CMPE 150 Winter 2009 Lecture 12 February 17 2009 P E Mantey CMPE 150 Introduction to Computer Networks Instructor Patrick Mantey mantey soe ucsc edu http www soe ucsc edu mantey Office Engr 2 Room 595J Office hours Tues 3 5 PM Mon 5 6 PM TA Anselm Kia akia soe ucsc edu Web site http www soe ucsc edu classes cmpe150 Winter09 Text Tannenbaum Computer Networks 4th edition available in bookstore etc Syllabus Today s Agenda Network Layer Congestion Control Quality of Service Reading Assignment Today Tannenbaum Chapter5 section 5 3 5 4 1 5 4 3 Thursday 5 6 Internet Protocol Internet Layering Level 5 Application Layer rlogin ftp SMTP POP3 IMAP HTTP Level 4 Transport Layer a k a Host to Host TCP UDP ARP ICMP etc Level 3 Network Layer a k a Internet IP Level 2 Data Link Layer MAC sub layer a k a Network Interface or Network Access Layer Level 1 Physical Layer Congestion Control Packets delivered Maximum capacity Packets sent Ideal network behavior Congestion With too much traffic attempted congestion occurs and performance can degrade significantly Routers and Congestion Slow router processors can contribute to congestion If processors are slow in managing queueing buffers updating routing tables etc this can slow down the packet movement through the router adding to congestion See Tannenbaum page 386 Link Bandwidth and Congestion Low bandwidth lines can also contribute to congestion Faster lines will move packets out of a router more quickly freeing up buffer space in the router Router Speed needs to match Link Speed If the data link speeds are upgraded then the router may become the bottleneck If the router speed is improve the links may become the bottlenect Challenge is to balance router speed and link speeds throughout the network General Principles of Congestion Control 1 2 3 Monitor the system and detect when and where congestion occurs Pass information to where action can be taken Adjust system operation and or hardware to correct the problem Network Congestion What is network congestion Too many packets in the network Router queues are always full Routers start dropping packets Congestion can fuel itself Packet drops lead to retransmissions More traffic May result in congestion collapse Close to 0 throughput Congestion metrics Packet loss Buffer queue lengths Packets timed out and retransmitted Packet delay average maximum std deviation Packet jitter Infinite Buffer Routers Intuition says add more memory to routers and that ll avoid congestion Nagle 1987 showed that infinite buffers actually make congestion worse More packets enqueued for long time they time out and are retransmitted but still transmitted by router Therefore more traffic Data Movement and Policies 5 26 Network Link congestion control Data link flow control Causes of Congestion Mismatch in capacity among different parts of the system Mismatch in link speeds R Mismatch in router processing capability Table lookup and update Queue management Congestion in one point of network tends to propagate backwards toward sender Congestion Control vs Flow Control Congestion control tries to ensure the network is able to carry offered traffic Involves hosts and intermediate routers Flow control ensures that the communication end points are able to keep up with one another Involves only the end points Congestion and Flow Control Often mixed because tend to use same feedback mechanisms Example slow down message received at host may be caused by receiver not being able to keep up with sender host or by network not being able to handle additional traffic Congestion Control Principles From control theory point of view Open loop solutions Open and closed loop solutions Avoidance approach Tries to make sure problem doesn t happen Doesn t take current network state into account Closed loop solutions Feedback loop Closed Loop Solutions 3 components Monitoring Feedback generation Operation adjustment Monitoring metrics Packet loss Average queue length Number of retransmitted packets Average packet delay Feedback Send information about the problem once it s detected Router that detects problem sends packet to traffic source s Special purpose bit in every packet that router sets when it detects congestion above certain level to warn neighbors Special probe messages to detect congested areas so they can be avoided Stability avoid oscillations Congestion Control Taxonomy Virtual Circuit Congestion Control Admission control once congestion is detected don t set up any more virtual circuits until some are released i e no more dial tone Route new circuits around congestion Reservations service agreement Wastes bandwidth Congestion Control in Virtual Circuit Subnets a A congested subnet b A redrawn subnet eliminates congestion and a virtual circuit from A to B Router monitor utilization Example line utilization for each line out of the router 0 u 1 unew uold 1 f Where f is the instantaneous measurement of utilization on that line Thus unew is a filtered smoothed measurement of the line utilization Hop by Hop Choke Packets for Datagrams a A choke packet that affects only the source b A choke packet that affects each hop it passes through Load Shedding If load continues to build up on a router to some level beyond the level where it sends choke packets IT DISCARDS SOME OF THE PACKETS Packets could be picked at random Or can look at packet age wine send old drops new packets milk sends new packets drops old Intelligent Load Shedding Senders applications put packets in priority classes Problem how to keep all senders from making all packets top priority Random Early Detection RED Routers should drop packets and thus signal senders before becoming overloaded Senders on wired networks interpret lost packets as sign of congestion Will then slow down their rate of transmission of packets Jitter Control a High jitter b Low jitter Router sends packets furthest behind schedule Quality of Service Requirements Techniques for Achieving Good Quality of Service Integrated Services Differentiated Services Label Switching and MPLS Quality of Service Requirements 5 30 Overprovisioning Allocate capacity in links routers in excess of need to guarantee all service gets the quality it requires Used on intranets internal to an organization Expensive wastes capacity Buffering Smoothing the output stream by buffering packets Open Loop Approaches Traffic Shaping Avoid traffic burstiness by forcing packets to be transmitted at more predictable rate Regulates average
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