SlideShare a Scribd company logo

Congestion Control in Networks

•
•
R
rapatil

Congestion control aims to prevent network overload by regulating data transmission. It is concerned with bottlenecks in packet-switched networks and sometimes there is no way to route around congested routers. Common congestion control techniques include flow control, router-based approaches like queue management algorithms, and host-based approaches where end hosts adjust their behavior in response to network conditions. Fair queuing algorithms like weighted fair queuing assign weights to flows to control bandwidth allocation.

Related slideshows

Congestion control
Congestion controlCongestion control
Congestion control
 
Congestion control 1
Congestion control 1Congestion control 1
Congestion control 1
 
Internet congestion
Internet congestionInternet congestion
Internet congestion
 
1 of 7
Download to read offline
Congestion Control Definitions and  Flow control:: keep a fast sender from overrunning a slow receiver. Resource Allocation  Congestion control:: the efforts made by network nodes to prevent or respond to overload conditions. Reference:- “Computer Networks A Systems Congestion control is intended to keep a fast Approach”, Third Edition, Peterson and sender from sending data into the network Davie, due to a lack of resources in the network {e.g., available link capacity, router buffers}. Morgan Kaufmann, 2003. 1 2 Congestion Control Congestion 3  Congestion control is concerned with the 6 bottleneck routers in a packet switched network. 1 4  Congestion control can be distinguished 8 from routing in that sometimes there is no 2 7 5 way to ‘route around’ a congested router. Copyright ©2000 The McGraw Hill Companies 3 4 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
Flows Source 1 10-M  flow :: a sequence of packets sent between a bps Ethe rnet Router source/destination pair and following the same Destination 1.5-Mbps T1 link route through the network. DDI Source 100 -Mb ps F  Connectionless flows within the TCP/IP model:: 2 The connection-oriented abstraction, TCP, is implemented at the transport layer while IP provides a connectionless datagram delivery Congestion in a packet-switched packet- service. network  With connectionless flows, there exists no state at the routers. 5 6 Flows Source 1 Router Destination  Connection-oriented flows (e.g., X.25) – 1 connection-oriented networks maintain hard state Router at the routers. Source 2  Soft state :: represents a middle ground where Router soft state is not always explicitly created and Destination 2 removed by signaling. Source  Correct operation of the network does not depend 3 on the presence of soft state, but soft state can permit the router to better handle packets. Multiple Flows passing through a set of routers 7 8 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
Service Lack of Congestion Control  Best-effort service :: The hosts are given no opportunity to ask for guarantees on a Controlled Throughput flow’s service.  QoS (Quality of Service) :: is a service Uncontrolled model that supports some type of guarantee for a flow’s service. Offered load 9 10 Congestion Control Taxonomy Congestion Control Taxonomy  Router-Centric  Reservation-Based – the hosts attempt to The internal network routers take responsibility for:   Which packets to forward reserve network capacity when the flow is  Which packets to drop or mark established.  The nature of congestion notification to the hosts.  The routers allocate resources to satisfy  This includes the Queuing Algorithm to manage the reservations or the flow is rejected. buffers at the router.  Host-Centric  The reservation can be receiver-based (e.g.,  The end hosts adjust their behavior based on RSVP) or sender-based. observations of network conditions.  (e.g., TCP Congestion Control Mechanisms) 11 12 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
