Chapter 4: Network Layer!Always keep the big picture in mind!CS118!2!HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP IP Ethernet interface Ethernet interface SONET interface SONET interface host host router router HTTP message TCP segment IP packet IP packet IP packet Schedule of the Quarter!CS118!3!3/29* Course intro, Ch 1 4/5 WEB, FTP 4/12 DNS 4/19 reliable data delivery 4/26 congestion control 3/31* Protocols, BW& delay, web, HTTP 4/7 email, P2P 4/14 transport protocols 4/21 TCP 4/28 Midterm T Th Week: 1 2 3 4 5 5/3 Internet Protocol (IP) 5/10 routing algorithms & protocols 5/17 multicast routing 5/24* hubs and switches 5/31 mobile networking 5/5 IP 5/12 Routing in the Internet 5/19 data link layer, Ethernet 5/26* wireless Networking 6/2 Review 6 7 8 9 10 T Th 6/4: Final exam * Lectures given by the TA Chapter 4: Network Layer!4. 1 Introduction!4.2 Virtual circuit and datagram networks!4.3 Whatʼs inside a router!4.4 IP: Internet Protocol!! Datagram format!! IPv4 addressing!! ICMP!! IPv6!4.5 Routing algorithms!! Link state!! Distance Vector!! Hierarchical routing!4.6 Routing in the Internet!! RIP!! OSPF!! BGP!4.7 Broadcast and multicast routing!Network Layer!4-4!Week 7!Week 6!Network layer!" network layer protocols in every host and router!" Transporting segments from sending to receiving host !" Source host: encapsulates segments into packets!" Destination host: delivers segments to transport layer!" Each router examines the header field of a packet passing through it!! Forwarding: move packets towards its destination!! Routing: calculate the best path to each destination!CS118!5!segment Network protocol header S D segment To transport protocol R R R Network layer:" Connection vs. connection-less service!" Virtual Circuit network provides connection-oriented service!! source-to-dest. path works in a way like telephone circuit!" Datagram network provides connectionless service!! Each packet finds its way to destination independently!" analogous to TCP vs. UDP at transport-layer, but:!! Network delivery service: host-to-host!! Hosts have no choice: a given network provides one or the other, but not both (as in transport layer)!CS118!6!Virtual circuit Network!" Use a signaling protocol to setup connection before data can flow!" every router on the path maintains “state” for each passing connection!! link, router resources (bandwidth, buffers) allocated to each VC!" each packet carries VC identifier (not destination host address!)!" VC number changes on each link!CS118!7!application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data Forwarding table!CS118!8!12 22 32 1 3 2 VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 2 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … … Forwarding table in Upper left router: Routers maintain connection state information!Internet: A Datagram Network!" hosts are connected to subnets!" subnets are interconnected by IP routers!" All hosts and routers speak IP!" IP provides two basic functions!! Assigning globally unique addresses to all connected points!! Best effort datagram delivery from source to destination hosts!# Fragmentation/reassembly of packets when needed!CS118!9!R1 ETH FDDI IP IP ETH R2 FDDI WLAN IP R3 WLAN ETH IP H1 IP ETH H8 What does IP do?!" Assiging a globally unique address to each attachment point!" Best effort packet delivery from any host to any other host!" Perform packet fragmentation and! reassembly when needed!CS118!10!browser Web server IP datagram format!CS118!11!ver Total length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier IP header checksum time to live source IP address IP version number header length upper layer protocol to deliver payload to head. len type of service flgs fragment offset protocol max number of remaining hops destination IP address Options (if any) 3 fields used for packet fragmentation/reassembly basic header IP Fragmentation & Reassembly!" Different subnets may have different MTUs (Maximum Transmission Unit)!" Sending host uses its local MTU size!" Routers fragment IP packets if the next link has a smaller MTU!! chop packets to the MTU size of next link!! further fragmentation down the path possible!" packet fragments are reassembled at destination host!CS118!12!reassembly H1 H2 R3 R2 R1 1300B MTU=532B 512B 276B MTU=1500B H1 sending an IP packet of 1300 byte data to H2:IP Fragmentation: An example!CS118!13!reassembly H1 H2 R3 R2 1300B 512B 276B data (1300 bytes) rest of the IP header 4 5 TOS 1320 7394 0 0 0 0 data (512 bytes) rest of the IP header 4 5 TOS 532 7394 0 0 1 0 data (512 bytes) rest of the IP header 4 5 TOS 532 7394 0 0 1 64 data (276 bytes) rest of the IP header 4 5 TOS 296 7394 0 0 0 128 Destination host: examming IP header fields: identifier: tell all pieces in the same packet 2 flag bits: if MF=0, the last fragment offset: tell whether there are holes missing in the middle MTU=532B IP Address structure!" IP address space: 2-level hierarchy!" What’s a network ? (from IP address’ perspective)!! Hosts & routers are on the same network if they can reach each other without going through a router!CS118!14!173.1.1.1 173.1.1.2 173.1.1.3 173.1.1.4 173.1.2.9 173.1.2.2 173.1.2.1 173.1.3.2 173.1.3.1 173.1.3.27 LAN 32-bits, uniquely identifies a host or router interface !– interface: connecting point between host/router and physical link!Network-ID host-ID 4 byte 173.1.1.1 = 10101101 00000001 00000001 00000001
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