Network Layer: Internet Protocol

Transcription

1 Network Layer: Internet Protocol

2 Motivation Heterogeneity Scale Intering IP is the glue that connects heterogeneous s giving the illusion of a homogenous one. Salient Features Each host is identified by a unique 32 bit identifier. Best Effort Service Model Global Addressing Scheme

3 Network layer functions transport packet from sending to receiving hosts layer protocols in every host, router three important functions: path determination: route taken by packets from source to dest. Routing algorithms switching: move packets from router s input to appropriate router output call setup: some architectures require router call setup along path before data flows application transport data link physical data link physical data link physical data link physical data link physical data link physical data link physical data link physical data link physical application transport data link physical

4 The Internet Model no call setup at layer routers: no state about end-to-end connections no -level concept of connection packets typically routed using destination host ID packets between same source-dest pair may take different paths application transport data link physical 1. Send data 2. Receive data application transport data link physical

5 The Internet: Service Model Connectionless Datagram based Best-effort delivery (unreliable service) packets are lost packets are delivered out of order duplicate copies of a packet may be delivered packets can be delayed for a long time

6 IP Internet Concatenation of Networks Network 1 (Ethernet) H7 R3 H8 H1 H2 H3 Network 2 (Ethernet) R1 Network 4 (point-to-point) R2 Protocol Stack H4 H5 Network 3 (FDDI) H6 H1 H8 TCP R1 R2 R3 TCP IP IP IP IP IP ETH ETH FDDI FDDI PPP PPP ETH ETH

7 IP datagram format IP protocol version number header length (32 bit words) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to ver head. len 16-bit identifier time to live type of service upper layer 32 bits flgs length fragment offset Internet checksum 32 bit source IP address 32 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.

8 IP Fragmentation & Reassembly links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs large IP datagram 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

9 IP Fragmentation and Reassembly length =4000 ID =x fragflag =0 offset =0 One large datagram becomes several smaller datagrams length =1500 ID =x fragflag =1 offset =0 length =1500 ID =x fragflag =1 offset =1480 length =1040 ID =x fragflag =0 offset =2960

10 ICMP: Internet Control Message Protocol used by hosts, routers, gateways to communication -level information error reporting: unreachable host,, port, protocol echo request/reply (used by ping) -layer above IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

11 IP Addressing: Introduction IP address: 32-bit identifier for host, router interface interface: connection between host, router and physical link router s typically have multiple interfaces host may have multiple interfaces IP addresses associated with interface, not host, router =

12 IP Addressing IP address: part (high order bits) host part (low order bits) What s a? (from IP address perspective) device interfaces with same part of IP address can physically reach each other without intervening router LAN consisting of 3 IP s (for IP addresses starting with 223, first 24 bits are address)

13 IP Addressing How to find the s? Detach each interface from router, host create islands of isolated s Interconnected system consisting of six s

14 0 IP Addresses given notion of, let s re-examine IP addresses: class-full addressing: class A B C host 10 host 110 host to to to D 1110 multicast address 32 bits to

15 IP addressing: CIDR classful addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that CIDR: Classless InterDomain Routing portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in portion of address part host part /23

16 IP addresses: how to get one? Hosts (host portion): hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol: dynamically get address: plug-and-play host broadcasts DHCP discover msg DHCP server responds with DHCP offer msg host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg

17 IP addresses: how to get one? Network ( portion): get allocated portion of ISP s address space: ISP's block /20 Organization /23 Organization /23 Organization / Organization /23

18 Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization /23 Organization /23 Organization /23 Organization Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet /23 ISPs-R-Us Send me anything with addresses beginning /16

19 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization /23 Organization / Organization /23 Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Send me anything with addresses beginning /16 or /23 Internet

20 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes

21 Getting a datagram from source to dest. IP datagram: misc fields source IP addr dest IP addr data datagram remains unchanged, as it travels source to destination addr fields of interest here A B routing table in A Dest. Net. next router Nhops E

22 Getting a datagram from source to dest. misc fields data Starting at A, given IP datagram addressed to B: look up net. address of B find B is on same net. as A link layer will send datagram directly to B inside link-layer frame B and A are directly connected A B Dest. Net. next router Nhops E

23 Getting a datagram from source to dest. misc fields data Starting at A, dest. E: look up address of E E on different A, E not directly attached routing table: next hop router to E is link layer sends datagram to router inside linklayer frame datagram arrives at continued.. A B Dest. Net. next router Nhops E

24 Getting a datagram from source to dest. Dest. next misc fields data router Nhops interface Arriving at , destined for look up address of E E on same as router s interface router, E directly attached link layer sends datagram to inside link-layer frame via interface datagram arrives at !!! A B E