Chapter 5. The Network Layer IP

Transcription

1 Chapter 5 The Netwrk Layer IP These slides are taken frm the bk Cmputer etwrking, A Tp Dwn Apprach Featuring the Internet by Kurse & Rss and frm the bk Cmputer etwrks by Andrew Tanenbaum.

2 The Netwrk Layer in the Internet The IP Prtcl IP Datagram Frmat IP Addresses NAT Internet Cntrl Prtcls ICMP ARP DHCP OSPF The Interir Gateway Ruting Prtcl BGP The Exterir Gateway Ruting Prtcl

3 Netwrk layer transprt segment frm sending t receiving hst n sending side encapsulates segments int datagrams n rcving side, delivers segments t transprt layer netwrk layer prtcls in every hst, ruter Ruter examines header fields in all IP datagrams passing thrugh it applicatin transprt netwrk data link physical netwrk data link physical netwrk data link physical netwrk data link physical netwrk data link physical netwrk data link physical netwrk data link physical netwrk data link physical netwrk data link physical applicatin transprt netwrk data link physical

4 The Internet Netwrk layer Hst, ruter netwrk layer functins: Transprt layer: TCP, UDP Netwrk layer Ruting prtcls path selectin RIP, OSPF, BGP frwarding table IP prtcl addressing cnventins datagram frmat packet handling cnventins ICMP prtcl errr reprting ruter signaling Link layer physical layer

5 IP datagram frmat IP prtcl versin number header length (bytes) type f data max number remaining hps (decremented at each ruter) upper layer prtcl t deliver paylad t hw much verhead with TCP? a)20 bytes f TCP b)20 bytes f IP c)= 40 bytes + app layer verhead ver head. len 16-bit identifier time t live type f service upper layer 32 bits flgs length fragment ffset header checksum 32 bit surce IP address 32 bit destinatin IP address Optins (if any) data (variable length, typically a TCP r UDP segment) ttal datagram length (bytes) fr fragmentatin/ reassembly E.g. timestamp, recrd rute taken, specify list f ruters t visit.

6 The IP Prtcl The IPv4 (Internet Prtcl) header.

7 IP Fragmentatin & Reassembly netwrk links have MTU (max.transfer size) - largest pssible link-level frame. different link types, different MTUs large IP datagram divided ( fragmented ) within net ne datagram becmes several datagrams reassembled nly at final destinatin IP header bits used t identify, rder related fragments reassembly fragmentatin: in: ne large datagram ut: 3 smaller datagrams

8 IP Fragmentatin and Reassembly Example a)4000 byte datagram b)mtu = 1500 bytes length =4000 ID =x fragflag =0 ffset =0 One large datagram becmes several smaller datagrams 1480 bytes in data field ffset = 1480/8 length =1500 length =1500 length =1040 ID =x ID =x ID =x fragflag =1 fragflag =1 fragflag =0 ffset =0 ffset =185 ffset =370

9 IP Addressing: intrductin IP address: 32-bit identifier fr hst, ruter interface interface: cnnectin between hst/ruter and physical link ruter s typically have multiple interfaces hst typically has ne interface IP addresses assciated with each interface =

10 IP Addresses IP address frmats.

11 Subnets What s a subnet? The parts f the netwrk is called subnet device interfaces with same subnet part f IP address can physically reach each ther withut intervening ruter IP address: subnet part (high rder bits) hst part (lw rder bits) subnet netwrk cnsisting f 3 subnets

12 Subnets A campus netwrk cnsisting f LANs fr varius departments.

13 Subnettting Subnetting is the prcess f allcating bits frm the hst prtin as a netwrk prtin A class B netwrk subnetted int 64 subnets.

14 Subnettting Subnet 1: : Subnet 2: : Subnet 3: :

15 IP addressing: CIDR CIDR: Classless InterDmain Ruting subnet prtin f address f arbitrary length address frmat: a.b.c.d/x, where x is # bits in subnet prtin f address subnet part hst part /23

16 IP addresses: hw t get ne? Q: Hw des hst get IP address? a) hard-cded by system admin in a file Wintel: cntrl-panel->netwrk->cnfiguratin- >tcp/ip->prperties UNIX: /etc/rc.cnfig b) DHCP: Dynamic Hst Cnfiguratin Prtcl: dynamically get address frm as server plug-and-play

17 IP addressing: the last wrd... Q: Hw des an ISP get blck f addresses? A: ICANN: Internet Crpratin fr Assigned Names and Numbers allcates addresses manages DNS assigns dmain names, reslves disputes

18 NAT: Netwrk Address Translatin rest f Internet lcal netwrk (e.g., hme netwrk) / All datagrams leaving lcal netwrk have same single surce NAT IP address: , different surce prt numbers Datagrams with surce r destinatin in this netwrk have /24 address fr surce, destinatin (as usual)

19 NAT: Netwrk Address Translatin Mtivatin: lcal netwrk uses just ne IP address as far as utside wrld is cncerned: range f addresses nt needed frm ISP: just ne IP address fr all devices can change addresses f devices in lcal netwrk withut ntifying utside wrld can change ISP withut changing addresses f devices in lcal netwrk devices inside lcal net nt explicitly addressable, visible by utside wrld (a security plus).

20 NAT: Netwrk Address Translatin Implementatin: NAT ruter must: utging datagrams: replace (surce IP address, prt #) f every utging datagram t (NAT IP address, new prt #)... remte clients/servers will respnd using (NAT IP address, new prt #) as destinatin addr. remember (in AT translatin table) every (surce IP address, prt #) t (NAT IP address, new prt #) translatin pair incming datagrams: replace (NAT IP address, new prt #) in dest fields f every incming datagram with crrespnding (surce IP address, prt #) stred in NAT table

21 NAT: Netwrk Address Translatin 2: NAT ruter changes datagram surce addr frm , 3345 t , 5001, updates table 2 NAT translatin table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 3345 D: , : hst sends datagram t , S: , 80 D: , : Reply arrives dest. address: , 5001 S: , 80 D: , : NAT ruter changes datagram dest addr frm , 5001 t , 3345

22 NAT: Netwrk Address Translatin 16-bit prt-number field: 60,000 simultaneus cnnectins with a single LANside address! NAT is cntrversial: ruters shuld nly prcess up t layer 3 (mdifies TCP lad. Layer 4) vilates end-t-end argument cnnectinless -> cnectin riented (if the NAT table crashes all TCP cnectins get lst. Prcesses d nt require TCP (prt # is nt avail.)

23 ICMP: Internet Cntrl Message Prtcl used by hsts & ruters t cmmunicate netwrk-level infrmatin errr reprting: unreachable hst, netwrk, prt, prtcl ech request/reply (used by ping) netwrk-layer abve IP: ICMP msgs carried in IP datagrams ICMP message: type, cde plus first 8 bytes f IP datagram causing errr

24 Internet Cntrl Message Prtcl 5-61 The principal ICMP message types.

25 Tracerute and ICMP a)surce sends series f UDP segments t dest First has TTL =1 Secnd has TTL=2, etc. Unlikely prt number b)when nth datagram arrives t nth ruter: Ruter discards datagram And sends t surce an ICMP message (type 11, cde 0, TTL expired ) Message includes name f ruter& IP address a)when ICMP message arrives, surce calculates RTT b)tracerute des this 3 times Stpping criterin a)udp segment eventually arrives at destinatin hst b)destinatin returns ICMP hst unreachable packet (type 3, cde 3, destinatin prt unreachable ) c)when surce gets this ICMP, stps.