publishers, San Fransisco, California
2 Professor Raj Jain’s homepage– http://www.cis.ohio-state.edu/~jain/
– e.g. course material from “Recent Advances in Networking (1999)”
3 Quality of Service Forum Homepage– http://www.qosforum.com/tech_resources.htm
– e.g. white papers on QoS and links to related sites