CS 5565 Network Architecture and Protocols Lecture 3 Godmar Back

Announcements Assignment: Created CS5565 Forum on Piazza Do Wireshark Lab 1 Don’t need to hand it in. Created CS5565 Forum on Piazza Use this to find a project partner All 4 projects will be done in groups of up to 2. CS 5565 Spring 2012

Summary Terminology: hosts (end systems), communication links, routers, transmission rates, packets, internet vs. intranet vs. the Internet Protocols: protocols define format, order of messages sent and received among network entities, and actions taken on msg transmission, receipt View from network edge: Client/server, peer2peer, other models Service view Communication infrastructure provide connection-oriented + connectionless service View from network core: Circuit-switching vs packet-switching Datagram network vs. virtual-circuit networks CS 5565 Spring 2012

How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn B A packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers CS 5565 Spring 2012

Four sources of packet delay 1. Nodal processing delay: check bit errors determine output link 2. Queuing delay time waiting at output link for transmission depends on congestion level of router A B propagation transmission nodal processing queueing CS 5565 Spring 2012

Queuing Delay Show Applet here Queuing delay depends on http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/queuing/queuing.html Queuing delay depends on traffic intensity nature of packet arrival process (bursts, periodic, periodic bursts, random intervals) CS 5565 Spring 2012

Queueing Delay traffic intensity = La/R R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R  1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite! CS 5565 Spring 2012

Queuing Analysis Source: Ts : mean service time Stallings Ts : mean service time Coefficient of variation determines increase in delay CS 5565 Spring 2012

Queuing Analysis (II) Depending on estimated coefficient of variance, pick appropriate server model M/M/1: both arrival rate & service time is “M”, Poisson process vs. negative exponentially distributed services In general: X/Y/n, where n number of servers & X, Y one of G: general independent M: negative exponential distribution D: deterministic (fixed length of service) Q.: what’s the expected queue length for a D/D/1 queue if arrival rate < service rate? CS 5565 Spring 2012

Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantities! CS 5565 Spring 2012

Transmission vs. Propagation Delay Show Applet here http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/transmission/delay.html Transmission delay depends on speed of link (10Mbps, 1000Mbps, …) Propagation delay depends on distance (and speed of light in medium) CS 5565 Spring 2012

Nodal delay dproc = processing delay dqueue = queuing delay typically a few microsecs or less dqueue = queuing delay depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs CS 5565 Spring 2012

Packet-switching: store-and-forward L R R R Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward Example: L = 7.5 Mbits R = 1.5 Mbps delay = ? (assume no propagation/processing/queuing delay) 15 seconds CS 5565 Spring 2012

End-to-end vs. nodal delay Question: in store-and-forward model, does end-to-end delay for a message of length L depend on the number/size of packets the message is split into? Let’s look at: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/message/messagesegmentation.html CS 5565 Spring 2012

End-to-end vs. nodal delay Question: in store-and-forward model, end-to-end delay for a message of length L depends on the number/size of packets the message is split into: Transmission times of packets can be overlayed if multiple packets are part of a message “Store-and-forward” model applies to packet, not message Consequence: packetization reduces transmission delay But you pay a price for header overhead CS 5565 Spring 2012

Circuit vs. Packet Switching (2) Circuit Switching Dedicated link bandwidth Dedicated switch capacity Low link utilization Low overall utilization Bounded delay variance Packet Switching Better Link utilization Better overall utilization Need for congestion control Need to identify to which “call” a packet belongs Smaller average delay than TDM High variance in delay CS 5565 Spring 2012

Latency vs Bandwidth Latency (Delay) lags Bandwidth [Patterson 2004] Similar pattern in many areas CS 5565 Spring 2012

Bandwidth Delay Product Delay d Bandwidth b Aka “size of pipe” Important in protocol design b in bps d in ms bxd in bytes Gigabit Ethernet 1G .150 18,750 100Mbit Ethernet 100M .7 8,750 Dialup Modem+Internet 56K 180 1,260 Cable Modem+Internet 4M 67,500 CS 5565 Spring 2012

Summary Transmission & Propagation Delay End-to-end delay in packet-switched networks Bandwidth-delay product CS 5565 Spring 2012