Chapter 4: Network Layer. TDTS06 Computer networks. Chapter 4: Network Layer. Network layer. Two Key Network-Layer Functions

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1 Chapter : Netork Laer TDTS06 Compter s Lectre : Netork laer II Roting algorithms Jose M. Peña, jospe@ida.li.se ID/DIT, LiU Chapter goals: nderstand principles behind laer serices: laer serice models forarding erss roting ho a roter orks roting (path selection) dealing ith scale adanced topics: IP6, mobilit instantiation, implementation in the Internet * Slides are modified J. F. Krose and K. W. Ross. Netork Laer - Netork Laer - Chapter : Netork Laer. Introdction. Virtal circit and datagram s. What s inside a roter. IP: Internet Protocol Datagram format IP addressing ICMP IP6. Roting algorithms Link state Distance Vector Hierarchical roting.6 Roting in the Internet RIP OSPF BGP.7 Broadcast and mlticast roting Netork laer transport segment sending to receiing host on sending side encapslates segments into datagrams on rcing side, deliers segments to transport laer laer protocols in eer host, roter roter eamines header fields in all IP datagrams passing throgh it application transport phsical phsical phsical phsical phsical phsical phsical phsical phsical phsical phsical phsical application transport phsical Netork Laer - Netork Laer - To Ke Netork-Laer Fnctions Interpla beteen roting and forarding forarding: moe packets roter s inpt to appropriate roter otpt roting: determine rote taken b packets sorce to dest. roting algorithms analog: roting: process of planning trip sorce to dest forarding: process of getting throgh single interchange ale in arriing packet s header roting algorithm local forarding table header ale otpt link Netork Laer - Netork Laer -6

2 Forarding table billion possible entries Longest prefi matching Destination ddress Range throgh Link Interface Prefi Match Link Interface otherise throgh throgh otherise Netork Laer -7 Eamples D: D: Which interface? Which interface? Netork Laer -8 Roter rchitectre Oerie To ke roter fnctions: rn roting algorithms/protocol (RIP, OSPF, BGP) forarding datagrams incoming to otgoing link Netork Laer -9 IP datagram format IP protocol ersion nmber header length (btes) tpe of data ma nmber remaining hops (decremented at each roter) pper laer protocol to delier paload to ho mch oerhead ith TCP? 0 btes of TCP 0 btes of IP = 0 btes + app laer oerhead bits head. tpe of er length len serice fragment 6-bit identifier flgs offset time to pper header lie laer checksm bit sorce IP address bit destination IP address Options (if an) data (ariable length, tpicall a TCP or UDP segment) total datagram length (btes) for fragmentation/ reassembl E.g. timestamp, record rote taken, specif list of roters to isit. Netork Laer -0 TCP segment strctre URG: rgent data (generall not sed) CK: CK # alid PSH: psh data no (generall not sed) RST, SYN, FIN: connection estab (setp, teardon commands) Internet checksm (as in UDP) bits sorce port # dest port # seqence nmber acknoledgement nmber head not Receie indo len sed UP RSF checksm Urg data pnter Options (ariable length) application data (ariable length) conting b btes of data (not segments!) # btes rcr illing to accept Chapter : Netork Laer. Introdction. Virtal circit and datagram s. What s inside a roter. IP: Internet Protocol Datagram format IP addressing ICMP IP6. Roting algorithms Link state Distance Vector Hierarchical roting.6 Roting in the Internet RIP OSPF BGP.7 Broadcast and mlticast roting Netork Laer -

3 Interpla beteen roting, forarding Graph abstraction roting algorithm local forarding table header ale otpt link Graph: G = (N,E) ale in arriing packet s header 0 N = set of roters = {,,,,, } E = set of links ={ (,), (,), (,), (,), (,), (,), (,), (,), (,) } Remark: Graph abstraction is sefl in other contets Eample: PP, here N is set of peers and E is set of TCP connections Netork Laer - Netork Laer - Graph abstraction: costs Roting lgorithm classification c(, ) = cost of link (, ) - e.g., c(,) = cost cold alas be, or inersel related to bandidth, or inersel related to congestion Cost of path (,,,, p ) = c(, ) + c(, ) + + c( p-, p ) Qestion: What s the least-cost path beteen and? Roting algorithm: algorithm that finds least-cost path Netork Laer - Global or decentralied information? Global: all roters hae complete topolog, link cost info link state algorithms Decentralied: roter knos phsicallconnected neighbors, link costs to neighbors iteratie process of comptation, echange of info ith neighbors distance ector algorithms Static or dnamic? Static: rotes change slol oer time Dnamic: rotes change more qickl periodic pdate in response to link cost changes Netork Laer -6 Link-State Roting lgorithm Dijsktra s lgorithm 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 table 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 -7 Initialiation: N' = {} for all nodes if adjacent to then D() = c(,) 6 else D() = 7 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' Netork Laer -8

