Chapter IV: Network Layer

Transcription

1 Chapter IV: Network Laer UG3 Computer Communications & Networks (COMN) Mungjin Lee Slides copright of Kurose and Ross

2 IP addresses: how to get one? Q: How does a host get IP address? hard-coded b sstem admin in a file Windows: control-panel->network->configuration->tcp/ip- >properties UNIX: /etc/rc.config DHCP: Dnamic Host Configuration Protocol: dnamicall get address as server plug-and-pla 7

3 DHCP: Dnamic Host Configuration Protocol goal: allow host to dnamicall obtain its IP address network server when it joins network can renew its lease on address in use allows reuse of addresses (onl hold address while connected/ on ) support for mobile users who want to join network (more shortl) DHCP overview: host broadcasts DHCP discover msg [optional] DHCP server responds with DHCP offer msg [optional] host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg 8

4 DHCP client-server scenario 3...0/ DHCP server arriving DHCP client needs address in this network 3...0/ /4 9

5 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 iaddr: transaction ID: 654 arriving client DHCP request DHCP offer src: , 68 dest:: , 67 iaddrr: transaction ID: 655 lifetime: 3600 secs src: 3...5, 67 dest: , 68 iaddrr: transaction ID: 654 lifetime: 3600 secs DHCP ACK src: 3...5, 67 dest: , 68 iaddrr: transaction ID: 655 lifetime: 3600 secs 30

6 DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet: address of first-hop router for client (i.e., default gatewa) name and IP address of DNS server network mask (indicating network versus host portion of address) 3

7 IP addresses: how to get one? Q: how does network get subnet part of IP addr? A: gets allocated portion of its provider ISP s address space ISP's block /0 Organiation /3 Organiation /3 Organiation / Organiation /3 3

8 Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organiation /3 Organiation /3 Organiation /3 Organiation /3.... Fl-B-Night-ISP ISPs-R-Us Send me anthing with addresses beginning /0 Send me anthing with addresses beginning /6 Internet 33

9 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organiation Organiation /3 Organiation /3 Organiation /3 Organiation /3.... Fl-B-Night-ISP ISPs-R-Us Send me anthing with addresses beginning /0 Internet Send me anthing with addresses beginning /6 or /3 34

10 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 35

11 NAT: network address translation rest of Internet local network (e.g., home network) 0.0.0/ all datagrams leaving local network have same single source NAT IP address: ,different source port numbers datagrams with source or destination in this network have 0.0.0/4 address for source, destination (as usual) 36

12 NAT: network address translation motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed ISP: just one IP address for all devices can change addresses of devices in local network without notifing outside world can change ISP without changing addresses of devices in local network devices inside local net not eplicitl addressable, visible b outside world (a securit plus) 37

13 NAT: network address translation : NAT router changes datagram source addr , 3345 to , 500, updates table NAT translation table WAN side addr LAN side addr , , 3345 S: , 500 D: , S: , 3345 D: , 80 : host sends datagram to , S: , 80 D: , : repl arrives dest. address: , 500 S: , 80 D: , : NAT router changes datagram dest addr , 500 to ,

14 NAT: network address translation 6-bit port-number field: 60,000 simultaneous connections with a single LANside address! NAT is controversial: routers should onl process up to laer 3 violates end-to-end argument NAT possibilit must be taken into account b app designers, e.g., PP applications address shortage should instead be solved b IPv6 39

15 ICMP: internet control message protocol used b hosts & routers to communicate networklevel information error reporting: unreachable host, network, port, protocol echo request/repl (used b ping) network-laer above IP: ICMP msgs carried in IP datagrams ICMP message: tpe, code plus first 8 btes of IP datagram causing error Tpe Code description 0 0 echo repl (ping) 3 0 dest. network unreachable 3 dest host unreachable 3 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 0 0 router discover 0 TTL epired 0 bad IP header 40

