CS Summer Lecture #23: Network and End-to-End layers

Transcription

1 S 4 - Summer 8 - Leture #3: Network nd End-to-End lyers Pst & Present! Lst time: pushing its out of/into hrdwre Link lyer. How to: enode its on wire (in. ntenn), prse its into pkets, ritrte etween s, nme s! Tody: network lyer (portle it pushing) nd end-toend (useful it pushing) lyers! Network lyer: given pket, get it to the other side of lrge (or HUGE) olletion of networks. Issue : Portility. Provide n interfe tht works ross heterogeneous networks. Issue : Slility. Provide nmes nd routing tht works with illions of end hosts. Moving pket from to! Swith: interonnets links to form lrger network swith! Two prts: Forwrding: Tking pkets rriving on n input nd sending them to the right output. T T3 Sts- Routing: olleting the informtion tht tells you possile routes to destintion (nd thus whih output link to send the pket on). T3 T Two onnetion models! onnetionless (or dtgrm ): Eh pket ontins enough informtion tht routers n deide how to get it to its finl destintion! onnetion-oriented (or virtul iruit ) First set up onnetion etween two nodes. Lel it (lled virtul iruit identifier (VI)). ll pkets rry lel. Virtul iruit swithing (wht TM does)! Forming iruit: Send onnetion request from to. ontins VI + ddress of. Rule: VI must e unique on the link its used on. Swith retes n entry mpping input messges with VI to output port. Swith piks new VI unique etween it nd next swith. 5 Virtul iruit forwrding! For eh VI swith hs tle whih mps input link to output link nd gives the new VI to use If s messges ome into swith on link nd go out on link 3 then the tle will e: Swith 5 (Input link,vi) (output link, new VI) (, ) (3, 5) (, ) (3, ) Swith Swith 3 Virtul iruit issues! Good: esy to ssoite resoures with flows n gurntee uffering nd dely, whih mkes qulity of servie gurntees (QoS) esy to provide. lso good: VI smll, mking per-pket overhed smll.! d: not good in the fe of rshes Doesn t hndle host rshes well: eh onnetion hs stte strewn throughout network. to lose onnetion, host must expliitly issue ter down. In generl, to survive filure, wnt to mke stuff s stteless s possile, trivilly eliminting ny storge mngement prolems. Doesn t hndle swith rshes well: hve to terdown nd reinitite new iruit.! Telephone network is onnetion-sed

2 Dtgrms! Simple ide: Don t set up onnetion, just mke sure eh pket ontins enough informtion to get it to destintion. Wht is this? omplete destintion ddress. In onnetionless network, you re lwys onneted. D. heriton! Forwrding: Swith retes forwrding tle, mpping destintions to output port (ignores input ports). When pket with destintion ddress in the tle rrives, it pushes it out on the pproprite output port. When pket with destintion ddress not in the tle rrives, it must find out more routing informtion (next prolem). Dtgrm exmple d S Forwrding tle for swith Destintion swith port d S S3 Dtgrm Trdeoffs! Good: No round-trip dely to setup onnetion. Eh pket forwrded independently of lst: if swith or link fils, n e routed round it. Resoures lloted dynmilly (dptively) rther thn sttilly ound t onnetion time. Lets eh flow hieve pek ndwidth of idle link.! d: usy link = unpreditle, wild servie flututions. Eh pket rries full destintion ddress, whih mkes per pket overhed higher.! Internet uses dtgrms ( Internet Protool or IP) Some prolems! Where do the forwrding tles ome from? ould hnd-enter into entrl tle. ut this doesn t work well if nodes rsh, nd s the numer of nodes goes to infinity (internet).! nd wht out sle???? Rell: size of forwrding tle grew O(hosts) this... suks. d S S S3 uilding routing tles! Routing = grph theory prolem. The grph: Nodes = swithes or hosts Edges = links, hve n ssoited ost whih pproximtes the desirility of sending trffi over the link 3 D 6 The routing prolem: find the lowest-ost pth etween ny two nodes where the ost of pth = sum of ll edges tht mke up the pth. E simple entrlized routing sheme! t retion time: Hve one entrl node K. Hve every swith send vetor ontining (neighor, ost) for eh of its outgoing links to K. From this informtion, K n ompute grph tht gives the topology of the network nd then whip out grph theory lgorithm to find shortest pth. K then sends this mtrix to ll swithes.! Nie nd simple! Prolem: didn t sle to lrge networks Rel networks were just too ig. K got rushed. W: entrliztion is the enemy of slility, so good routing protools re distriuted.! (Now: fster hrdwre? entrlized OK for Stnford?)

