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Computer Networks Guest Lecture in COS 318 Jennifer Rexford http://www.cs.princeton.edu/~jrex Goal of the Lecture • Brief introduction to data networking – Best-effort service and the hourglass model – From sending packets to downloading Web pages – Internet addressing, routing, and topology • Teaser for COS 461, offered next term – MW 1:30-2:50pm Best-Effort Packet-Delivery Service IP Service Model: Best-Effort Packet Delivery • Packet switching – Send data in packets – Header with source & destination address • Best-effort delivery – Packets may be lost – Packets may be corrupted – Packets may be delivered out of order source destination IP network IP Service Model: Why Packets? • Data traffic is bursty – Logging in to remote machines – Exchanging e-mail messages • Don’t want to waste reserved bandwidth – No traffic exchanged during idle periods • Better to allow multiplexing – Different transfers share access to same links • Packets can be delivered by most anything – RFC 2549: IP over Avian Carriers (aka birds) • … still, packet switching can be inefficient – Extra header bits (“envelope”) for every packet IP Service Model: Why Best-Effort? • It’s easier not to make promises – Don’t reserve bandwidth and memory – Don’t do error detection and correction – Don’t remember from one packet to next • Easier to survive failures – Transient disruptions are okay during failover • … but, applications do want efficient, accurate transfer of data in order, in a timely fashion IP Service Model: Best-Effort is Enough • No error detection or correction – Receiver can discard corrupted packets – Sender can send the packets again • Successive packets may not follow the same path – Okay as long as packets reach the destination • Packets can be delivered out-of-order – Receiver can put packets back in order • Packets may be lost or arbitrarily delayed – Sender can send the packets again • No network congestion control (beyond “drop”) – Sender can slow down in response to loss or delay Layering in the IP Protocols: Hourglass HTTP Telnet FTP DNS Transmission Control Protocol (TCP) User Datagram Protocol (UDP) Internet Protocol SONET Ethernet RTP ATM Transport Protocols: Between End Hosts Transmission Control Protocol (TCP) • Communication service (socket) – Ordered, reliable byte stream – Simultaneous transmission in both directions • Key mechanisms at end hosts – – – – Retransmit lost and corrupted packets Discard duplicate packets and put packets in order Flow control to avoid overloading the receiver buffer Congestion control to adapt sending rate to network load TCP connection source network destination Opening and Closing a TCP Connection B A time • Three-way handshake to establish connection – Host A sends a SYN to the host B – Host B returns a SYN and acknowledgement – Host A sends an ACK to acknowledge the SYN ACK • Four-way handshake to close the connection – Finish (FIN) to close and receive remaining bytes , or – Reset (RST) to close and not receive remaining bytes Lost and Corrupted Packets • Detecting corrupted and lost packets – Error detection via checksum on header and data – Sender sends packet, sets timeout, and waits for ACK – Receiver sends ACKs for received packets – Sender infers loss from timeout or duplicate ACKs • Retransmission by sender – Sender retransmits lost/corrupted packets – Receiver reassembles and reorders packets – Receiver discards corrupted and duplicated packets TCP Flow and Congestion Control • Window-based flow control – Sender limits number of outstanding bytes (window size) – Receiver window ensures data does not overflow receiver • Adapting to network congestion congestion window – Congestion window tries to avoid overloading the network (increase with successful delivery, decrease with loss) – TCP connection starts with small initial congestion window congestion avoidance slow start time User Datagram Protocol (UDP) • Some applications do not want or need TCP – Avoid overhead of opening/closing a connection – Avoid recovery from lost/corrupted packets – Avoid sender adaptation to loss/congestion • Example applications that use UDP – Multimedia streaming applications – Domain Name System (DNS) queries/replies • Dealing with the growth in UDP traffic – Interference with TCP performance – Pressure to apply congestion control – Future routers may enforce “TCP-friendly” behavior Converting Host Names to Numerical Addresses Domain Name System (DNS) • Properties of DNS – Hierarchical name space divided into zones – Translation of names to/from IP addresses – Distributed over a collection of DNS servers • Client application – Extract server name (e.g., from the URL) – Invoke system call to trigger DNS resolver code – E.g., gethostbyname() on “www.cs.princeton.edu” • Server application – Extract client IP address from socket – Optionally invoke system call to translate into name – E.g., gethostbyaddr() on “12.34.158.5” Domain Name System unnamed root com edu org generic domains bar uk ac zw arpa country domains ac inaddr west east cam 12 foo my usr 34 my.east.bar.edu usr.cam.ac.uk 56 12.34.56.0/24 DNS Resolver and Local DNS Server Root server 3 4 Application DNS cache 5 1 10 DNS resolver DNS query 2 6 Local DNS server Top-level domain server 7 DNS response 9 8 Second-level domain server Caching based on a time-to-live (TTL) assigned by