16 Traceroute and ICMP source sends series of UDP segments to dest first set has TTL = second set has TTL=, etc. unlikel port number when nth set of datagrams arrives to nth router: router discards datagrams and sends source ICMP messages (tpe, code 0) ICMP messages includes name of router & IP address 3 probes 3 probes when ICMP messages arrives, source records RTTs stopping criteria: v UDP segment eventuall arrives at destination host v destination returns ICMP port unreachable message (tpe 3, code 3) v source stops 3 probes 4

17 IPv6: motivation initial motivation: 3-bit address space soon to be completel allocated additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fied-length 40 bte header no fragmentation allowed 4

18 IPv6 datagram format priorit: identif priorit among datagrams in flow flow Label: identif datagrams in same flow. (concept of flow not well defined) net header: identif upper laer protocol for data ver pri flow label paload len net hdr hop limit source address (8 bits) destination address (8 bits) data 3 bits 43

19 Other changes IPv4 checksum: removed entirel to reduce processing time at each hop options: allowed, but outside of header, indicated b Net Header field ICMPv6: new version of ICMP additional message tpes, e.g. Packet Too Big multicast group management functions 44

20 Transition IPv4 to IPv6 not all routers can be upgraded simultaneousl no flag das how will network operate with mied IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as paload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr UDP/TCP paload IPv4 paload IPv6 datagram IPv4 datagram 45

21 Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 phsical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 46

22 Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 phsical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 flow: X src: A dest: F data src:b dest: E Flow: X Src: A Dest: F src:b dest: E Flow: X Src: A Dest: F flow: X src: A dest: F data data data A-to-B: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 E-to-F: IPv6 47

23 Interpla between routing, forwarding routing algorithm local forwarding table dest address output link address-range address-range address-range 3 address-range 4 3 routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router IP destination address in arriving packet s header 3 48

24 Graph abstraction 5 graph: G = (N,E) u v 3 3 w 5 N = set of routers = { u, v, w,,, } E = set of links ={ (u,v), (u,), (v,), (v,w), (,w), (,), (w,), (w,), (,) } 49

25 Graph abstraction: costs u 5 v 3 3 w 5 c(, ) = cost of link (, ) e.g., c(w,) = 5 cost could alwas be, or inversel related to bandwidth, or inversel related to congestion cost of path (,, 3,, p ) = c(, ) + c(, 3 ) + + c( p-, p ) ke question: what is the least-cost path between u and? routing algorithm: algorithm that finds that least cost path 50

26 Routing algorithm classification Q: global or decentralied information? global: all routers have complete topolog, link cost info link state algorithms decentralied: router knows phsicallconnected neighbors, link costs to neighbors iterative process of computation, echange of info with neighbors distance vector algorithms Q: static or dnamic? static: routes change slowl over time dnamic: routes change more quickl periodic update in response to link cost changes 5

27 A Link-State Routing Algorithm Dijkstra s algorithm net topolog, link costs known to all nodes accomplished via link state broadcast all nodes have same info computes least cost paths one node ( source ) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k dest. s notation: c(,): link cost node to ; = if not direct neighbors D(v): current value of cost of path source to dest. v p(v): predecessor node along path source to v N': set of nodes whose least cost path definitivel known 5

28 Dijkstra s Algorithm Initialiation: N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = 7 8 Loop 9 find w not in N' such that D(w) is a minimum 0 add w to N' update D(v) for all v adjacent to w and not in N' : D(v) = min( D(v), D(w) + c(w,v) ) 3 /* new v is either old v or known 4 shortest path w plus cost w to v */ 5 until all nodes in N' 53

29 Dijkstra s algorithm: eample Step N' D(v) p(v) D(w) p(w) D() p() D() p() D() p() u 7,u 3,u 5,u uw 6,w 5,u,w uw 6,w,w 4, uwv 0,v 4, uwv, uwv 9 notes: v v construct shortest path tree b tracing predecessor nodes ties can eist (can be broken arbitraril) u 5 3 w v 54

30 Dijkstra s algorithm: another eample 55 Step N' u u u uv uvw uvw D(v),p(v),u,u,u D(w),p(w) 5,u 4, 3, 3, D(),p(),u D(),p(), D(),p() 4, 4, 4, u w v

