Section 6.1: Resource Allocation Issues Chapter 6: Congestion - - PDF document

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Chapter 6: Congestion Control and Resource Allocation

CS/ECPE 5516: Comm. Network

• Prof. Abrams

Spring 2000

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Section 6.1: Resource Allocation Issues

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How to prevent traffic jams

• Traffic lights on freeway entrance during rush hour

n Controls and reduces congestion after it develops n Congestion Control

• Schedule each car’s trip on highway so congestion never

develops

n Resource allocation

• Carpool Lane w/ traffic lights

n Gives higher priority to carpool vehicles n Congestion could still develop n Combination of Resource allocation & Congestion Control

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What’s wrong with doing ONLY…

• Resource Allocation?
• Congestion control?

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Is routing a congestion control mechanism?

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How do these differ…

• Congestion control

vs.

• Flow control

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Another term…

• Fairness:

n Attempt to allocate resources to customers in way

that some property is maintained…

n Perhaps equal resource use n Perhaps a guaranteed fraction of resource use n Perhaps a guarantee that eventually each customer will be

served

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Objectives of Congestion Control

• Two main objectives of congestion control

n Delay … Achieve balance between rejecting traffic

(which decreases throughput) and keeping delay low

n Fairness

• Secondary objective of congestion control

n Buffer overflow … Prevent buffer overflow that can

lead to deadlock or extreme throughput degradation

n Related to delay and fairness

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Which One does the Telephone Network Do?

• Congestion control?
• Resource allocation?

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Ways to Impose Congestion Control (1)

• Call blocking

n A “session” is blocked from entering the network n Traditionally used in circuit-switched networks

n “Busy signal”

n Used in ATM

n CBR, VBR, or ABR session requests connection with guaranteed

bandwidth and/or maximum latency -- quality of service (QoS)

n Network will accept or block connection n Network could offer a “degraded” connection that may be acceptable to

application

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Ways to Impose Congestion Control (2)

• Packet (cell) discarding

n Packets discarded when buffers are (nearly) full n Discarding can be selective

n Packets from low priority sessions n Packets from sessions exceeding their allotted bandwidth n Packets likely to be discarded elsewhere

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Ways to Impose Congestion Control (3)

• Packet blocking

n Individual packets can be prevented from entering

network

n Buffer for later transmission (high priority) n Discard (low priority)

n It’s better to prevent packet from entering network

than to enter, user resources, and be discarded

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Ways to Impose Congestion Control (4)

• Packet scheduling

n Network nodes may selectively expedite or delay

packet transmission

n Selection criteria

n Delay low priority traffic in favor of high priority traffic n Use round-robin schedule to ensure fairness n Delay packets that will use congested resources elsewhere

in network

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How easy is it to do congestion control/resource allocation on…

• Connection-oriented
• Connection-less
• Flow-based (see next slide…)

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Flows Vs. Datagrams

• Datagram->packets can all take different routes
• Flows->packets follow same route, even without

connection setup

• Routers recognize flow

n perhaps via flow-identifier in packet headers (IPv6) n perhaps by flow set-up message n perhaps by seeing same source/dest layer3 addresses

• Routers maintain soft state

n created/deleted by router, not by signaling

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6.2: Example of Resource Allocation with Flows

• Fair Queuing (FQ)

n One flow per queue n Packets vary in size; service flows with small packets

more often …

Round- Robin Service

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6.1.2: Taxonomy of Approaches to Resource Alloc. & Congestion

Router Centric Host Centric Reservation-Based Feedback-Based Rate-Based Window-Based

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Taxonomy of Approaches (2)

• Router-centric versus host-centric: Where is the main

functionality of congestion control?

n Hosts -- at the edge of the network n Routers -- inside the network

• Reservation-based versus feedback-based

n Hosts can reserve capacity

n Network accepts or rejects (blocks)

n Network can provide feedback to hosts

n Explicit – bit in header indicating congestion n Implicit – dropped packet

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Taxonomy of Approaches (3)

• Window-based versus rate-based

n Window-based: hosts are controlled by limiting their

send window size

n Messages update the size of the window

n Rate-based: hosts transmit at some rate

n Example: video stream needs 1.5 Mbps

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Where is TCP?

