Assignments. Computer Networks LECTURE 7 Network Layer: Routing and Addressing. Network Layer Function. Internet Architecture

Transcription

1 ompter Netorks LETURE Netork Laer: Roting and ddressing ssignments Project : Web Pro Serer DUE OT Sandha Darkadas Department of ompter Science Uniersit of Rochester Internet rchitectre Bottom-p: phsical: electromagnetic signals on the ire link: data transfer beteen neighboring netork elements encoding, framing, error correction, access control for shared links netork: host-to-host connectiit roting, addressing transport: host-host data transport reliable data transport, congestion control, flo control application: anthing o ant to do on compter netorks application transport netork link phsical transport packet sending to receiing hosts roting: determine a path sorce to dest and rote packets along the path addressing: niqel identif each node in the netork netork connecting deices called roters participate in netork protocols links connect adjacent hosts, roters Netork Laer Fnction 9//06 S / - Fall 0 9//06 S / - Fall 0

2 Roting Principles ROUTING PROTOOLS Roting protocol Goal: determine good path (seqence of roters) thr netork sorce to dest. Graph abstraction for roting algorithms: graph nodes are hosts or roters graph edges are links link cost: dela, $ cost, or congestion leel B D good path: E F tpicall means minimm cost path 9//06 6 Roting lgorithm lassification Global information: all roters hae complete topolog, link cost info link state algorithm Decentralied: roter knos connected neighbors, link costs to neighbors echange of info ith neighbors to learn remote parts of the netork, ma take man learning ronds distance ector algorithm 9//06 link-state roting algorithm Dijkstra s algorithm net topolog, link costs knon to all nodes accomplished ia link state broadcast all nodes hae same info comptes least cost paths one node ( sorce ) to all other nodes gies forarding for that node iteratie: after k iterations, kno least cost path to k dest. s notation: c(,): link cost node to ; = if not direct neighbors D(): crrent ale of cost of path sorce to dest. p(): predecessor node along path sorce to N': set of nodes hose least cost path definitiel knon Netork Laer: ontrol Plane -8

3 Dijsktra s algorithm Initialiation: N' = {} for all nodes if adjacent to then D() = c(,) 6 else D() = 8 Loop 9 find not in N' sch that D() is a minimm 0 add to N' pdate D() for all adjacent to and not in N' : D() = min( D(), D() + c(,) ) /* ne is either old or knon shortest path pls cost to */ ntil all nodes in N' Dijkstra s algorithm: eample D() D() D() D() D() Step N' p() p() p() p() p() 0,,, 6,,, 6,,, 0,,, notes: constrct shortest path tree b tracing predecessor nodes ties can eist (can be broken arbitraril) 8 9 Netork Laer: ontrol Plane -9 Netork Laer: ontrol Plane Netork Laer - Netork Laer -

4 Dijkstra s algorithm: another eample Dijkstra s algorithm: eample () Step 0 N' D(),p(),,, D(),p(),,,, D(),p(), D(),p(), D(),p(),,, reslting shortest-path tree : reslting forarding in : * heck ot the online interactie eercises for more eamples: Netork Laer: ontrol Plane - destination link (,) (,) (,) (,) (,) Netork Laer: ontrol Plane - Dijkstra s lgorithm: ompleit Dijkstra s lgorithm: Stabilit lgorithm compleit: n nodes, e links Each iteration: need to check all nodes,, not in N n*(n-)/ checks: O(n ) Update -more-hop paths: O(e) Total: O(n +e), or O(n ) Using Fibonacci heap to find minimm distance node O(nlogn + e) 9//06 Oscillations possible: e.g., link cost = amont of carried traffic +e D B e e initiall +e 0 0 +e D +e B D B e Soltions: asnchronos (at different time) adjstments across roters s cost metric (independent of roting polic) +e 0 D +e B 0 0 recompte roting recompte recompte 9//06 6

5 Link state roting: Netork Roting Dijkstra s algorithm efficient approach to calclate least cost rotes all roters need complete topolog, link cost info costl (or impossible) to acqire sch information in large netorks Decentralied roting: distribted, asnchronos, iteratie roter onl needs to kno phsicall-connected neighbors, link costs to neighbors learn more b info echanges beteen neighbor roters Distance Vector Roting B D Roting (at each host): the net hop for each destination in the netork Distance ector roting: roting can be deried the distance ector at each node distance ectors can be maintained in a decentralied fashion E F 9//06 S / - Fall 0 9//06 S / - Fall 0 8 Distance ector algorithm Bellman-Ford eqation let d () := cost of least-cost path to then d () = min {c(,) + d () } cost neighbor to destination neighbor min taken oer all neighbors of Bellman-Ford eample clearl, d () =, d () =, d () = B-F eqation sas: d () = min { c(,) + d (), c(,) + d (), c(,) + d () } = min { +, +, + } = node achieing minimm is net hop in shortest path, sed in forarding Netork Laer: ontrol Plane -9 Netork Laer: ontrol Plane -0