Congestion Control Taxonomy Evaluation Criteria  Feedback-Based - The transmission rate is adjusted (via window size) according to feedback received from  Evaluation criteria are needed to decide how well a the sub network. network effectively and fairly allocates resources.  Explicit feedback – FECN  Implicit feedback – router packet drops.  Effective measures – throughput, utilization,  Window-Based - The receiver sends an advertised efficiency, delay, queue length, goodput and power. window to the sender or a window advertisement can throughput α be used to reserve buffer space in routers. Power = --------------  Rate-Based – The sender’s rate is controlled by the delay receiver indicating the bits per second it can absorb. Where 0 < α < 1 13 14 Fairness Congestion Control  Jain’s fairness index (at the router)  Queuing algorithms determine: For any given set of user throughputs (x1, x2,…xn ), the (x x2 fairness index to the set is defined:  How packets are buffered.  Which packets get transmitted.  n  2  Which packets get marked or dropped.   xi  f(x1, x2, …, xn) =  i 1   Indirectly determine the delay at the router. n n  x i2  Queues at outgoing links drop/mark packets to  Max-min fairness i 1 implicitly signal congestion to TCP sources. Essentially ‘borrow’ from the rich-in-performance to help the poor-in- performance  Remember to separate queuing policy from queuing For example, CSFQ mechanism. 15 16 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
Congestion Control Drop Tail Router [FIFO] (at the router)  Some of the possible choices in queuing algorithms: • First packet to arrive is first to be transmitted.  FIFO (FCFS) also called Drop-Tail • FIFO queuing mechanism that drops packets from  Fair Queuing (FQ) the tail of the queue when the queue overflows.  Weighted Fair Queuing (WFQ) • Introduces global synchronization when packets are  Random Early Detection (RED) dropped from several connections.  Explicit Congestion Notification (ECN). • FIFO is the scheduling mechanism, Drop Tail is the policy 17 18 Priority Queuing Priority Queuing  Mark each packet with a priority (e.g., in  Problem:: high priority packets can TOS (Type of Service field in IP) ‘starve’ lower priority class packets.  Implement multiple FIFO queues, one for  Priority queuing is a simple case of each priority class. “differentiated services” [DiffServ].  Always transmit out of the highest priority  One practical use in the Internet is to non-empty queue. protect routing update packets by giving  Still no guarantees for a given priority them a higher priority and a special queue class. at the router. 19 20 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
Fair Queuing [FQ] Flow 1  The basic problem with FIFO is that it does Flow 2 Round-robin not separate packets by flow. service Flow 3  Another problem with FIFO :: an “ill- behaved” flow can capture an arbitrarily large share of the network’s capacity. Flow 4 Idea:: maintain a separate queue for each flow, and Fair Queuing (FQ) services these queues in a round-robin fashion. Fair Queuing 21 22 Fair Queuing [FQ] Fair Queuing [FQ]  FQ simulates bit-by-bit behavior by using timestamps (too many details for here!).  “Ill-behaved” flows are segregated into  One can think of FQ as providing a guaranteed their own queue. minimum share of bandwidth to each flow.  There are many implementation details for  FQ is work-conserving in that the server is never idle as long as there is a customer in the queue. FQ, but the main problem is that packets * Note: The per-flow state information kept at are of different lengths  simple FQ is not the router is expensive (it does not scale). fair!!  Ideal FQ:: do bit-by-bit round-robin. 23 24 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
Weighted Fair Queuing [WFQ] WFQ idea:: Assign a weight to each flow (queue) such that the weight logically specifies the number of bits to transmit each time the router services that queue.  This controls the percentage of the link capacity that the flow will receive.  The queues can represent “classes” of service and this becomes DiffServ.  An issue – how does the router learn of the weight assignments?  Manual configuration  Signaling from sources or receivers. 25 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/

