Principles of Congestion Control

Transport Layer3-1Principles of Congestion ControlCongestion:informally: too many sources sending too much data too fast for network to handledifferent from flow control!lost packets (buffer overflow at routers)long delays (queueing in router buffers)a top-10 problem!

Transport Layer3-2Causes/costs of congestion: scenario 1 two senders, two receiversone router, infinite buffers no retransmission

large delays when congestedmaximum achievable throughput

unlimited shared output link buffersHost Alin : original dataHost BloutTransport Layer3-3Causes/costs of congestion: scenario 2 one router, finite buffers sender retransmission of lost packet

finite shared output link buffersHost Alin : original dataHost Bloutl'in : original data, plus retransmitted dataTransport Layer3-4Causes/costs of congestion: scenario 2 costs of congestion: more work (retrans) for given goodputunneeded retransmissions: link carries multiple copies of pkt

R/2R/2linloutb.R/2R/2linlouta.R/2R/2linloutc.R/4R/3Transport Layer3-5Causes/costs of congestion: scenario 3 four sendersmultihop pathstimeout/retransmit

finite shared output link buffersHost Alin : original dataHost Bloutl'in : original data, plus retransmitted dataTransport Layer3-6Causes/costs of congestion: scenario 3 Another cost of congestion: when packet dropped, any upstream transmission capacity used for that packet was wasted!

Host AHost BloutTransport Layer3-7Approaches towards congestion controlEnd-end congestion control:no explicit feedback from networkcongestion inferred from end-system observed loss, delayapproach taken by TCPNetwork-assisted congestion control:routers provide feedback to end systemssingle bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)Two broad approaches towards congestion control:Network Layer4-8Network layertransport segment from sending to receiving host on sending side encapsulates segments into datagramson rcving side, delivers segments to transport layernetwork layer protocols in every host, routerRouter examines header fields in all IP datagrams passing through it

networkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalNetwork Layer4-9Key Network-Layer Functionsforwarding: move packets from routers input to appropriate router outputrouting: determine route taken by packets from source to dest. Routing algorithms

analogy:routing: process of planning trip from source to destforwarding: process of getting through single interchange

Network Layer4-101230111value in arrivingpackets headerrouting algorithmlocal forwarding tableheader valueoutput link01000101011110013221Interplay between routing and forwardingNetwork Layer4-11Network service modelQ: What service model for channel transporting datagrams from sender to rcvr?Example services for individual datagrams:guaranteed deliveryGuaranteed delivery with less than 40 msec delayExample services for a flow of datagrams:In-order datagram deliveryGuaranteed minimum bandwidth to flow guaranteed maximum jitterSecurity service