cisc 7332x t6 c15a: congestion control and quality of service€¦ · c15a: congestion control and...
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CISC 7332X T6
C15a: Congestion Control and Quality of Service
Hui Chen
Department of Computer & Information Science
CUNY Brooklyn College
12/4/2018 1CUNY | Brooklyn College
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The Network Layer
• Responsible for delivering packets between endpoints over multiple links
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Physical
Link
Network
Transport
Application
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Congestion
• Overloading of network (when too much traffic is offered)
• Packet switching: store-and-forward
• Need memory to store packets
• Need processing power to forward packets
• e.g., too many packets arriving at a router,exceed its capacity, result in lost of packets (no memory to store the packets) or packet delay
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Performance Degradation
• Performance degrades due to loss or retransmissions
• Goodput (=useful packets) trails offered load
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Handling Congestion: Layered Approach• Effectively handling congestion requires that
the Network and upper (e.g., Transport) layers work together
• Congestion control approaches in Network Layer
• Traffic-aware routing
• Admission control
• Traffic throttling
• Load shedding
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Congestion Control Approaches
• Network must do its best with the offered load
• Different approaches at different timescales
• Nodes should also reduce offered load (Transport)
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Traffic-Aware Routing
• Choose routes depending on traffic, not just topology
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Traffic-Aware Routing: Example
• e.g., use EI for West-to-East traffic if CF is loaded
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Traffic-Aware Routing: Challenges• Favorable routes changing quickly, results in
wild oscillation in routing tables.
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Admission Control
• Admission control allows a new traffic load only if the network has sufficient capacity, e.g., with virtual circuits
• Can combine with looking for an uncongested route
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Admission Control: Example
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Network with some congested nodes
Uncongested portion and route AB around congestion
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Traffic Throttling
• Congested routers signal hosts to slow down traffic
• ECN (Explicit Congestion Notification) marks packets and receiver returns signal to sender
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Traffic Throttling
• Example: using the Exponentially Weighted Moving Average on queuing delay inside routers
• dnew = dold + (1 - ) s
• where d is delay and s is queue length
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Load Shedding
• When all else fails, network will drop packets (shed load)
• Can be done end-to-end or link-by-link
• But which packet to drop?
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Link-by-Link Load Shedding
• Link-by-link produces rapid relief
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End-to-end Load Shedding
• End-to-end takes longer to have an effect, but can better target the cause of congestion
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Questions?
• Concept of congestion and congestion control
• Network layer approaches
• Traffic-aware routing
• Admission control
• Traffic throttling
• Load shedding
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Quality of Service
• Application requirements
• Traffic shaping
• Packet scheduling
• Admission control
• Integrated services
• Differentiated services
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Application Requirements
• Different applications care about different properties
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“High” means a demanding requirement, e.g., low delay
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QoS on Network Layer
• Network provides service with different kinds of QoS (Quality of Service) to meet application requirements
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Network Service Application
Constant bit rate Telephony
Real-time variable bit rate Videoconferencing
Non-real-time variable bit rate Streaming a movie
Available bit rate File transfer
Example of QoS categories from ATM networks
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Traffic Shaping
• Traffic shaping regulates the average rate and burstiness of data entering the network
• Lets users make guarantees: “my transmission pattern will look like this, can you handle it?”
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Shape traffic here
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Token/Leaky Bucket
• Token/Leaky bucket limits both the average rate (R) and short-term burst (B) of traffic
• For token, bucket size is B, water enters at rate R and is removed to send; opposite for leaky.
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Leaky bucket(need not full to send)
Token bucket(need some water to send)
to send
to send
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Effect of Token Bucket
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Shaped by R=200 Mbps B=9600 KB
Shaped by R=200 Mbps B=0 KB
Host trafficR=200 Mbps B=16000 KB
Smaller bucket size delays traffic and reduces burstiness
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Packet Scheduling
• Without packet scheduling, packets are handled in the FIFO (first in first out) manner
• Packet scheduling divides router/link resources among traffic flows other than FIFO
• Different queueing discipline
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Round-Robin Fair Queueing
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Weighted-Fair Queueing
• Fair Queueing approximates bit-level fairness with different packet sizes; weights change target levels
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Weighted-Fair Queueing: Example
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Packets may be sent out of arrival order
Finish virtual times determine transmission order
Fi = max(Ai, Fi-1) + Li/W
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Admission Control
• Admission control takes a traffic flow specification and decides whether the network can carry it
• Sets up packet scheduling to meet QoS
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Example flow specification
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Admission Control: Example
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Admission Control: Example
• Construction to guarantee bandwidth B and delay D:
• Shape traffic source to a (R, B) token bucket
• Run WFQ with weight W / all weights > R/capacity
• Holds for all traffic patterns, all topologies
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Integrated Services
• Design with QoS for each flow
• Handle multicast traffic.
• Admission with RSVP (Resource reSerVationProtocol):
• Receiver sends a request back to the sender
• Each router along the way reserves resources
• Routers merge multiple requests for same flow
• Entire path is set up, or reservation not made
• Require advanced setup to establish each flow
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Integrated Services: Example
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R3 reserves flow from S1
R3 reserves flow from S2
R5 reserves flow from S1; merged with R3 at H
Merge
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Differentiated Services
• Design with classes of QoS
• Customers buy what they want
• Expedited class is sent in preference to regular class
• Less expedited traffic but better quality for applications
• No requirement on setting up path. Each router implements it indepedently
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Differentiated Services: Example
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Differentiated Services: Implementation• Customers mark desired class on packet
• ISP shapes traffic to ensure markings are paid for
• Routers use WFQ to give different service levels
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Differentiated Services: Implementation
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Questions?
• Application requirements
• Quality of Services (QoS) provisioning
• Traffic shaping
• Packet scheduling
• Admission control
• Integrated services
• Differentiated services
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