4 Dijkstra s algorithm: eample Dijkstra s algorithm: eample () Step 0 N' D(),p(),,, D(),p(),,,, D(),p(), D(),p(), D(),p(),,, Reslting shortest-path tree : Netork Laer -9 Reslting forarding table in : destination link (,) (,) (,) (,) (,) Each roter shold eecte the algorithm to compte its table!!! Netork Laer -0 Dijkstra s algorithm, discssion lgorithm compleit: n nodes each iteration: need to check all nodes,, not in N n(n+)/ comparisons: O(n ) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amont of carried traffic Soltion: roters send link adertisements at random times +e C e e initiall The algorithm is re-rn de to e.g. cost changes. +e 0 0 +e C 0 recompte roting 0 +e 0 0 C +e recompte +e 0 0 +e C e recompte Netork Laer - Distance Vector lgorithm Bellman-Ford Eqation (dnamic programming) Define d () := cost of least-cost path to Then d () = min {c(,) + d () } here min is taken oer all neighbors of Netork Laer - Bellman-Ford eample Distance Vector lgorithm Clearl, d () =, d () =, d () = B-F eqation sas: d () = min { c(,) + d (), c(,) + d (), c(,) + d () } = min { +, +, + } = Node that achiees minimm is net hop in shortest path forarding table Netork Laer - D () = estimate of least cost to Node knos each neighbor : c(,) Node maintains distance ector D = [D (): є N ] Node also maintains its neighbors distance ectors For each neighbor, maintains D = [D (): є N ] Netork Laer -

5 Distance ector algorithm () Distance Vector lgorithm () Basic idea: From time-to-time, each node sends its on distance ector estimate to neighbors snchronos When a node receies ne DV estimate neighbor, it pdates its on DV sing B-F eqation: D () min {c(,) + D ()} for each node N Under minor, natral conditions, the estimate D () conerge to the actal least cost d () 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 - Netork Laer -6 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table 0 7 node table node table D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = time 7 Netork Laer -7 D () = min{c(,) + D (), c(,) + D ()} D () = min{c(,) + = min{+0, 7+} = D (), c(,) + D ()} node table = min{+, 7+0} = node table node table time Netork Laer -8 Distance Vector: link cost changes Distance Vector: 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 time t 0, detects the link-cost change, pdates its DV, and informs its neighbors. t time t, receies the pdate and pdates its table. It comptes a ne least and sends its neighbors its DV. t time t, receies s pdate and pdates its distance table. s least costs do not change and hence does not send an message to. Netork Laer -9 Link cost changes: good nes traels fast bad nes traels slo - cont to infinit problem! iterations before algorithm stabilies: see tet Poisoned reerse: 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? No Netork Laer -0

6 Comparison of LS and DV algorithms Hierarchical Roting Message compleit LS: ith n nodes, E links, O(nE) msgs sent DV: echange beteen neighbors onl Speed of Conergence LS: O(n ) algorithm reqires O(nE) msgs DV: conergence time aries ma be roting loops cont-to-infinit problem No clear inner. Both sed in the internet. Robstness: hat happens if roter malfnctions? LS: node can adertise incorrect link cost each node comptes onl its on table DV: DV node can adertise incorrect path cost each node s table sed b others error propagate thr Netork Laer - Or roting std ths far - idealiation all roters identical flat not tre in practice scale: ith 00 million destinations: can t store all dest s in roting tables! roting table echange old samp links! administratie atonom internet = of s each admin ma ant to control roting in its on Netork Laer - Hierarchical Roting Interconnected Ses aggregate roters into regions, atonomos sstems (S) roters in same S rn same roting protocol intra-s roting protocol roters in different S can rn different intra- S roting protocol Gatea roter Direct link to roter in another S c a b S a c d b Intra-S Roting algorithm S Forarding table Inter-S Roting algorithm c a b S forarding table configred b both intra- and inter-s roting algorithm intra-s sets entries for internal dests inter-s & intra-s sets entries for eternal dests Netork Laer - Netork Laer - Inter-S tasks sppose roter in S receies datagram destined otside of S: roter shold forard packet to gatea roter, bt hich one? c a b S a c d b S mst:. learn hich dests are reachable throgh S, hich throgh S. propagate this reachabilit info to all roters in S Job of inter-s roting! S c a b S Netork Laer - Eample: Setting forarding table in roter d sppose S learns (ia inter-s protocol) that sbnet reachable ia S (gatea c) bt not ia S. inter-s protocol propagates reachabilit info to all internal roters. roter d determines intra-s roting info its interface, sa I, that is on the least cost path to c. installs forarding table entr (,I) c a b S a c d b S c a b S Netork Laer -6 6

7 Eample: Choosing among mltiple Ses no sppose S learns inter-s protocol that sbnet is reachable S and S. to configre forarding table, roter d mst determine toards hich gatea it shold forard packets for dest. this is also job of inter-s roting protocol! c a b S a c d b S c a b S Netork Laer -7 Eample: Choosing among mltiple Ses no sppose S learns inter-s protocol that sbnet is reachable S and S. to configre forarding table, roter d mst determine toards hich gatea it shold forard packets for dest. this is also job of inter-s roting protocol! hot potato roting: send packet toards closest of to roters. Learn inter-s protocol that sbnet is reachable ia mltiple gateas Use roting info intra-s protocol to determine costs of least-cost paths to each of the gateas Hot potato roting: Choose the gatea that has the smallest least cost More details in the net lectre Determine forarding table the interface I that leads to least-cost gatea. Enter (,I) in forarding table Netork Laer -8 7