6 Distance ector algorithm D () = estimate of least cost to maintains distance ector D = [D (): є N ] node : knos each neighbor : c(,) maintains its neighbors distance ectors. For each neighbor, maintains D = [D (): є N ] Distance ector algorithm ke idea: time-to-time, each node sends its on distance ector estimate to neighbors hen receies ne DV estimate neighbor, it pdates its on DV sing B-F eqation: D () min {c(,) + D ()} for each node N nder minor, natral conditions, the estimate D () conerge to the actal least cost d () Netork Laer: ontrol Plane - Netork Laer: ontrol Plane - Distance ector algorithm iteratie, asnchronos: each local iteration cased b: local link cost change DV pdate message neighbor distribted: each node notifies neighbors onl hen its DV changes neighbors then notif their neighbors if necessar each node: ait for (change in local link cost or msg neighbor) recompte estimates if DV to an dest has changed, notif neighbors Netork Laer: ontrol Plane - node node node D () = min{c(,) + D (), c(,) + D ()} = min{+0, +} = time D () = min{c(,) + D (), c(,) + D ()} = min{+, +0} = Netork Laer: ontrol Plane - 6

7 node node node D () = min{c(,) + D (), c(,) + D ()} = min{+0, +} = time D () = min{c(,) + D (), c(,) + D ()} = min{+, +0} = Netork Laer: ontrol Plane - Distance ector: link cost changes link cost changes: node detects local link cost change pdates roting info, recalclates distance ector if DV changes, notif neighbors good nes traels fast 0 t 0 : detects link-cost change, pdates its DV, informs its neighbors. t : receies pdate, pdates its, comptes ne least, sends its neighbors its DV. t : receies s pdate, pdates its distance. s least costs do not change, so does not send a message to. * heck ot the online interactie eercises for more eamples: Netork Laer: ontrol Plane -6 Distance Vector to Roting Table destination ia E D () B D B B Otgoing link to se, cost, D, D, Link cost changes: Distance Vector: Link ost hanges node detects local link cost change recompte pdates local distance ector if cost change in least cost path, notif neighbors X Y Z 0 algorithm terminates D D D, Distance ector Roting 8

8 Distance Vector: Link ost hanges Distance ector: link cost changes pathological case: recrsie distance ector pdates applies onl to link cost increase bad nes settles slol 60 X Y 0 Z algorithm contines on! link cost changes: node detects local link cost change bad nes traels slo - cont to infinit problem! iterations before algorithm stabilies: see tet poisoned reerse: 60 0 If Z rotes throgh Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y on t rote to X ia Z) ill this completel sole cont to infinit problem? 9 Netork Laer: ontrol Plane -0 Distance Vector: Link ost hanges Roting Loops If Z rotes throgh Y to get to X : Z tells Y its (Z s) distance to X is infinit (so Y on t rote to X ia Z) Will this completel sole the problem of recrsie distance ector pdates? 60 X 0. Y Z 0 U 0. algorithm terminates In packet sitching netorks, each node comptes its on roting independentl What if? Y s net hop to X is Z; and Z s net hop to X is Y. Y X Z Roting loops dring recrsie distance ector pdates roting mis-behaiors 8

9 omparison of LS and DV algorithms The Internet Netork Laer message compleit LS: ith n nodes, E links, O(nE) msgs sent DV: echange beteen neighbors onl conergence time aries speed of conergence LS: O(n ) algorithm reqires O(nE) msgs ma hae oscillations DV: conergence time aries ma be roting loops cont-to-infinit problem robstness: hat happens if roter malfnctions? LS: node can adertise incorrect link cost each node comptes onl its on DV: DV node can adertise incorrect path cost each node s sed b others error propagate thr netork Netork Laer: ontrol Plane - Netork laer Roting protocols RIP, OSPF, BGP Transport laer: TP, UDP The rest of the IP protocol addressing conentions packet format packet handling conentions Link laer IMP protocol error reporting roter signaling phsical laer IP ddressing: Introdction IP address: -bit identifier for each host, roter interface DDRESSING Interface: connecting point into each data link roter tpicall has mltiple interfaces host often has single interface = //06 6 9

10 IP Netork and Hierarchical ddressing IP ddresses: Original Standard What s an IP netork? can phsicall reach each other ithot interening roter (interening sitches?) IP address: netork part (high order bits); host part (lo order bits) deices ith same netork part of IP address are in the same IP netork IP netork netork consisting of IP netorks Gien notion of netork, let s re-eamine IP addresses: class -based addressing: class B D 0netork host 0 netork host 0 netork host 0 mlticast address bits to to to to //06 9//06 8 IP ddressing: IDR IP ddress Depletion (DHP) lass -based addressing: inefficient se of address space, address space ehastion e.g., class B netork allocated enogh addresses for 6K hosts, een if onl K hosts in that netork IDR: classless addressing netork portion of address of arbitrar length address format: a.b.c.d/, here is # bits in netork portion netork part / host part ddress depletion -bit address space soon to be sed p. Obseration: not eer host is online at a gien time. DHP: allo host to dnamicall obtain its IP address netork serer hen it joins netork can rene its lease on address in se allos rese of addresses (onl hold address hile connected) 9//06 9 9//06 0 0

11 IP ddress Depletion (NT) IP ddress Depletion (NT) Obserations: lot of traffic is local lthogh IP addresses are fe, possible (IPaddr, port) tples are more abndant and the can identif commnication end point rest of Internet local netork (e.g., home netork) / ll IP packets leaing local Packets ith sorce or netork hae same single sorce NT IP destination in this netork address: , hae / addresses for different sorce port nmbers sorce, destination Problem: inbond connection not alloed! 9//06 9//06 Disclaimer Parts of the lectre slides are adapted and coprighted b James Krose and Keith Ross and those b Prof. Kai Shen. The slides are intended for the sole prpose of instrction of compter netorks at the Uniersit of Rochester. ll coprighted materials belong to their original oner(s).