Operating Systems Lesson 10

Networking Communications protocol is the set of standard rules for ◦ Data representation ◦ Signaling ◦ Authentication ◦ Error detection Usually split into layers ◦ Higher layer protocols use services from low level ◦ High layer protocols adds complexity ◦ Low-level protocols are usually unaware of higher-layer payload content

OSI Model Data unitLayerFunction Host layers Data 7. ApplicationApplicationNetwork process to application 6. PresentationPresentationData representation and encryption 5. SessionSessionInterhost communication Segment4. TransportTransportEnd-to-end connections and reliability (TCP) Media layers Packet/Datagram3. NetworkNetwork Path determination and logical addressing (IP) Frame2. Data linkData linkPhysical addressing (MAC & LLC) Bit1. PhysicalPhysicalMedia, signal and binary transmission

Routing/Switching Usually there is no direct connection between every 2 devices (PC, phones- hosts) Communications data travels a path of several nodes( e.g routers/switches) Each node forwarded (routes) data to the next node over a communication link(hop)

Packet vs. Circuit switching Packet switching ◦ Packets (discrete blocks of data) are routed between nodes over data links shared with other traffic ◦ Usually “ best effort” Circuits switching ◦ Establishes a fixed bandwidth circuit (or channel) between nodes and terminals before the users may communicate ◦ Bandwidth is reserved even if data is not transmitted

Transmission modes Broadcast Multicast Unicast

TCP/IP protocol stack

Ethernet: Bus A network card, network adapter, LAN Adapter or NIC Several Stations connected by wire (bus), “everyone sees everyone” Each station has unique address (MAC) burned into network card Need to know each other address to communicate Nothing is guaranteed

Ethernet: Star HUB: Connect devices as a single segment Switch: Inspect and forward packets to appropriate port

Ethernet: CSMA/CD

CSMA/CA

IP address Numerical address ( human readable form) Global unique addresses to be reached from outside world Local addresses to use within the enterprise can be reused by different networks Host with local addresses can reach global hosts using Network Address Translation (NAT) mechanism

IP Routing network id costnext hop Windows shell: ROUTE PRINT If Destination Address & Mask==Network Id send to specified gateway through specified NIC

ARP (Address Resolution Protocol) IP router decided what is the next hop according to its routing table Knows IP but Ethernet needs MAC address Broadcast IP and wait for response from host that has this IP ARP –a in windows shell

DHCP (Dynamic Host Configuration Protocol) What happens when you plug-in network cable into wall? Your PC need to know its IP, IP of a gateway and other parameters DHCP server accept broadcasts DHCP server assigns IP and provides addresses of all required services IPCONFIG in Windows Shell

IP is Best-Offer: Fault-events ◦ data corruption ◦ lost data packets ◦ duplicate arrival ◦ out-of-order packet delivery

UDP over IP (User Datagram) Over IP Ports to distinguish applications at the same host No guaranteed delivery May arrive out of order (different routing path) LightWeight (less delay, processing) Usage: Audio streaming

TCP over IP (TCP/IP) Transmission Control Protocol (TCP) Guaranteed delivery ◦ Packets have sequence number (SQN) ◦ Receiver send acknowledge (ACK) Port number in addition to IP address ◦ To distinguish between applications Mechanisms for establishment and termination of a connection

DNS (Dynamic Name System) Sometimes want to assign symbolic name to a host (tau.ac.il) DNS server lookup IP address by DNS name Multilevel lookup (ac.il is an address of DNS server which manage universities) DNS server is obtained by DHCP DNS tables are cached everywhere

Windows Networking Tools IPCONFIG ◦ IP configuration ARP –a ◦ ARP table ROUTE PRINT ◦ IP routing table PING ◦ Ping host and DNS resolve TRACERT ◦ Routing path to host NSLOOKUP ◦ DNS lookup NETSTAT ◦ Open TCP/UDP ports

Example Connect a laptop to the wall socket (Ethernet), open browser and go to 1.laptop detects that cable is connected 2.Laptop broadcasts DHCP request asking for IP configuration (e.g. IP address, IP routing table, DNS server address etc.) 3.DHCP server responses with IP configuration 4.Browser needs to translate symbolic address (google.com) to the numeric IP through sending a UDP request to DNS server 5.DNS server address is known (received through DHCP), need to know next gateway to reach it over IP 6.Lookup next gateway on the route to DNS using IP routing table 7.Got IP of the next gateway from IP table, need MAC address to send over Ethernet 8.Send ARP broadcast to find out MAC address of the gateway, save response in the table. Send DNS UDP request to the gateway through Ethernet. 9.After DNS server responds with IP of the Google’s web site, browser establishes TCP connection to port 80(HTTP) of the server 10. Repeat steps 6-8 this type for Google IP: Find next gateway on the way to Google web site, send ARP if required to find gateway’s MAC address