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  Max packet size is 1500 bytes  Packets can be reordered, corrupted, dropped  Anyone can sniff packets from the network

Virtual/physical interfaces Hardware OS Applications

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

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

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

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

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?

Virtual/physical interfaces Hardware OS Applications

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

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

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

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.

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

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?

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)

Traceroute example   Unix traceroute utility

Virtual/physical interfaces Hardware OS Applications

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   High-level interface  Provide the destination hostname  crocus.cs.duke.edu

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

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

Example:  nslookup in interactive mode

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 ?  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

Virtual/physical interfaces Hardware OS Applications

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

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

Virtual/physical interfaces Hardware OS Applications

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

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

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

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

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

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

Virtual/physical interfaces Hardware OS Applications

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, …)

Ordered messages  Do what for a message that arrives out of order?  (0, 1, 3, 2, 4) a.Save #3 and deliver after #2 is delivered  (this is what TCP does) b.Drop #3, deliver #2, deliver #4 c.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.

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

Virtual/physical interfaces Hardware OS Applications

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

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.”

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)

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.

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

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 1.Corruption  drops 2.Drops  duplicates 3.Drop any duplicate messages (very simple)

Virtual/physical interfaces Hardware OS Applications

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

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

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

Byte streams  UDP makes boundaries visible  TCP makes boundaries invisible  (loop until you get everything you need)  How to know # of bytes to receive? 1.Size is contained in header 2.Read until you see a pattern (sentinel) 3.Sender closes connection

Sentinels  Idea: m essage 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?