the DNS server responsible for the host name to reduce latency in DNS translation. Building Applications on Top (e.g., Web) Application-Layer Protocols • Messages exchanged between applications – Syntax and semantics of the messages between hosts – Tailored to the specific application (e.g., Web, e-mail) – Messages transferred over transport connection (e.g., TCP) • Popular application-layer protocols – Telnet, FTP, SMTP, NNTP, HTTP, … GET /index.html HTTP/1.1 Client HTTP/1.1 200 OK Server Example: Many Steps in Web Download Browser cache DNS resolution TCP open 1st byte response Last byte response Sources of variability of delay • Browser cache hit/miss, need for cache revalidation • DNS cache hit/miss, multiple DNS servers, errors • Packet loss, round-trip time, server accept queue • RTT, busy server, CPU overhead (e.g., CGI script) • Response size, receive buffer size, congestion • … downloading embedded image(s) on the page IP Suite: End Hosts vs. Routers host host HTTP message HTTP TCP segment TCP router IP Ethernet interface HTTP IP packet Ethernet interface IP TCP router IP packet SONET interface SONET interface IP IP packet Ethernet interface IP Ethernet interface Routers, Addressing, and Forwarding What is a Router? • A computer with… – Multiple interfaces – Implementing routing protocols – Packet forwarding • Wide range of variations of routers – Small LinkSys device in a home network – Linux-based PC running router software – Million-dollar high-end routers with large chassis • … and links – Serial line – Ethernet – Packet-over-SONET Network Components Links Interfaces Fibers Ethernet card Switches/routers Large router Wireless card Coaxial Cable Telephone switch Inside a High-End Router Processor Line card Line card Line card Line card Switching Fabric Line card Line card Happy Routers Make Happy Packets • Routers forward packets – Forward incoming packet to outgoing link – Store packets in queues – Drop packets when necessary • Routers compute paths – Routers run routing protocols – Routers compute forwarding tables • A famous quotation from RFC 791 – “A name indicates what we seek. An address indicates where it is. A route indicates how we get there.” -- Jon Postel IP Addressing • 32-bit number in dotted-quad notation (12.34.158.5) • Divided into network & host portions (left and right) • 12.34.158.0/24 is a 24-bit prefix with 28 addresses 12 34 158 5 00001100 00100010 10011110 00000101 Network (24 bits) Host (8 bits) whois –h whois.arin.net 128.112.136.35 OrgName: Princeton University OrgID: PRNU Address: Office of Information Technology Address: 87 Prospect Avenue City: Princeton StateProv: NJ PostalCode: 08544-2007 Country: US NetRange: 128.112.0.0 - 128.112.255.255 CIDR: 128.112.0.0/16 NetName: PRINCETON NetHandle: NET-128-112-0-0-1 Parent: NET-128-0-0-0-0 NetType: Direct Allocation RegDate: 1986-02-24 Packet Forwarding • Forwarding tables in IP routers – Maps each IP prefix to next-hop link(s) • Destination-based forwarding – Packet has a destination address – Router identifies longest-matching prefix – Cute algorithmic problem: very fast lookups forwarding table destination 12.34.158.5 4.0.0.0/8 4.83.128.0/17 12.0.0.0/8 12.34.158.0/24 126.255.103.0/24 outgoing link Serial0/0.1 Internet Topology and Routing Autonomous Systems (ASes) Path: 6, 5, 4, 3, 2, 1 4 3 5 2 7 1 6 Web server Client Internet Routing Architecture • Divided into Autonomous Systems – Distinct regions of administrative control – Routers/links managed by a single “institution” – Service provider, company, university, … • Hierarchy of Autonomous Systems – Large, tier-1 provider with a nationwide backbone – Medium-sized regional provider with smaller backbone – Small network run by a single company or university • Interaction between Autonomous Systems – Internal topology is not shared between ASes – … but, neighboring ASes interact to coordinate routing Autonomous System Numbers AS Numbers are 16 bit values. Currently around 20,000 in use. • • • • • • • • • Level 3: 1 MIT: 3 Harvard: 11 Yale: 29 Princeton: 88 AT&T: 7018, 6341, 5074, … UUNET: 701, 702, 284, 12199, … Sprint: 1239, 1240, 6211, 6242, … … Interdomain Routing (Between ASes) • ASes exchange info about who they can reach – IP prefix: block of destination IP addresses – AS path: sequence of ASes along the path • Policies configured by the network operator – Path selection: which of the paths to use? – Path export: which neighbors to tell? “I can reach 12.34.158.0/24 via AS 1” “I can reach 12.34.158.0/24” 2 1 data traffic 12.34.158.5 3 data traffic Inside an AS: Abilene Internet2 Backbone Intradomain Routing (Within an AS) • Routers exchange topology information – Routers compute “next hop” to other routers – Path chosen based on link weights (shortest path) • Link weights configured by network operator – … to control the flow of traffic 2 3 2 1 1 1 3 5 4 3 Funny Things About the Internet • Nobody really knows how big it is – No global registry of the topology • Hard to know what traffic it carries – New applications try to hide their identity • Built based on trust in others – Do congestion control, announce only the addresses you own, and so on • Operators do a lot of things manually – Half of outages are caused by operator error • Diagnosing performance problems is hard – So many things can go wrong, in so many places Learn More • COS 461, spring 2006 – MW 1:30-2:50pm