Network layer" 1DT066! Distributed Information Systems!! Chapter 4" Network Layer!! goals: "

Transcription

1 1DT066! Distributed Information Systems!! Chapter 4" Network Layer!! Network layer" goals: "! understand principles behind layer services:" " layer service models" " forwarding versus routing" " how a router works and routing algorithms" " syntax an semantics of IP addresses" " address resolution services" " Network Layer 4-

2 Outline and book chapters" 4.1 introduction" 4. virtual circuit and datagram s" 4.3 what s inside a router" 4.4 IP: Internet Protocol" " datagram format" " IPv4 addressing" Network Layer 4-3 Network layer"! forwards packets from sending to receiving host "! on sending side: encapsulates transport packets into datagrams!! on receiving side: delivers packets to transport layer"! layer protocols exist in every host & router"! router examines header fields in all datagrams" application transport application transport pp " Network Layer 4-4

3 Two key -layer functions"! forwarding: move packets from router s input to appropriate router output"! routing: determine route taken by packets from source to dest. " " " routing algorithms" pp , " Network Layer 4-5 Interplay between routing and forwarding" routing algorithm local forwarding table header value output link routing algorithm determines end-end-path through forwarding table determines local forwarding at this router value in arriving packet s header pp " Network Layer 4-6

4 Connection vs connection-less services"! datagram provides -layer connectionless service"! virtual-circuit provides -layer connection service" pp 339" Network Layer 4-7 Virtual circuits: Signaling and flow"! Signaling to setup a virtual circuit, reserve resources, e.g. line capacity and buffers at each router. Establish state."! The flow of data packets starts. "! Signaling to supervise flow (e.g. Route/link failure)"! Signaling to tear down circuit & release resources" application transport 5. data flow begins" 6. receive data" 4. call connected" 3. accept call" 1. initiate call". incoming call" application transport Pp " Network Layer 4-8

5 Datagram s"! no call setup at layer"! routers: no state about end-to-end connections" " no -level concept of connection "! packets forwarded using destination host address, looked up at all encountered routers." application transport 1. send datagrams. receive datagrams application transport pp " Network Layer 4-9 The Internet layer" host, router layer functions:" transport layer: TCP, UDP layer routing protocols! path selection RIP, OSPF, BGP forwarding table IP protocol! addressing conventions datagram format packet handling conventions ICMP protocol! error reporting router signaling link layer layer pp " Network Layer 4-10

6 IP datagram format" IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead?! 0 bytes of TCP! 0 bytes of IP! = 40 bytes + app layer overhead ver head. len 16-bit identifier time to live type of service upper layer 3 bits flgs length fragment offset header checksum 3 bit source IP address 3 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly e.g. timestamp, record route taken, specify list of routers to visit. pp " Network Layer 4-11 IP fragmentation, reassembly"! links have MTU (Max.Transfer Unit) - largest possible link-level frame" " different link types, different MTUs "! large IP datagram are divided ( fragmented ) within net" " one datagram becomes several datagrams" " reassembled only at final destination" " IP header bits used to identify, order related fragments" reassembly fragmentation: in: one large datagram out: 3 smaller datagrams pp " Network Layer 4-1

7 IP addressing: introduction"! IP address: 3-bit identifier for host == router interface "! interface: connection between host/router and link" " router s typically have multiple interfaces" " host s typically has one or two interfaces (e.g., wired Ethernet, wireless 80.11)"! IP addresses associated with each interface! = pp " Network Layer 4-13 Dotted-decimal Notation 3-bit host addresses are often given in a dotted-decimal notation for convenience: The 3bits are split into four groups of 8bits. Each 8-bit value is translated into decimal form. Example: The dotted-decimal notation for is a) Translate the following addresses: pp " Network Layer 4-14

