CSE331: Introduction to Networks and Security Lecture 13 Fall 2002

CSE331 Fall Announcements Reminder: –Project 1 due on Monday, Oct. 7 th –In-class midterm Wednesday, Oct. 9 th Monday’s Class –Further Topics in Networking –Review / Question & Answer

CSE331 Fall Recap Application Level Protocols –SMTP –HTTP –SNMP Congestion control Resource Management Quality of Service Today

CSE331 Fall Sharing Resources How do we effectively & fairly share resources on the net? –Bandwidth of the links –Buffers space in routers in switches Many competing users –What does “fairly” mean?

CSE331 Fall Contention and Congestion Packets contend at a router for use of a link –Multiple packets are enqueued at the router Congestion is when packets are dropped because the queue is full –Wasted resources –Can lead to timeouts/retransmission Problem is resource allocation

CSE331 Fall Congestion In Packet-switched Networks Router Sources Destination

CSE331 Fall Network Resource Allocation Challenges –Distributed resources are hard to coordinate –Only way to coordinate is through the network itself! –Not isolated to single level of the protocol hierarchy –Not always possible to “route around” congestion –Bottleneck not always visible from the source Resource allocation –Attempt to meet competing demands of applications –Not always possible!

CSE331 Fall Flows A flow is a sequence of packets sent along the same route between a source and dest. Connectionless Flows –No per-flow state at the routers –Example: Pure datagram model Connection Oriented Flows –Necessary per-flow state at the routers –Explicitly created/removed by signalling –Example: Virtual Circuit Switching –Potentially does not scale Soft-state Flows –Some (not strictly necessary) per-flow state –Example: Routing information in Learning Bridges

CSE331 Fall Multiple flows Source 1 Source 2 Source 3 Dest. 1 Dest. 2 Router

CSE331 Fall Router- vs. Host-Centric Router-centric –Each router selects packets to forward & packets to drop –Routers inform hosts about network conditions Host-centric –Hosts observe network behavior by watching ACKs, Timeouts, ICMP messages, etc. –Adjust behavior accordingly Not mutually exclusive approaches

CSE331 Fall Reservation vs. Feedback Reservation –End hosts ask network for certain amount of capacity –If request can’t be satisfied, router rejects the flow –Examples: measure MTU or link capacities –Router-centric approach Feedback –End hosts send data without reserving capacity –Adjust behavior based on feedback –Explicit feedback: TCP flow control –Implicit feedback: Packet losses

CSE331 Fall Throughput, Delay and Load Network load is a measure of total link utilization Ideally we would –Maximize throughput –Minimize delay Increasing #packets in network lengthens queues, which increases delay. Power = Throughput/Delay

CSE331 Fall Power vs. Load Ratio of Throughput/Delay as a function of network load Difficult to control load in fine-grained ways Need stable mechanism: avoid thrashing Load Throughput/Delay Optimal Load

CSE331 Fall Fair Resource Allocation What does “Fair” mean? –Equal share of resources for all flows? –Proportional to how much you pay for service? –Should we take route length into account? Router

CSE331 Fall FIFO Queuing First-in First-out –Scheduling discipline: determines order Tail Drop –If queue is full, most recent packet to arrive is dropped –Drop policy: which packets are dropped Most widely used in Internet routers –Pushes congestion control & resource allocation to end hosts (TCP) –Does not discriminate between flows –Trusts end hosts to “share” – but no one is forced to use TCP, for example.

CSE331 Fall Priority Queuing Simple variant on FIFO –Use the IP Type of Service header field as a priority –Send all higher priority packets in the queue before sending lower priority packets Problems –Starvation of low-priority flows –Who sets priorities? (Not end user!)

CSE331 Fall Fair Queing Strategy –Maintain a separate queue for each flow being handled by the router –Individual queues are treated FIFO with tail-drop –Queues are handled round-robin Flow 1 Flow 2 Flow 3 Round Robin

CSE331 Fall Fair Queuing Continued Designed to be used with end-to-end congestion control –Doesn’t restrict transmission rates of end hosts –Badly-behaved end hosts only hurt themselves Details –Different packet sizes complicates “fairness” –Link is never idle (as long as there is data to send) –If N flows are transmitting, each gets maximum of 1/N bandwidth

CSE331 Fall Congestion Avoidance Mechanisms Try to prevent congestion before it occurs –Unlike TCP, which reacts to existing congestion Strategy 1: Routers watch their queues –Routers set a bit in outgoing packets if avg. queue length > 1 –Receiver copies bit into its ACK –Sender increases/decreases send window based on # of packets that report congestion –Called the DECbit algorithm

CSE331 Fall Congestion Avoidance Continued Strategy 2: Random Early Detection –Router monitors queue length –If length > dropLevel then drop packet with certain probability –Source times out on dropped packets –TCP causes send window to decrease –Much tuning of parameters to optimize performance

CSE331 Fall Quality of Service Issues Sometimes best effort is not enough Application requirements –Real time: data must arrive within certain time constraints to be useful Telephony, video conferencing –Jitter (variation in arrival times of packets) is bad Audio/visual data need low jitter –Packet loss:can it be tolerated or not? Mpeg can interpolate missing frames Remote robot surgeon cannot tolerate packet loss

CSE331 Fall Playback Buffer Example Time Sequence # Packet Generation Playback Packet Arrival delay buffer

CSE331 Fall Integrated Services (RSVP) Proposed in Service Classes –Guaranteed arrival service For delay intolerant applications Guarantee a maximum delay –Controlled Load For loss tolerant, adaptive applications Emulate lightly loaded network

CSE331 Fall Implementation Mechanisms Flowspecs –Describe the kind of service needed “I need maximum delay of 100ms” “I need to use controlled load service” Admission Control –Network decides whether it can provide the desired service Resource Reservation –Mechanism to exchange info about requests Packet Scheduling –Manage queuing and scheduling.