Router Centric Host Centric Reservation-Based Feedback-Based Rate-Based Window-Based

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6.4: Alternatives to TCP…

• Congestion Control

n What TCP uses n Slow start creates congestion to detect it! n Why? Tries to solve problem at end hosts only

• Congestion Avoidance

n Routers + end hosts cooperate n Routers signal developing problems n End hosts can throttle back before congestion occurs

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Congestion Avoidance Example: DECbit

• Add one bit (congestion bit) to packet header
• Router measures ave. queue length
• Router sets bit=1 if queue length >1
• Receiver returns congestion bit in ack header
• Sender:

n if <50% of acks in window have bit=1,

CWND=CWND*7/8

n else CWND = CWND+1

• vs. TCP’s 1/2

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Congestion Avoidance Example: Random Early Detection (RED)

• Due to Sally Floyd and Van Jacobson
• No explicit congestion bit like DECbit
• Instead, router intentionally drops packet to

signal end host to slow down!

• So router drops packets sooner than it would

have to

• “Random” in name means packets from random

flows are dropped, to be fair

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More on Random Early Detection (RED)

• Router computes ave q length:

n AveLen = (1-λ)AveLen + λ LenSample

(Look familiar?)

Pr(drop) AveLen Max Min 1.0

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Miscellaneous facts on RED

• Factor λ is based on RTT for typical connections:

If router drops packet, takes as much as RTT for end host to slow down. So router shouldn’t adapt faster than RTT.

• With random drops, flows using most bandwidth

have most drops – a notion of fairness

• In ATM, packet requires multiple cells. If RED

drops one cell, drop all cells.

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6.5: Quality of Service

• Certain traffic has performance requirements

n Real-time (e.g., max packet latency for voice call) n Multimedia (e.g., frame rate for video)

• IETF’s 2 service classes (“Integrated Services”)

n Guaranteed Service

n Voice calls cannot tolerate packets late by a certain amount

n Controlled Load

n Streaming media player can slowly adjust its buffer size to

fit performance of channel.

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Some QoS parameters

• Data rate
• Average latency (delay)
• Variance in delay or jitter

n Jitter is the instantaneous difference in two

synchronized events or streams

• Error rate

n Bit error rate (BER) n Packet error rate (PER) – entire packet is lost

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QoS Terminology…

• Flowspec:

n “I want controlled load” or “jitter<125µs” or

“my ave. sending rate is 1Mbps, with bursts up to 10Kb long”

• Admission control:

n We’ve got six 1Mbps connections; we cannot admit a seventh

• Resource reservation:

n Allocate resources based on QoS needs

• Packet scheduling:

n Determine when to transmit a packet at hosts & routers

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Resource Reservation Protocol -- RSVP (1)

• For unicast or multicast
• Step 1: senders send PATH message requesting

service

• Step 2: receivers decide what to give senders, and

make reservation with RESV message

n Receiver decision is opposite of a traditional phone call

• Routers use “soft state”

n Reservation state is cached in routers n Periodically refreshed when receivers send RESV

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Resource Reservation Protocol -- RSVP (2)

Src 1 Src 2 Rcvr A

RESV RESV RESV PATH PATH

Rcvr B

Joe is sending video at 10 fps with flowID 97

I’ll listen to Joe on flowID 97 at 5fps

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Differentiated Services

• You just saw Integrated Services:

n Allocate resources to individual flows n RSVP run for every “call”, and routers must keep

track of many flows.

• Differentiated services:

n Just two service classes, period: normal & premium n Packets identify which they belong to with a bit

• Rationale: Internet has trouble with 1 class today,

so just increase number classes to 2!