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Congestion Control An Overview

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The aggregate demand for network resources exceeds the available capacity of a link. ... Action before the network becomes overloaded. Congestion Control (Reactive) ... – PowerPoint PPT presentation

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Title: Congestion Control An Overview


1
Congestion Control An Overview
  • -Jyothi Guntaka

2
Congestion
  • What is congestion ?
  • The aggregate demand for network resources
    exceeds the available capacity of a link.
  • Effects of congestion
  • Performance Degradation
  • Multiple packet loss
  • Low link utilization (low throughput)
  • High queueing delay
  • Congestion collapse

3
Approaches
  • Congestion Avoidance (Proactive)
  • Action before the network becomes overloaded.
  • Congestion Control (Reactive)
  • Action after the network is overloaded.

4
Implicit vs. Explicit feedback
  • Implicit feedback Congestion Control
  • Network drops packets when congestion occurs..
  • Source infer congestion implicitly.
  • time-out, duplicated ACKs, etc.
  • Example end-to-end TCP congestion Control.
  • Simple to implement but inaccurate.
  • implemented only at Transport layer (e.g., TCP)

5
Implicit vs. Explicit feedback (contd)
  • Explicit feedback Congestion Control
  • Network component (e.g., router) provides
    congestion indication explicitly to sources.
  • use packet marking, or RM cells (in ATM ABR
    control)
  • Examples DECbit, ECN, ATM ABR CC, etc.
  • Provide more accurate information to sources .
  • But is more complicated to implement.
  • Need to change both source and network algorithm
  • Need cooperation between sources and network
    component

6
Importance of Congestion Control - Issues
  • Fairness
  • No universal (mathematical) definition for
    fairness.
  • Depends on how many relevant dependencies are
    included in the model.
  • All being equal concept (in best effort networks)
  • Everybody gets equal service.
  • All resources available to everybody.
  • Each is expected to respect others and behave
    accordingly.
  • When a new connection is added, everybody gets a
    little bit worse service.

7
Issues (contd)
  • Economical aspect (in QoS enabled networks)
  • You should get what you pay for.
  • Old flows should not experience harm if a new
    flow is accepted.

8
TCP Congestion Control
  • Use end-to-end congestion control
  • use implicit feedback.
  • e.g., time-out, triple duplicated ACKs, etc.
  • use window based flow control.
  • self-clocking
  • slow-start and congestion avoidance
  • Examples
  • TCP Tahoe, TCP Reno, TCP Vegas, etc.

9
AIMD
  • Additive Increase/Multiplicative Decrease (AIMD)
  • Objective adjust to changes in the available
    capacity.
  • New state variable per connection
    CongestionWindow.
  • Limits how much data source has in transit.
  • TCP source sending no faster than the slowest
    component. (network or destination host) can
    tolerate.
  • Idea
  • Increase CongestionWindow when congestion goes
    down.
  • Decrease CongestionWindow when congestion goes up.

10
Random Early Detection (RED)
  • RED
  • Use network algorithm to detect incipient
    congestion.
  • Design goals
  • minimize packet loss and queueing delay
  • avoid global synchronization
  • maintain high link utilization
  • removing bias against bursty source
  • Achieve goals by
  • randomized packet drop
  • queue length averaging

11
Explicit Congestion Notification (ECN)
  • Current congestion indication
  • use packet drop to indicate congestion.
  • source infers congestion implicitly.
  • ECN
  • to give less packet drop and better performance.
  • use packet marking rather than drop.
  • need cooperation between sources and network.

12
High Bandwidth-Delay Product Environments
  • TCP congestion control performs poorly as
    bandwidth or delay increases.
  • Proposed an eXplicit Control Protocol (XCP).
  • Small queues
  • Almost no drops
  • Improved fairness
  • Scalable (no per-flow state)

13
TCP vs. XCP
  • TCP
  • AIMD controls both congestion and fairness.
  • XCP
  • To control congestion MIMD is used which shows
    fast response.
  • To control fairness AIMD is used which converges
    to fairness.
  • XCP provides a joint design of end-systems and
    routers and it can co-exist with TCP.

14
TCP friendly congestion control
  • TCP friendly a protocol that behaves like TCP
  • Backs off if congestion and uses a fair share of
    resources.
  • Protocol that obeys TCP long term throughput
    relation.
  • Internet requirement new transport protocols
    must be TCP friendly
  • Backs off if congestion and uses a fair share of
    resources.
  • Applies also to application layer protocols
    transmitting over UDP, e.g., real time telephony
    or streaming applications.
  • Rate control implemented on top of UDP as part of
    application.

15
TCP friendly congestion control (contd)
  • Non-TCP friendly
  • A protocol that takes more than its fair share of
    bandwidth (greedy).
  • May cause fluctuations in network load and result
    in congestion collapse.
  • How to protect your protocol against non-TCP
    friendly greedy protocols?
  • RED is designed to solve this problem to some
    extent.

16
Research
  • Develop a multipath protocol dealing with
    congestion control.
  • Further study on measuring available bandwidth
    and other network path characteristics.
  • Congestion control in wireless networks.
  • Sensor networks.
  • Completely wireless and hybrid wireless networks.

17
Acknowlegements
  • Some of the slides from
  • Improving Adaptability and Fairness in Internet
    Congestion Control.
  • Congestion Control for High Bandwidth-Delay
    Product Environments.
  • Lecture notes on Congestion Control
    (http//www.tct.hut.fi/opetus/s38188/2003/lect/188
    lect8net.pdf).
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