School of Information Technologies TCP Congestion Control NETS3303/3603 Week 9

School of Information Technologies Outline What is congestion? Approaches to congestion control TCP congestion control TCP vs UDP

School of Information Technologies Principles of Congestion Control Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations: –lost packets (buffer overflow at routers) –long delays (queueing in router buffers) a top-10 problem!

School of Information Technologies Causes/costs of congestion two senders, two receivers one router, infinite buffers no retransmission large delays when congested maximum achievable throughput unlimited shared output link buffers Host A in : original data Host B out

School of Information Technologies Costs of congestion Reality: routers have finite buffers –So packets can be lost Congestion causes: –sender retransmit of lost packet –retransmission of delayed (not lost) packet –when packets dropped, any transmission capacity used for that packet was wasted!

School of Information Technologies Congestion Control routers may drop packets as its service is best effort –Implies congestion routers don’t have effective mechanism to indicate congestion to sender –ICMP source quench is not it... assumption: –packet loss due to damage is small, therefore TCP assumes it means congestion since ACKs do not come back

School of Information Technologies Approaches towards congestion control End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems –single bit indicating congestion (TCP/IP ECN, ATM) –explicit rate sender should send at Two broad approaches towards congestion control:

School of Information Technologies TCP Congestion Control end-end control (no network assistance) Assumes long delays (packet loss) is due to congestion Uses successive retransmissions as measure of congestion –Reduces effective window as retransmissions increase Effective window is minimum of receiver’s advertisement and computed quantity known as the congestion window (cwnd)

School of Information Technologies Congestion Control II TCP uses slow start and multiplicative decrease to deal with congestion Van Jacobson 1988 outlined these ideas slow-start roughly: whenever starting traffic or recovering from congestion, start cwnd at the size of a single segment and increase it (up to a point) as ACKs show up

School of Information Technologies Slow Start Used when starting traffic or when recovering from congestion Self-clocking startup to increase transmission rate rapidly as long as no packets are lost When starting traffic, initialize the cwnd to the size of a single MSS Increase cwnd by size of 1 segment each time an ACK arrives without retransmission – till ssthresh

School of Information Technologies Slow Start II When connection begins, increase rate exponentially until first loss event: –double cwnd every RTT –done by incrementing cwnd for every ACK received Summary: initial rate is slow but ramps up exponentially fast Host A one segment RTT Host B time two segments four segments

School of Information Technologies Congestion Avoidance Slow start is used until cwnd reaches ssthresh Exponential growth is stopped Above threshold, slow down and increase cwnd by 1 segment per RTT

School of Information Technologies TCP Congestion control ssthresh CW Size Time slow-start congestion avoidance

School of Information Technologies Retransmissions TCP uses both Go back-N and selective repeat for retransmissions If timeout (=> congestion): –Go Back-N and go into slow start If an isolated error (more next slide) –Selective repeat and go to ssthresh, enter congestion avoidance

School of Information Technologies Fast Retransmit Rule RTO expires up to secs after segment dropped If an isolated error, may be can do better than being too conservative! Fast retransmission –Sender uses three duplicate ACKs as trigger –Sender retransmits ‘‘early’’ before timeout –Sender reduces cwnd to half (instead of 1)

School of Information Technologies CC Refinements After 3 dup ACKs: –cwnd is cut in half –window then grows linearly But after timeout event: –cwnd instead set to 1 MSS; –window then grows exponentially –to a threshold, then grows linearly 3 dup ACKs indicates network capable of delivering some segments timeout before 3 dup ACKs is “more alarming” Philosophy:

School of Information Technologies Summary of TCP Congestion Control When cwin is below SSThresh, sender in slow-start phase, window grows exponentially. When cwin is above SSThresh, sender is in congestion- avoidance phase, window grows linearly. When a triple duplicate ACK occurs, SSThresh set to cwin/2 and cwin set to SSThresh. When timeout occurs, SSThresh set to cwin/2 and cwin is set to 1 MSS.

School of Information Technologies Congestion and Routers with RED routers might use an obvious queue-drop mechanism –too many packets; drop packets at end of queue call this a“tail-drop” policy –on heavily multiplexed router TCP connections may lose many packets and be forced into slow-start routers may use Random Early Detection (or RED) - basically randomly discard packets in queue at certain thresholds –thus avoid tail-drop policy

School of Information Technologies Comparison Of UDP and TCP

School of Information Technologies Summary Of TCP Major transport service in the Internet (85% of traffic) Connection oriented Provides end-to-end reliability Uses adaptive retransmission Includes facilities for flow control and congestion avoidance Uses 3-way handshake for connection startup and shutdown