3 Link stte routing (sort of used in Internet)! si ide: Every node knows how to reh its diret neighors. If this informtion n e disseminted to every node, then we will hve enough informtion to good routes.! Relies on two mehnisms: Relile flooding of link-stte informtion. lultion of routes from sum of ll umulted knowledge (uses modified form of Dijkstr s lgorithm).! link stte pket: [ ID of reting node, list of (neighor, ost), sequene numer, time to live]. Sequene numer: monotonilly inresing integer used to order link stte pkets. Time-to-live: mke sure pket doesn t irulte forever. node-level view of relile flooding reeive(pkt) If lredy hve opy of LSP from pkt.id if pkt s sequene numer <= opy s disrd pkt else derement pkt.ttl reple opy with pkt forwrd pkt to ll links esides the one tht we reeived it on # done every minutes or so gen_lsp() inrement node s sequene # y one reompute ost vetor send reted LSP to ll neighors Slle routing! Prolem: our routing tles grow with the numer of nodes. This is rel prolem. Wht ws the use? Our ddresses re flt. Every router needs n entry for eh. Solution: hierrhy! (or, strutured grouping)! Hierrhil ddressing: Divide network into zones. Lel these uniquely. (,, ) Hve node ddresses inlude the zone tht the node is in. (mke su-zones nd su-su-zones s needed). Top level routers know how to forwrd pkets to the router in hrge of zone. Zone routers know how to forwrd to every node in their domin (or to the next level down). Exmple: hierrhil ddressing. S. Forwrding tle for swith Destintion swith port S 3 S3 Zone 3.. Zone Exmple: the internet protool (IP)! IP ddresses form -level hierrhy Two prts: network nd host. network tells whih network host is on. Hosts on sme network hve sme prefix network 3-its host Mps well to internetwork (network of multiple networks) IP(V4) ddresses re 3 its. re inluded in every IP pkt.! Three lsses:,, net host 7 4 its net host 4 6 its net host 3 8 its RP: Mpping IP ddresses to link-level (LL)! We n forwrd IP pket to physil network, ut how to get it to host on tht network? e.g., need trnsltion etween IP ddress of host nd its Ethernet ddress so tht the router n enpsulte the pket in n Ethernet pket nd send it to host. How to get these mppings? ddress resolution protool! Router (or swith) keeps tle of (IP->LL) mppings. If it gets pket for n IP ddress not in this RP he it rodsts query ontining the IP ddress. Every host heks if its IP ddress mthes nd, if so, sends response with its link-level ddress k to origintor. This n work in the reverse: RRP/OOTP/DHP (RP he entries re ged. Why?)

4 IP: est-effort, host-to-host protool! IP = portle, onnetionless (dtgrm) protool! Host-to-host: IP gives eh host glolly unique IP ddress! est effort servie model Host gives dtgrm to IP; IP does its est to deliver it. No ttempt is mde to reover from lost, reordered, duplited, or orrupted pkets. Synthesize reliility t higher levels (wht out dely?)! IP provides portility y: ommon pket formt tht gets used on ll networks. Invisily trnslting, splitting nd ressemling pket s it trverses over different physil networks. glol, network-wide ddress spe. Portle dtgrms! Every dtgrm rries enough informtion to forwrd pket info Sr ddr dst ddr dt! IP gol: omine mny physilly distint networks into one logil network. How? Every host nd router in logil network must understnd IP pkets; every router e le to forwrd them. Key: est effort servie model. out the simplest servie you n sk for from the underlying network (IP gol: to run over nything )! Network independene? frgmenttion nd ressemly Frgmenttion nd ressemly! Prolem: physil networks hve different MTUs mximum trnsmission unit : Ethernet: 5, FDDI 45, TM 48(!) hoie : pket smll enough to fit in nything?! hoie : frgmenttion nd ressemly If pket > MTU of network, split (frgment) into piees. router Put ddress into eh piee, long with id + yte offset so it n e put k together (ressemled) y host. How to pik initil pket size? (Hint: usully pkets intended for mhines on sme network). Summry: IP s mehnisms for slility! Hook mny networks together? illions of hosts + lots of weird onstrints.! How to hndle illions of hosts? Hierrhil ddresses. network id host id Routers only need to know how to forwrd pket to other networks, rther thn to ll hosts. lled hierrhil ggregtion: ondenses ll hosts on entire network into single integer (the network #).! How to ommodte weird physil networks? Its onnetionless, est-effort servie model: too stringent servie model won t work in rel world. IP philosophy: mke undemnding enough tht just out ny network n provide the neessry servie. End-to-end lyer - Mking the network useful/usle! Network Lyer Got our its from one end of olletion of networks to the other. r r3 r f» Used glolly unique nmes, routers, nd portle messge strtion. Exmple: the internet protool (IP)» Gret, exept... how n proess use it?! Mking it useful/usle: end-to-end lyer Interfe etween pplitions nd network Some suessful end-to-end protools Turning est effort into usle interfe (or, deriving TP from first priniples) e Suess story #: UDP! Unrelile/User Dtgrm Protool: est-effort, proessto-proess Lives on top of IP: dds orruption detetion nd ports so pkets n e ddressed to proess on host. (note there re mny other fields in heder) UDP IP dt ksum sr port dst port Hdr ksum sr IP ddr dst IP ddr Host: IP ddr = Port 8 Port 8 9

