Lecture 16: Transport Protocols & Network Congestion Control - Prof. John Musacchio, Assignments of Accounting

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ISM 50 - Business Information Systems

Lecture 16

Instructor: John Musacchio

UC Santa Cruz

November 15, 2005

Class announcements „

Assignment 5 due Thursday

„

Reading for Thursday (Nov 17

th

‰ Messerschmitt Ch 9 (273-289) „

Student Presentations Thursday (Nov 17

th

‰ Ian Rogen ‰ Kee-Yip Chan

Review of last time „ Physical Layer ‰ Send bits using signals „ Link Layer ‰ Sending separate frames with a bit link ‰ Medium Access Control „ Ethernet Hub vs Switch ‰ MAC Addresses „ Network Layer ‰ Packet Forwarding based on Address ‰ Routing Tables ‰ IP Addresses vs. MAC Addresses ‰ Encapsulation of IP packets within Ethernet Frames

Review of Last time „ Link Sharing ‰ Time Division Multiplexing ‰ Statistical Multiplexing „ Transport Layer ‰ TCP – acknowledgements and congestion control ‰

UDP

Transmission Control Protocol (TCP) „

Retransmit mechanism for reliability ‰ Receiver sends acknowledgements to sender ‰ If a packet is lost, source fails to get ACK, and thenretransmits. „ Congestion control ‰ If congestion perceived (by lost packets) ‰ Source reduces its send rate „ When loss, sender reduces send rate by half „ Otherwise slowly increases ACK Packet 1

Packet 2Packet 2

TCP cont’d „ TCP port numbers ‰ TCP Header has a “port” number field ‰ Helps host sort out how to route packets toapplications Port 80 Packet TCP Header IP Header Payload Email Client Port 80 Port 143 Your Computer

Network congestion „ Traffic can overload links ‰ Failure of statistical multiplexing „ Congestion must be limited in some fashion

Carriedtraffic Offeredtraffic Network “capacity” Increasing portion ofnetwork traffic is resentpackets Congestion instability Socialoptimum

Quality of Service (QoS) -metrics „ Latency – the time it takes a packet to travel from a senderto receiver. „ Throughput – the rate of the connection in bps. „ Loss – the fraction of packets that get lost. „ Jitter – How much the latency varies over time. Audiocoder Audio decoder Packetlatency

Achieving QoS „ Increase the capacity of the network a lot „ TDMA instead of statistical multiplexing ‰ That way traffic from one connection does noteffect the quality of another ‰ But, we lose the benefits of statisticalmultiplexing. „ Priority Scheduling? ‰ Analogy: first class check-in vs. coach check-in

Pricing TodayEnd User „ Flat Rate most common ‰ pays an ISP a flat rate per month ‰ does not depend on use

Pricing Alternatives „ Usage based pricing ‰ Charge some amount per megabit sent. ‰ Those that use more thus, pay more. ‰ This is done today for the phone network, aswell as data connections over cell phones. ‰ Advantages?

Pricing AlternativesCongestion Pricing „ Idea studied a lot in research community, ‰ Pay more when links are congested. ‰ This gives an incentive to reduce usage ‰ If fined grained enough, congestion prices could be analternative to TCP congestion control. ‰ Some proposed schemes allow users with greater needsto outbid those with less need.

Congestion PricingAdvantages? „ More revenue for provider „ Allows users with sensitive applications topay more to get the service they require. Disadvantages? „ Complexity ‰ This is why it has not caught on.