Network layer. Two Key Network-Layer Functions. Datagram Forwarding table. IP datagram format. IP Addressing: introduction

Transcription

1 Netork laer transport segment sending to receiing host on sending side encapslates segments into grams on rcing side, deliers segments to transport laer laer protocols in eer host, roter roter eamines header fields in all IP grams passing throgh it application transport phsical phsical phsical phsical phsical phsical phsical phsical phsical phsical phsical phsical application transport phsical To Ke Netork-Laer Fnctions 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 Netork Laer - Netork Laer - Datagram Forarding table roting algorithm local forarding table dest address otpt link address-range address-range address-range address-range IP destination address in arriing packet s header billion IP addresses, so rather than list indiidal destination address list range of addresses (aggregate table entries) Datagram Forarding table Destination ddress Range throgh throgh throgh otherise Link Interface 0 Netork Laer - Netork Laer - IP gram format IP protocol ersion nmber header length (btes) tpe of ma nmber remaining hops (decremented at each roter) pper laer protocol to delier paload to 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) (ariable length, tpicall a TCP or UDP segment) total gram length (btes) for fragmentation/ reassembl IP ddressing: introdction IP address: -bit identifier for host, roter interface interface: connection beteen host/roter and phsical link roter s tpicall hae mltiple interfaces host tpicall has one interface IP addresses associated ith each interface = Netork Laer - Netork Laer -6

2 Sbnets Sbnets...0/...0/ IP address: sbnet part (high order bits) host part (lo order bits) What s a sbnet? deice interfaces ith same sbnet part of IP address can phsicall reach each other ithot interening roter sbnet... consisting of sbnets Recipe to determine the sbnets, detach each interface its host or roter, creating islands of isolated s each isolated is called a sbnet....0/ Sbnet mask: / Netork Laer - Netork Laer -8 IP addressing: CIDR CIDR: Classless InterDomain Roting sbnet portion of address of arbitrar length address format: a.b.c.d/, here is # bits in sbnet portion of address sbnet host part part / IP addresses: ho to get one? Q: Ho does a host get IP address? hard-coded b sstem admin in a file Windos: control-panel->->configration- >tcp/ip->properties UNIX: /etc/rc.config DHCP: Dnamic Host Configration Protocol: dnamicall get address as serer plg-and-pla Netork Laer -9 Netork Laer -0 IP addressing: the last ord... Q: Ho does an ISP get block of addresses? : ICNN: Internet Corporation for ssigned Names and Nmbers allocates addresses manages DNS assigns domain names, resoles disptes IP6 Initial motiation: -bit address space soon to be completel allocated. dditional motiation: header format helps speed processing/forarding header changes to facilitate QoS IP6 gram format: fied-length 0 bte header no fragmentation alloed Netork Laer - Netork Laer -

3 IP6 Header (Cont) Priorit: identif priorit among grams in flo Flo Label: identif grams in same flo. (concept of flo not ell defined). Net header: identif pper laer protocol for er pri flo label paload len net hdr hop limit sorce address (8 bits) destination address (8 bits) Other Changes IP Checksm: remoed entirel to redce processing time at each hop Options: alloed, bt otside of header, indicated b Net Header field ICMP6: ne ersion of ICMP additional message tpes, e.g. Packet Too Big mlticast grop management fnctions bits Netork Laer - Netork Laer - Transition From IP To IP6 Not all roters can be pgraded simltaneos no flag das Ho ill the operate ith mied IP and IP6 roters? Tnneling: IP6 carried as paload in IP gram among IP roters Tnneling Logical ie: Phsical ie: B E F tnnel IP6 IP6 IP6 IP6 B E F IP6 IP6 IP IP IP6 IP6 Netork Laer - Netork Laer -6 Tnneling Logical ie: Phsical ie: B E F tnnel IP6 IP6 IP6 IP6 B C D E F IP6 IP6 IP IP IP6 IP6 Flo: X Src: Src:B Dest: E Flo: X Src: Src:B Dest: E Flo: X Src: Flo: X Src: Rotingalgoritmer Hr skapas innehållet i rotingtabellerna?? -to-b: IP6 B-to-C: IP6 inside IP B-to-C: IP6 inside IP E-to-F: IP6 Netork Laer - Netork Laer -8

4 Graph abstraction Graph abstraction: costs Graph: G = (N,E) 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 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 -9 Netork Laer -0 Roting lgorithm classification Link-State Roting lgorithm 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 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 - Netork Laer - Dijsktra s lgorithm 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' Netork Laer - 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 -

5 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 table in : destination link (,) (,) (,) (,) (,) Netork Laer - Netork Laer -6 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 +e D B C e e initiall +e 0 D +e B 0 C 0 recompte roting 0 +e D 0 0 B C +e recompte +e 0 D +e B 0 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 -8 Bellman-Ford eample 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 Distance Vector lgorithm 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 ] Netork Laer -9 Netork Laer -0

6 Distance ector algorithm () Distance Vector lgorithm () Basic 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 () 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 - D () = min{c(,) + D (), c(,) + D ()} = min{+0, +} = node table 0 node table node table D () = min{c(,) + D (), c(,) + D ()} = min{+, +0} = time Netork Laer - D () = min{c(,) + D (), c(,) + D ()} D () = min{c(,) + = min{+0, +} = D (), c(,) + D ()} node table = min{+, +0} = node table node table time Netork Laer - 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 0 : detects link-cost change, pdates its DV, informs its neighbors. t : receies pdate, pdates its table, comptes ne least, sends its neighbors its DV. t : receies s pdate, pdates its distance table. s least costs do not change, so does not send a message to. 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? 60 0 Netork Laer - Netork Laer -6 6

7 Comparison of LS and DV algorithms 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 table DV: DV node can adertise incorrect path cost each node s table sed b others error propagate thr Netork Laer -