Congestion Control An Overview -Jyothi Guntaka

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

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

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)

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

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.

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.

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.

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.

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

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.

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)

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.

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.

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.

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.

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 ( ct/188lect8net.pdf).