8 Datagram forwarding table" routing algorithm local forwarding table dest address output link address-range 1 address-range address-range 3 address-range billion IP addresses, so rather than list individual destination address" list range of addresses" (aggregate table entries)" IP destination address in arriving packet s header 1 3 pp " Network Layer 4-15 Datagram forwarding table" Destination Address Range through through Link Interface through otherwise 3 Q: but what happens if ranges don t divide up so nicely? " pp 11-17" Network Layer 4-16

9 Longest prefix matching" longest prefix matching! when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address." Destination Address Range *** ********* ********* *** ********* otherwise! examples: DA: DA: pp 11-17" Link interface which interface?" which interface?" Network Layer 4-17 Subnets" ! IP address: " " subnet part - high order bits" " host part - low order bits "! what s a subnet?! " device interfaces with same subnet part of IP address" " can ly reach each other without intervening router! subnet part pp 11-17" subnet consisting of 3 subnets /4 host part Network Layer 4-18

10 IP addresses: how to get one?" Q: How does a host get IP address?" "! hard-coded by system admin in a file" " Windows: control-panel->->configuration->tcp/ ip->properties" " UNIX: /etc/rc.config"! : Dynamic Host Configuration Protocol: dynamically get address from server" " " pp " Network Layer 4-19 : example" UDP IP Eth Phy UDP IP Eth Phy router with server built into router! connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use "! request encapsulated in UDP, encapsulated in IP, encapsulated in 80.1 Ethernet" "! Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running server"! Ethernet demuxed to IP demuxed, UDP demuxed to " pp " Network Layer 4-0

11 : example" UDP IP Eth Phy UDP IP Eth Phy router with server built into router! DCP server formulates ACK containing client s IP address, IP address of first-hop router for client, name & IP address of DNS server"! encapsulation of server, frame forwarded to client, demuxing up to at client"! client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router" pp " Network Layer 4-1 client-server scenario" server: discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriving client request offer src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 lifetime: 3600 secs src: , 67 dest: , 68 yiaddrr: transaction ID: 654 lifetime: 3600 secs ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs pp " Network Layer 4-

12 In TCP we talked about the 3-way handshake to make sure both sender and receiver agree that a connection is established. Why does need a fourstep process? o The fourth step is needed to agree on the lifetime of the address lease. o Several servers can answer with offers; therefore an additional step to select an offer of a particular server. o There are four steps because there are four parameters to be transferred: address, subnet mask, first hop router, local DNS server. o To avoid that two hosts configure the same address. Network Layer 4-3 NAT: address translation" rest of Internet local (e.g., home ) / all datagrams leaving local" have same single source NAT IP address: ,different source port numbers" datagrams with source or " destination in this " have /4 address for " source, destination (as usual)" pp " Network Layer 4-4

13 NAT: address translation" motivation: local uses just one IP address as far as outside world is concerned:" " range of addresses not needed from ISP: just one IP address for all devices" " can change addresses of devices in local without notifying outside world" " can change ISP without changing addresses of devices in local " " devices inside local net not explicitly addressable, visible by outside world (a security plus)" " pp " Network Layer 4-5 NAT: address translation" : NAT router changes datagram source addr from , 3345 to , 5001, updates table NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 80 D: , : reply arrives dest. address: , S: , 3345 D: , 80 1 S: , 80 D: , : host sends datagram to , : NAT router changes datagram dest addr from , 5001 to , 3345 pp " Network Layer 4-6

14 NAT: address translation"! 16-bit port-number field: " " 60,000 simultaneous connections with a single LAN-side address!"! NAT is controversial:" " routers should only process up to layer 3" " violates end-to-end argument" NAT possibility must be taken into account by app designers, e.g., PP applications" " address shortage should instead be solved by IPv6" pp " Network Layer 4-7 NAT traversal problem"! client wants to connect to server with address " " server address local to LAN (client can t use it as destination addr)" " only one externally visible NATed address: "! solution1: statically configure NAT to forward incoming connection requests at given port to server" " e.g., ( , port 500) always forwarded to port 5000" client? NAT router pp " Network Layer 4-8