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CPS110: Networks

CPS110: Networks. Landon Cox March 20, 2008. 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

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CPS110: Networks

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  1. CPS110: Networks Landon Cox March 20, 2008

  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 Applications Messages Procedure calls Device independence Many types of NICs Route across networks Deliver only on LAN Symbolic host names MAC addresses Large messages Small messages Process to process NIC to NIC Ordered messages Unordered messages Reliable messaging Unreliable messaging Byte streams Distinct messages Secure transmission Insecure transmission OS Hardware

  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) HTTP (web) SMTP (email) SSH (login) Applications RPC Abstraction UDP TCP Abstraction IP Abstraction Ethernet ATM PPP Hardware

  6. OSI model • Open Systems Interconnections Layer 7 Applications Applications Layer 6 Presentation Presentation Layer 5 Session Session Layer 4 Transport Transport Layer 3 Network Network Layer 2 DataLink DataLink Layer 1 Physical 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 Applications Route across networks Deliver only on LAN OS Hardware

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

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

  15. Internet graph A B C E D G F 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 Applications Symbolic host names MAC addresses OS Hardware

  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 Applications Large messages Small messages OS Hardware

  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 1 Router Computer 2 IP IP IP Ethernet Ethernet ATM ATM

  27. Virtual/physical interfaces Applications Process-to-process NIC-to-NIC OS Hardware

  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 • Exam regrades back on Tuesday • Project 2 also due on Tuesday • Four groups have submitted • Any questions?

  34. Virtual/physical interfaces Applications Reliable messages Unreliable messages Byte streams Distinct messages Ordered messages Unordered messages OS Hardware

  35. Ordered messages • Networks can re-order IP messages • E.g. Send: A, B. Arrive: B, A • How should we fix this? • Assign sequence numbers (0, 1, 2, 3, 4, …)

  36. Ordered messages • Do what for a message that arrives out of order? • (0, 1, 3, 2, 4) • Save #3 and deliver after #2 is delivered • (this is what TCP does) • Drop #3, deliver #2, deliver #4 • Deliver #3, drop #2, deliver #4 b. and c. are ordered, but not reliable (messages are dropped). Relies on the reliability layer to handle lost messages.

  37. Ordered messages • For a notion of order, first need “connections” • Why? • Must know which messages are related to each other • Idea in TCP • Open a connection • Send a sequence of messages • Close the connection • Opening a connection ties two sockets together • Connection is socket-to-socket unique: only these sockets can use it • Sequence numbers are connection specific

  38. Virtual/physical interfaces Applications Reliable messages Unreliable messages Byte streams Distinct messages Ordered messages Unordered messages OS Hardware

  39. Reliable messages • Usually paired with ordering • TCP provides both ordering and reliability • Hardware interface • Network drops messages • Network duplicates messages • Network corrupts messages • Application interface • Every message is delivered exactly once

  40. Detecting and fixing drops • How to fix a dropped message? • Have sender re-send it • How does sender know it’s been dropped? • Have receiver tell the sender • Receiver may not know it’s been sent • Like asking in the car, • “If we left you at the theater, speak up.”

  41. Detecting and fixing drops • Have receiver acknowledge each message • Called an “ACK” • If sender doesn’t get an ACK • Assume message has been dropped • Resend original message • Is this ok for the sender to assume? • No. ACKs can be dropped too (or delayed)

  42. Detecting and fixing drops • Possible outcomes • Message is delayed or dropped • ACK is delayed or dropped • Strategy • Deal with all as though message was dropped • Worst case if message wasn’t dropped after all? • Need to deal with duplicate messages • How to detect and fix duplicate messages? • Easy. Just use the sequence number and drop duplicate.

  43. What about corruption? • Messages can also be corrupted • Bits get flipped, etc • Especially true over wireless networks • How to deal with this? • Add a checksum (a little redundancy) • Checksum usually = sum of all bits • Drop corrupted messages

  44. What about corruption? • Dropping corrupted messages is elegant • Transforms problem into a dropped message • We already know how to deal with drops • Common technique • Solve one problem by transforming it into another • Corruption  drops • Drops  duplicates • Drop any duplicate messages (very simple)

  45. Virtual/physical interfaces Applications Reliable messages Unreliable messages Byte streams Distinct messages Ordered messages Unordered messages OS Hardware

  46. Byte streams • Hardware interface • Send information in discrete messages • Application interface • Send data in a continuous stream • Like reading/writing from/to a file

  47. Byte streams • Many apps think about info in distinct messages • What if you want to send more data than fits? • UDP max message size is 64 KB • What if data never ends? • Streamed media • TCP provides “byte streams” instead of messages

  48. Byte streams • Sender writes messages of arbitrary size • TCP breaks up the stream into fragments • Reassembles the fragments at destination • Receiver sees a byte stream • Fragments are not visible to either process • Programming the receiver • Must loop until certain number of bytes arrive • Otherwise, might get first fragment and return

  49. Byte streams • UDP makes boundaries visible • TCP makes boundaries invisible • (loop until you get everything you need) • How to know # of bytes to receive? • Size is contained in header • Read until you see a pattern (sentinel) • Sender closes connection

  50. Sentinels • Idea: message is done when special pattern arrives • Example: C strings • How do we know the end of a C string? • When you reach the null-termination character (‘\0’) • Ok, now say we are sending an arbitrary file • Can we use ‘\0’ as a sentinel? • No. The data payload may contain ‘\0’ chars • What can we do then?

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