Semester Copyright USM EEE449 Computer Networks Congestion En. Mohd Nazri Mahmud MPhil (Cambridge, UK) BEng (Essex, UK) Room 2.14

Semester Copyright USM What Is Congestion? congestion occurs when the no of packets being transmitted through the network approaches the packet handling capacity of the network congestion control aims to keep no of packets below a level at which performance falls off dramatically a data network is a network of queues generally 80% utilization is critical finite queues mean data may be lost

Semester Copyright USM Queues at a Node

Semester Copyright USM Interaction of Queues

Semester Copyright USM Ideal Network Utilization

Semester Copyright USM Effects of Congestion - No Control

Semester Copyright USM Mechanisms for Congestion Control

Semester Copyright USM Backpressure if node becomes congested it can slow down or halt flow of packets from other nodes –cf. backpressure in blocked fluid pipe –may mean that other nodes have to apply control on incoming packet rates –propagates back to source can restrict to high traffic logical connections used in connection oriented nets that allow hop by hop congestion control

Semester Copyright USM Choke Packet a control packet –generated at congested node –sent to source node –eg. ICMP source quench from router or destination source cuts back until no more source quench message sent for every discarded packet, or anticipated is a rather crude mechanism

Semester Copyright USM Implicit Congestion Signaling transmission delay increases with congestion hence a packet may be discarded source detects this implicit congestion indication useful on connectionless (datagram) networks –eg. IP based

Semester Copyright USM Explicit Congestion Signaling network alerts end systems of increasing congestion end systems take steps to reduce offered load Backwards –congestion avoidance notification in opposite direction to packet required Forwards –congestion avoidance notification in same direction as packet required

Semester Copyright USM Explicit Signaling Categories Binary –a bit set in a packet indicates congestion Credit based –indicates how many packets source may send –common for end to end flow control Rate based –supply explicit data rate limit –nodes along path may request rate reduction

Semester Copyright USM Traffic Management fairness –provide equal treatment of various flows quality of service –different treatment for different connections reservations –traffic contract between user and network –carry best-effort or discard excess traffic

Semester Copyright USM Congestion Control in Packet Switched Networks send control packet to some or all source nodes –requires additional traffic during congestion rely on routing information –may react too quickly end to end probe packets –adds to overhead add congestion info to packets in transit –either backwards or forwards

Semester Copyright USM Congestion Control in TCP/IP networks Credit-based flow control also used for congestion control –recognize increased transit times & dropped packets –react by reducing flow of data Three categories –retransmission timer management –window management –Active queue management

Semester Copyright USM Retransmission Timer Management static timer likely too long or too short estimate round trip delay by observing pattern of delay for recent segments set time to value a bit greater than estimate simple average over a number of segments exponential average using time series RTT Variance Estimation

Semester Copyright USM Window Management slow start –larger windows cause problem on connection created –at start limit TCP to 1 segment –increase when data ACK, exponential growth dynamic windows sizing on congestion –when a timeout occurs perhaps due to congestion –set slow start threshold to half current congestion window –set window to 1 and slow start until threshold –beyond threshold, increase window by 1 for each RTT

Semester Copyright USM Slow start TCP transmission is constrained by –awnd = allowed window (the number of segments that TCP is currently allowed to send without receiving acknowledgement –Cwnd = congestion window used during startup and to reduce flow during period of congestion –Credit = the amount of unused credit granted in the most recent acknowledgement When a new connection is opened, TCP initialises cwnd=1 Each time an ACK is received, the value of cwnd is increased by 1. Cwnd increases exponentially; may be too agressive awnd = MIN [credit,cwnd]

Semester Copyright USM Dynamic window sizing on congestion Begins with a slow start, followed by a linear growth in cwnd When time out occurs –Set a slow-start threshold equal to half the current congestion window; ssthresh = cwnd/2 –Set cwnd =1 and perform slow start process until cwnd = ssthreshold. –For cwnd >= ssthresh, increase cwnd by one for each round trip time.

Semester Copyright USM Window Management

Semester Copyright USM Fast Retransmit Fast Recovery retransmit timer rather longer than RTT if segment lost TCP slow to retransmit fast retransmit –if receive 4 ACKs for same segment (ie 3 duplicate ACKs) then immediately retransmit since likely lost fast recovery –lost segment means some congestion –halve window then increase linearly –avoids slow-start

Semester Copyright USM Active Queue Management Dropping packets is inefficient Burst are inevitable. Keeping queue size small and actively managing queues improves a router's ability to absorb bursts without dropping excessive packets useful to detect impending congestion conditions and actively manage congestion before it gets out of hand. technique in which routers actively drop packets from queues as a signal to senders that they should slow down

Semester Copyright USM Active Queue Management Recovering from many dropped packets is more difficult than recovering from a single dropped packet Large queue can translate into delay. Active queue management allows queues to be smaller, which improves throughput Lock-out occurs when a host fills a queue and prevents other hosts from using the queue. Active queue management can prevent this condition.

Semester Copyright USM Random Early Drop (RED) uses statistical methods to drop packets in a probabilistic way before queues overflow slows a source down enough to keep the queue steady and reduces the number of packets that would be lost when a queue overflows makes two important decisions- when to drop packets and what packets to drop keeps track of an average queue size and drops packets when the average queue size grows beyond a defined threshold The average size is recalculated every time a new packet arrives at the queue RED makes packet-drop decisions based on two parameters: –Minimum threshold (minth) Specifies the average queue size below which no packets will be dropped. –Maximum threshold (maxth) Specifies the average queue size above which all packets will be dropped

Semester Copyright USM Explicit Congestion Notification in TCP/IP network Explicit Congestion Notification is an extension proposed to RED which marks a packet instead of dropping it when the average queue size is between minth and maxth Upon receipt of a congestion marked packet, the TCP receiver informs the sender (in the subsequent ACK) about incipient congestion which will in turn trigger the congestion avoidance algorithm at the sender the sender triggers its congestion avoidance algorithm by halving its congestion window, cwnd, and updating its congestion window threshold value ssthresh. Once it has taken these appropriate steps, the sender sets the CWR bit on the next data outgoing packet to tell the receiver that it has reacted to the (receiver's) notification of congestion. The receiver reacts to the CWR by halting the sending of the congestion notifications, ECN-Echo (ECE) to the sender if there is no new congestion in the network