More Related Content

Congestion Control in Networks

  • 1. Congestion Control Definitions and  Flow control:: keep a fast sender from overrunning a slow receiver. Resource Allocation  Congestion control:: the efforts made by network nodes to prevent or respond to overload conditions. Reference:- “Computer Networks A Systems Congestion control is intended to keep a fast Approach”, Third Edition, Peterson and sender from sending data into the network Davie, due to a lack of resources in the network {e.g., available link capacity, router buffers}. Morgan Kaufmann, 2003. 1 2 Congestion Control Congestion 3  Congestion control is concerned with the 6 bottleneck routers in a packet switched network. 1 4  Congestion control can be distinguished 8 from routing in that sometimes there is no 2 7 5 way to ‘route around’ a congested router. Copyright ©2000 The McGraw Hill Companies 3 4 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
  • 2. Flows Source 1 10-M  flow :: a sequence of packets sent between a bps Ethe rnet Router source/destination pair and following the same Destination 1.5-Mbps T1 link route through the network. DDI Source 100 -Mb ps F  Connectionless flows within the TCP/IP model:: 2 The connection-oriented abstraction, TCP, is implemented at the transport layer while IP provides a connectionless datagram delivery Congestion in a packet-switched packet- service. network  With connectionless flows, there exists no state at the routers. 5 6 Flows Source 1 Router Destination  Connection-oriented flows (e.g., X.25) – 1 connection-oriented networks maintain hard state Router at the routers. Source 2  Soft state :: represents a middle ground where Router soft state is not always explicitly created and Destination 2 removed by signaling. Source  Correct operation of the network does not depend 3 on the presence of soft state, but soft state can permit the router to better handle packets. Multiple Flows passing through a set of routers 7 8 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
  • 3. Service Lack of Congestion Control  Best-effort service :: The hosts are given no opportunity to ask for guarantees on a Controlled Throughput flow’s service.  QoS (Quality of Service) :: is a service Uncontrolled model that supports some type of guarantee for a flow’s service. Offered load 9 10 Congestion Control Taxonomy Congestion Control Taxonomy  Router-Centric  Reservation-Based – the hosts attempt to The internal network routers take responsibility for:   Which packets to forward reserve network capacity when the flow is  Which packets to drop or mark established.  The nature of congestion notification to the hosts.  The routers allocate resources to satisfy  This includes the Queuing Algorithm to manage the reservations or the flow is rejected. buffers at the router.  Host-Centric  The reservation can be receiver-based (e.g.,  The end hosts adjust their behavior based on RSVP) or sender-based. observations of network conditions.  (e.g., TCP Congestion Control Mechanisms) 11 12 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
  • 4. Congestion Control Taxonomy Evaluation Criteria  Feedback-Based - The transmission rate is adjusted (via window size) according to feedback received from  Evaluation criteria are needed to decide how well a the sub network. network effectively and fairly allocates resources.  Explicit feedback – FECN  Implicit feedback – router packet drops.  Effective measures – throughput, utilization,  Window-Based - The receiver sends an advertised efficiency, delay, queue length, goodput and power. window to the sender or a window advertisement can throughput α be used to reserve buffer space in routers. Power = --------------  Rate-Based – The sender’s rate is controlled by the delay receiver indicating the bits per second it can absorb. Where 0 < α < 1 13 14 Fairness Congestion Control  Jain’s fairness index (at the router)  Queuing algorithms determine: For any given set of user throughputs (x1, x2,…xn ), the (x x2 fairness index to the set is defined:  How packets are buffered.  Which packets get transmitted.  n  2  Which packets get marked or dropped.   xi  f(x1, x2, …, xn) =  i 1   Indirectly determine the delay at the router. n n  x i2  Queues at outgoing links drop/mark packets to  Max-min fairness i 1 implicitly signal congestion to TCP sources. Essentially ‘borrow’ from the rich-in-performance to help the poor-in- performance  Remember to separate queuing policy from queuing For example, CSFQ mechanism. 15 16 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
  • 5. Congestion Control Drop Tail Router [FIFO] (at the router)  Some of the possible choices in queuing algorithms: • First packet to arrive is first to be transmitted.  FIFO (FCFS) also called Drop-Tail • FIFO queuing mechanism that drops packets from  Fair Queuing (FQ) the tail of the queue when the queue overflows.  Weighted Fair Queuing (WFQ) • Introduces global synchronization when packets are  Random Early Detection (RED) dropped from several connections.  Explicit Congestion Notification (ECN). • FIFO is the scheduling mechanism, Drop Tail is the policy 17 18 Priority Queuing Priority Queuing  Mark each packet with a priority (e.g., in  Problem:: high priority packets can TOS (Type of Service field in IP) ‘starve’ lower priority class packets.  Implement multiple FIFO queues, one for  Priority queuing is a simple case of each priority class. “differentiated services” [DiffServ].  Always transmit out of the highest priority  One practical use in the Internet is to non-empty queue. protect routing update packets by giving  Still no guarantees for a given priority them a higher priority and a special queue class. at the router. 19 20 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
  • 6. Fair Queuing [FQ] Flow 1  The basic problem with FIFO is that it does Flow 2 Round-robin not separate packets by flow. service Flow 3  Another problem with FIFO :: an “ill- behaved” flow can capture an arbitrarily large share of the network’s capacity. Flow 4 Idea:: maintain a separate queue for each flow, and Fair Queuing (FQ) services these queues in a round-robin fashion. Fair Queuing 21 22 Fair Queuing [FQ] Fair Queuing [FQ]  FQ simulates bit-by-bit behavior by using timestamps (too many details for here!).  “Ill-behaved” flows are segregated into  One can think of FQ as providing a guaranteed their own queue. minimum share of bandwidth to each flow.  There are many implementation details for  FQ is work-conserving in that the server is never idle as long as there is a customer in the queue. FQ, but the main problem is that packets * Note: The per-flow state information kept at are of different lengths  simple FQ is not the router is expensive (it does not scale). fair!!  Ideal FQ:: do bit-by-bit round-robin. 23 24 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/
  • 7. Weighted Fair Queuing [WFQ] WFQ idea:: Assign a weight to each flow (queue) such that the weight logically specifies the number of bits to transmit each time the router services that queue.  This controls the percentage of the link capacity that the flow will receive.  The queues can represent “classes” of service and this becomes DiffServ.  An issue – how does the router learn of the weight assignments?  Manual configuration  Signaling from sources or receivers. 25 Created with Print2PDF. To remove this line, buy a license at: http://www.binarynow.com/