1. Internet Routing COS 598A Jennifer Rexford http://www.cs.princeton.edu/~jrex/teaching/spring2005 Tuesdays/Thursdays 11:00am-12:20pm

2. Who am I, and Who are You? Who am I? –Joined the CS faculty in Feb 2005 (i.e., today) –Worked for 8.5 years at AT&T Labs—Research –Research on routing protocols, network measurement, and network operations Who are you, and what do you do? –Introductions…

3. What is Internet Routing? The glue that holds the Internet together How routers know where to forward packets How operators control the load on their links How networks achieve business relationships 1 2 3 4 5 6 7 ClientWeb server

4. What Does This Course Cover? Internet architecture –Best-effort packet-delivery service –Intradomain and interdomain routing Network topology –Inside a network, and between networks Traffic engineering –Getting the traffic to go where you want Convergence –Delay to respond to change –Whether the protocol ever converges

5. What Does the Course Cover? (Continued) Routers –Router hardware and software –Router configuration –Scaling to many destinations, routers, & networks Measurement –Monitoring the routing protocols –Characterizing the routing system –Troubleshooting routing problems Routing protocol security New architectural directions

6. Emphasis of the Course Not so much on the protocols –…though we will cover BGP, OSPF, IS-IS, MPLS, and various other acronyms of the day Or on the routers –…though we will talk about how routers work But more on how people manage routing –Selecting which protocols to use –Deciding how to set the parameters –Troubleshooting problems as they arise –Preventing attacks –…

7. Structure of the Course Classroom time –Mixture of lecture and discussion of papers Readings –Selected research papers and surveys –Videocasts of presentations (e.g., from NANOG) –Optional short “food for thought” reading each week Course project –Literature survey, measurement or simulation study, protocol design, theoretical analysis, etc. Grading –Final course project (written report and oral presentation) –Class participation (written reviews, class discussion, etc.)

8. Today, and Thursday Goal –Explain IP best-effort delivery model Today –What is the service model? –How can you do anything useful with this? Thursday –How do the routers support the service model? –How do the routing protocols work?

9. 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

10. 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 on every packet

11. IP Packet Structure 4-bit Version 4-bit Header Length 8-bit Type of Service (TOS) 16-bit Total Length (Bytes) 16-bit Identification 3-bit Flags 13-bit Fragment Offset 8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum 32-bit Source IP Address 32-bit Destination IP Address Options (if any) Payload 20-byteHeader usually IPv4 usually 20 bytes fragments more later error check header

12. IP Service Model: Why Best-Effort? It’s easier not to make promises –Don’t need to reserve bandwidth and memory –Don’t need to do error detection & correction –Don’t need to 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

13. IP Service Model: Best-Effort is Enough No error detection or correction –Higher-level protocol can provide error checking Successive packets may not follow the same path –Not a problem as long as packets reach the destination Packets can be delivered out-of-order –Receiver can put packets back in order (if necessary) Packets may be lost or arbitrarily delayed –Sender can send the packets again (if desired) No network congestion control (beyond “drop”) –Sender can slow down in response to loss or delay

14. Layering in the IP Protocols Internet Protocol Transmission Control Protocol (TCP) User Datagram Protocol (UDP) Telnet HTTP SONETATM Ethernet RTPDNS FTP

15. 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 sourcenetworkdestination TCP connection

16. Source and Destination Port Numbers Motivation for port numbers –Unique identifier of the TCP connection on each end –Necessary to (de)multiplex packets at the end-points Assigning port numbers –Port numbers below 1024 are assigned –Well-known port numbers for common applications Web client contacting a web server –Browser click results in creation of a TCP socket –Client machine assigns an available port (>=1024) –Client machine requests a connection with the server –Open TCP connection to port 80 at the server

17. Opening and Closing a TCP Connection 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 SYN SYN ACK ACK Data FIN ACK time A B FIN ACK

18. 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

19. 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 tries to avoid overloading the network (increase with successful delivery, decrease with loss) –TCP connection starts with small initial congestion window time congestion window slow start congestion avoidance

20. 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

21. 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.foo.com” Server application –Extract client IP address from socket –Optionally invoke system call to translate into name –E.g., gethostbyaddr() on “12.34.158.5”

22. Domain Name System comeduorgac uk zw arpa unnamed root bar westeast foomy ac cam usr in- addr 12 34 56 generic domainscountry domains my.east.bar.edu usr.cam.ac.uk 12.34.56.0/24

23. DNS Resolver and Local DNS Server Application DNS resolver Local DNS server 1 10 DNS cache DNS query 2 DNS response 9 Root server 3 4 Top-level domain server 5 6 Second-level domain server 7 8 Caching based on a time-to-live (TTL) assigned by the DNS server responsible for the host name to reduce latency in DNS translation.

24. 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, … ClientServer GET /index.html HTTP/1.1 HTTP/1.1 200 OK

25. Example: Many Steps in Web Download Browser cache DNS resolution TCP open 1 st 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, high RTT, 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

26. IP Suite: End Hosts vs. Routers HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP Ethernet interface Ethernet interface SONET interface SONET interface host router HTTP message TCP segment IP packet This course focuses on the routers…

27. 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

28. Reading for Thursday Two classic papers –End-to-end arguments in system design (1984) –Design philosophy of the DARPA Internet protocols (1988) New perspectives on success of the Internet –Tussle in cyberspace: Defining tomorrow’s Internet (2002)

29. Backup Slides TTL and traceroute

30. Time-to-Live Field Potential robustness problem –Routing loops can cause packets to cycle forever –Confusing if the packet arrives much later Time-to-live field in packet header –TTL field decremented by each router on the path –Packet is discarded when TTL field reaches 0… –…and send “timer expired” message to source

31. Traceroute: Measuring the Forwarding Path Time-To-Live field in IP packet header –Source sends a packet with a TTL of n –Each router along the path decrements the TTL –“TTL exceeded” sent when TTL reaches 0 Traceroute tool exploits this TTL behavior source destination TTL=1 Time exceeded TTL=2 Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message