Midterm ReviewLecture 1A Taxonomy of Communication NetworksPacket Switching: Statistical MultiplexingPacket Switching versus Circuit SwitchingDatagram Packet SwitchingVirtual-Circuit Packet SwitchingLecture 2Internet structure: network of networksFour sources of packet delayDelay in packet-switched networksInternet protocol stackApplication LayerHTTP connectionsResponse Time of HTTPFTP: separate control, data connectionsElectronic Mail: SMTP [RFC 2821]DNS name serversDNS exampleWeb caches (proxy server)Caching example (3)Transport LayerDemultiplexingUDP: User Datagram Protocol [RFC 768]Rdt1.0: reliable transfer over a reliable channelRdt2.0: channel with bit errorsrdt2.0: FSM specificationrdt2.0 has a fatal flaw!rdt2.1: sender, handles garbled ACK/NAKsrdt2.2: a NAK-free protocolrdt3.0: channels with errors and lossrdt3.0 senderGo-Back-NSelective RepeatSelective repeat: sender, receiver windowsTCP segment structureTCP Round Trip Time and TimeoutTCP Flow control: how it worksTCP Congestion ControlSummary: TCP Congestion ControlRefinementWhy is TCP fair?Delay modelingFixed congestion window (1)Fixed congestion window (2)HTTP ModelingSlide 47Slide 48Midterm Review• Homework 1 graded, solutions postedIn class, 9:30-11 am, Th. 2/2Close BookOne 8.5” by 11” sheet of paper permitted (single side)Lecture 1•Internet Architecture•Network Protocols•Network Edge•A taxonomy of communication networks•The fundamental question: how is data transferred through net (including edge & core)?•Communication networks can be classified based on how the nodes exchange information:A Taxonomy of Communication NetworksCommunication NetworksSwitchedCommunication NetworkBroadcastCommunication NetworkCircuit-SwitchedCommunication NetworkPacket-SwitchedCommunication NetworkDatagram NetworkVirtual Circuit NetworkTDMFDMPacket Switching: Statistical MultiplexingSequence of A & B packets does not have fixed pattern  statistical multiplexing.In TDM each host gets same slot in revolving TDM frame.ABC10 MbsEthernet1.5 MbsDEstatistical multiplexingqueue of packetswaiting for outputlinkPacket Switching versus Circuit Switching•Is it true for Packet Switching or Circuit Switching?–Network resources (e.g., bandwidth) divided into “pieces” for allocation–Great for bursty data–Resource piece idle if not used by owning call (no sharing)–Have excessive congestion: packet delay and loss–Protocols needed for reliable data transfer, congestion controlDatagram Packet Switching•Each packet is independently switched–Each packet header contains destination address which determines next hop–Routes may change during session•No resources are pre-allocated (reserved) in advance•Example: IP networksVirtual-Circuit Packet Switching•Hybrid of circuit switching and packet switching–All packets from one packet stream are sent along a pre-established path (= virtual circuit)–Each packet carries tag (virtual circuit ID), tag determines next hop•Guarantees in-sequence delivery of packets•However, packets from different virtual circuits may be interleaved•Example: ATM (Asynchronous Transfer Mode) networksLecture 2•Network access and physical media•Internet structure and ISPs •Delay & loss in packet-switched networks•Protocol layers, service modelsInternet structure: network of networks•“Tier-3” ISPs and local ISPs –last hop (“access”) network (closest to end systems)–Tier-3: Turkish Telecom, Minnesota Regional NetworkTier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPLocal and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of InternetFour sources of packet delay•1. processing: –check bit errors–determine output linkABpropagationtransmissionprocessingqueueing•2. queueing–time waiting at output link for transmission –depends on congestion level of routerDelay in packet-switched networks3. Transmission delay:•R=link bandwidth (bps)•L=packet length (bits)•time to send bits into link = L/R4. Propagation delay:•d = length of physical link•s = propagation speed in medium (~2x108 m/sec)•propagation delay = d/sABpropagationtransmissionprocessingqueueingNote: s and R are very different quantities!Internet protocol stack•application: supporting network applications–FTP, SMTP, STTP•transport: host-host data transfer–TCP, UDP•network: routing of datagrams from source to destination–IP, routing protocols•link: data transfer between neighboring network elements–PPP, Ethernet•physical: bits “on the wire”applicationtransportnetworklinkphysicalApplication Layer•Principles of app layer protocols•Web and HTTP•FTP•Electronic Mail: SMTP, POP3, IMAP•DNS•Socket Programming•Web CachingHTTP connectionsNonpersistent HTTP•At most one object is sent over a TCP connection.•HTTP/1.0 uses nonpersistent HTTPPersistent HTTP•Multiple objects can be sent over single TCP connection between client and server.•HTTP/1.1 uses persistent connections in default mode• HTTP Message, Format, Response, Methods• HTTP cookiesResponse Time of HTTPNonpersistent HTTP issues:•requires 2 RTTs per object•OS must work and allocate host resources for each TCP connection•but browsers often open parallel TCP connections to fetch referenced objectsPersistent HTTP•server leaves connection open after sending response•subsequent HTTP messages between same client/server are sent over connectionPersistent without pipelining:•client issues new request only when previous response has been received•one RTT for each referenced objectPersistent with pipelining:•default in HTTP/1.1•client sends requests as soon as it encounters a referenced object•as little as one RTT for all the referenced objectsFTP: separate control, data connections•FTP client contacts FTP server at port 21, specifying TCP as transport protocol•Client obtains authorization over control connection•Client browses remote directory by sending commands over control connection.•When server receives a command for a file transfer, the server opens a TCP data connection to client•After transferring one file, server closes connection.FTPclientFTPserverTCP control connectionport 21TCP data connectionport 20•Server opens a second TCP data connection to transfer another file.•Control connection: “out of band”•FTP server maintains