Midterm ReviewIn class, 9:30-11 am, Tu. 2/10Close BookOne 8.5” by 11” sheet of paper permitted (single side)Lecture 1• Internet Architecture• Network Protocols•Network Edge• A taxonomy of communication networks•Thefundamental 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 NetworkDatagramNetworkVirtual Circuit NetworkTDMFDMPacket Switching: Statistical MultiplexingSequence of A & B packets does not have fixed pattern Îstatistical 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• Packet Switching– Network resources (e.g., bandwidth) divided into “pieces” for allocation– Resource piece idleif not used by owning call (no sharing)– Dividing link bandwidth into “pieces”– Great for bursty data– NOT efficient !• Circuit Switching: – 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 customersofhigher 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 (~2x108m/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 “state”: current directory, earlier authenticationElectronic Mail: SMTP [RFC 2821]• uses TCP to reliably transfer email message from client to server, port 25• direct transfer: sending server to receiving server• three phases of transfer–handshaking (greeting)– transfer of messages–closure•command/response interaction–commands:ASCII text– response: status code and phrase• messages must be in 7-bit ASCIIDNS name servers• no server has all name-to-IP address mappingslocal name servers:– each ISP, company has local (default) name server– host DNS query first goes to local name serverauthoritative name server:– for a host: stores that host’s IP address, name– can perform name/address translation for that host’s name Why not centralize DNS?• single point of failure• traffic volume• distant centralized database• maintenancedoesn’t scale!DNS exampleRoot name server:•may not know authoritative name server•may know intermediate name server:who to contact to find authoritative name serverrequesting hostsurf.eurecom.frwww.cs.nwu.eduroot name