31 Dijkstra s algorithm: eample () resulting shortest-path tree u: v w u resulting forwarding table in u: destination v w link (u,v) (u,) (u,) (u,) (u,) 56

32 Dijkstra s algorithm, discussion algorithm compleit: n nodes each iteration: need to check all nodes, w, not in N n(n+)/ comparisons: O(n ) more efficient implementations possible: O(nlogn) oscillations possible: e.g., suppose link cost equals amount of carried traffic: D A +e e C initiall e B A +e 0 D 0 +e C B given these costs, find new routing. resulting in new costs 0 D A 0 +e 0 0 C +e B given these costs, find new routing. resulting in new costs A +e 0 D 0 +e C B given these costs, find new routing. resulting in new costs 0 57

33 Distance vector algorithm Bellman-Ford equation (dnamic programming) let d () := cost of least-cost path to then v d () = min {c(,v) + d v () } cost neighbor v to destination neighbor v min taken over all neighbors v of 58

34 Bellman-Ford eample u 5 v 3 3 w 5 clearl, d v () = 5, d () = 3, d w () = 3 B-F equation sas: d u () = min { c(u,v) + d v (), c(u,) + d (), c(u,w) + d w () } = min { + 5, + 3, 5 + 3} = 4 node achieving minimum is net hop in shortest path, used in forwarding table 59

35 Distance vector algorithm D () = estimate of least cost to maintains distance vector D = [D (): є N ] node : knows each neighbor v: c(,v) maintains its neighbors distance vectors. For each neighbor v, maintains D v = [D v (): є N ] 60

36 Distance vector algorithm ke idea: time-to-time, each node sends its own distance vector estimate to neighbors when receives new DV estimate neighbor, it updates its own DV using B-F equation: D () min v {c(,v) + D v ()} for each node N v under minor, natural conditions, the estimate D () converges to the actual least cost d () 6

37 Distance vector algorithm iterative, asnchronous: each local iteration caused b: local link cost change DV update message neighbor distributed: each node notifies neighbors onl when its DV changes neighbors then notif their neighbors if necessar each node: wait for (change in local link cost or msg neighbor) recompute estimates if DV to an dest has changed, notif neighbors 6

38 node table node table 0 7 node table 7 0 time 63

39 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table node table 0 7 node table 7 0 time 64

40 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table node table 0 7 node table 7 0 time 65

41 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table node table 0 7 node table 7 0 time 66

42 node table D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 3 node table 0 7 node table 7 0 time 67

43 node table D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 3 node table 0 7 node table 7 0 time 68

44 node table D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 3 node table 0 7 node table 7 0 time 69

45 node table node table D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 3 7 node table time 70

46 Distance vector: link cost changes link cost changes: v v v node detects local link cost change updates routing info, recalculates distance vector if DV changes, notif neighbors 4 50 good news travels fast t 0 : detects link-cost change, updates its DV, informs its neighbors. t : receives update, updates its table, computes new least, sends its neighbors its DV. t : receives s update, updates its distance table. s least costs do not change, so does not send a message to. 7

47 Distance vector: link cost changes link cost changes: v v v node detects local link cost change bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies Can reach X with cost of 6 (via Z) Can reach X with cost of 8 (via Z) Can reach X with cost of 7 (via Y) Can reach X with cost of 9 (via Y) 7

48 Distance vector: link cost changes link cost changes: v v v node detects local link cost change bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies poisoned reverse: v If Z routes through Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y won t route to X via Z) v will this completel solve count to infinit problem? 73

49 Comparison of LS and DV algorithms message compleit LS: with n nodes, E links, O(nE) msgs sent DV: echange between neighbors onl convergence time varies speed of convergence LS: O(n ) algorithm requires O(nE) msgs ma have oscillations DV: convergence time varies ma be routing loops count-to-infinit problem robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes onl its own table DV: DV node can advertise incorrect path cost each node s table used b others error propagate thru network 74