1. CPS110: Networks Landon Cox March 25, 2009

2. Network hardware reality  Lots of different network interface cards (NICs)  3Com/Intel, Ethernet/802.11x  Each NIC has a fixed hardware address  MAC address: 01:10:C6:CE:8E:42  Send packet to LAN by specifying MAC address  Max packet size is 1500 bytes  Packets can be reordered, corrupted, dropped  Anyone can sniff packets from the network

3. Virtual/physical interfaces Hardware OS Applications

4. Distributed computing  Try to make multiple computers look like one  We won’t really cover  Take CPS 214  Distributed shared memory  Distributed file systems  Parallelizing compilers  Process migration

5. Protocol layers NFS (files) NFS (files) HTTP (web) HTTP (web) SMTP (email) SMTP (email) SSH (login) SSH (login) RPC TCP UDP IP Ethernet ATM PPP Applications Abstraction Hardware

6. OSI model  Open Systems Interconnections Applications Layer 7 Presentation Layer 6 Session Layer 5 Transport Layer 4 Network Layer 3 DataLink Layer 2 Physical Layer 1 Applications Presentation Session Transport Network DataLink Physical

7. Network layers (the stack)  Build higher-level services on simpler ones  IP over Ethernet  TCP over IP  HTTP over TCP  Why build in layers?  Could have 0 layers (build directly on top of HW)  What would happen?  Have to build from scratch each time HW changes  E.g. one firefox for wired NIC, one for wireless NIC

8. Network layers (the stack)  Build higher-level services on simpler ones  IP over Ethernet  TCP over IP  HTTP over TCP  Why build in layers?  Could have 1 layer (OS provides single layer)  What would happen?  Better to let applications choose functionality they need  Unneeded features usually cost something (performance)  E.g. would you ever not need reliable communication?

9. Virtual/physical interfaces Hardware OS Applications

10. Routing  HW lets us send to neighbor on same LAN  Single-hop route  Want to send to computer on another LAN  Multi-hop route  IP (Internet Protocol) handles this

11. Local-area network  Typically, switched Ethernet  Messages delivered using  Ethernet MAC address  E.g. 00:0D:56:1E:AD:BB  Unique to physical card (like a serial number)  Switch knows all connected computers’ MAC addresses Ethernet switch

12. Routing  Can’t put all computers on one switch!  Think of the wiring logistics  Want to connect two LANs together  Use a machine that straddles two networks  Called a router or gateway or bridge  LANs and routers form the Internet

13. Internet graph A A C C B B D D E E F F G G Each letter is a router, possibly with a LAN connected to it.

14. Internet graph Each node is an Autonomous System (AS). Can think of as an ISP.

15. Internet graph A A C C B B D D E E F F G G How does D know how to get to router G? Should it send messages to E, C, or F?

16. Internet routing is imprecise  Internet has no centralized state  Makes it (supposedly) more fault-tolerant  Routing is hard when a network is  Large (a lot to track)  Dynamic (connections change quickly)  Incentives to lie (make money by accepting traffic)  The Internet exhibits all three  Basic idea  Routers propagate info about the graph to each other  BGP (Border Gateway Protocol)

17. Traceroute example  www.kernel.org  Unix traceroute utility

18. Virtual/physical interfaces Hardware OS Applications

19. Naming other computers  Low-level interface  Provide the destination MAC address  00:13:20:2E:1B:ED  Middle-level interface  Provide the destination IP address  152.3.140.183  High-level interface  Provide the destination hostname  crocus.cs.duke.edu

20. Translating hostname to IP addr  Hostname  IP address  Performed by Domain Name Service (DNS)  Used to be a central server  /etc/hosts at SRI  What’s wrong with this approach?  Doesn’t scale to the global Internet

21. DNS  Centralized naming doesn’t scale  Server has to learn about all changes  Server has to answer all lookups  Instead, split up data  Use a hierarchical database  Hierarchy allows local management of changes  Hierarchy spreads lookup work across many computers

22. Example: www.cs.duke.edu  nslookup in interactive mode

23. Translating IP to MAC addrs  IP address  MAC address  Performed by ARP protocol  Only done after you get to the right LAN  How does a router know the MAC address of 152.3.140.183?  ARP (Address Resolution Protocol)  If it doesn’t know the mapping, broadcast through switch  “Whoever has this IP address, please tell me your MAC address”  Cache the mapping  “/sbin/arp”  Why is broadcasting over a LAN ok?  Number of computers connected to a switch is relatively small

24. Virtual/physical interfaces Hardware OS Applications

25. Message sizes  Hardware interface  Max Ethernet message size is 1500 bytes  Application interface  IP maximum packet size is 64 kbytes  What if the route narrows?  Start at Ethernet max of 1500 bytes  Could traverse ATM w/ max of 53 bytes

26. Message sizes  IP layer fragments larger MTU to smaller MTU Computer 1Computer 2Router IP Ethernet IP Ethernet ATM IP ATM

27. Virtual/physical interfaces Hardware OS Applications

28. Processes vs machines  IP is machine-to-machine  E.g. crocus.cs.duke.edu  www.kernel.org  Process abstraction  Each app thinks it has its own machine  Give each process multiple virtual NICs

29. Processes vs machines  Hardware interface  One network endpoint per machine  Application interface  Multiple network endpoints per machine  Sockets  Software endpoints for communication  Like virtual network cards

30. Sockets  Another example of virtualized hardware  Thread  virtual processor  Address space  virtual memory  Endpoint/socket  virtual NIC  NIC and socket both have unique identifiers  NIC: MAC address  Socket: ‹hostname, port number›  bind () assigns a port number to a host’s socket

31. Sockets  OS allows apps to program sockets  E.g. BSD sockets  WinSock has pretty much same interface  Processes name each other via sockets  Each message includes a destination ‹host, port›  Tells routers which computer gets message  Tells dst computer which process gets message

32. Sockets  OS can multiplex multiple connections over one NIC  Kinds of sockets: UDP (datagrams), TCP (ordered, reliable)

33. Course administration  Project 2 due on Friday  Two groups are done  Many more are very close  Normal office hours Thursday and Friday  Use Friday discussion section to answer P2 questions  Any questions?