5 Suess story #: TP Lost pkets! Trnsmission ontrol protool Relile, in-order, proess-to-proess, two-wy yte strem. messge! Like UDP: Lyered on top of IP; dds ports nd dt heksums. time TP IP! Unlike UDP in two wys onnetion sed. The importnt differene: reovers from pket loss, duplition, orruption, reordering. Next: nswering how???? from first priniples.! Some uses: Trffi urst uses router uffers to overflow. Link orrupts pket (e.g. wireless: up to 4% pket loss). How to fix? Lost ks Delyed ks = pket duplition M k M time k M! Wht will the result e?! Possile uses: ongestion in the network tht delys pket. too-short (how ig should it e nywy?) Delyed ks tke Spontneous genertion ( insertion ) M M k k M M M k k M! Wht will the symptom e?! Possile uses? Messges nd Ks re numered,, 3, messge M from previous (losed) onnetion wnders round network nd then shows up t end host.

6 Summry of mehnisms Summry: synthesized reliility (M, id) (k, id) (M, s3) (k, s3) (M, s3) (k, id) (M, id) (k, s3) (M, s3) Disrd (M, s3) (M, s9)! Eh messge *logilly* hs two things: messge #, ordering msg with respet to ll messges. unique for ll time session id. ore: +retrnsmission to hndle lost pkets. Use msg # to detet duplite & reordered pkets; ssoites K s with pkets; onnetion setup: three wy hndshke (open, seq # x) (k x, seq # y) Sliding window lgorithm! We ve een sending dt: send, k, send, k,... This is lled stop nd go protool. It s simple, ut delivers relly low ndwidth (eh pket hs lteny of lest round-trip time). Sliding window llows multiple pkets to e in flight. (k y)! TP is two-wy (duplex) pipe There is different sequene numer for eh diretion symmetri open: server does pssive listen, lient does n tive open (sends first messge). k pkts -4! Prolem: hve to mke sure to not overwhelm Hve window = # of ytes is willing to uffer. Wht we n nd nnot do! We n Synthesize reliility on n unrelile network. Eliminte jitter on ursty network (given enough uffering t the )! We nnot Rerete low-lteny responses on network tht n indue ritrry ltenies. Is there nything else?! n impossiility: onsensus on n unrelile network onsensus: you nd I gree on it of informtion. fmous result: The two rmy prolem. The two (stik figure) rmy prolem n et either or, ut not oth. nd wnt to gree on time to simultneously ttk

7 Mking nmes pretty! IP ddresses re not very memorle Internet uses the domin nme system (DNS) to lyer (sort of) humn onsumle nmes on top of them. DNS type nmes: ls.mit.edu, pod.stnford.edu, et. hierrhil. Gives slility nd distriuted nming.! Simplified resolving pod.stnford.edu to IP ddress: host nme IP ddr Lol nme server pod.stnford.edu stnford s IP ddr pod.stnford.edu pod s IP ddr Root nme server stnford nme server Protool: nming + setup + gurntees! ddressing: identifying s IP = host-to-host: every host hs unique IP ddress. UDP & TP = proess-to-proess: extend IP with port tht is oupled to proess within host. Telephone: host-to-host : every onnetion point ssoited with unique phone numer.! onnetion model: IP, UDP, Ethernet: onnetionless. Send pket. Telephone, TM, TP: onnetion-sed. Expliitly set up onnetion efore sending dt. Ter down onnetion when done. (Mkes riher servie models esier.)! Dt delivery model: one dt entrusted to protool, wht gurntees re mde? Some suessful dt delivery models! est-effort: IP: pkets n e lost, delyed & pket dt orrupted. UDP: lyers orruption detetion on top of IP. Ethernet: my lose pkets, detets orruption. TM: n lose pkets, detets orruption & mkes reordering less likely.! Relile, in-order yte strem: TP Two-wy yte strem. Prevents: loss, duplition, orruption, reorder.! Mostly relile, qulity-of-servie: telephone Two-wy voie servie with smll end-to-end delys. Gurntees epted lls will run to ompletion. (esier to uild on TM thn non-onnetion sed) Future: hllenging the ssumptions of IP! Glolly unique ddresses? Geogrphil routing?! IPV4 ddress spe running out (ir 993) IPV6 igger (8-it) ddresses. Huge! Noody uses it. Insted: NT. Everyone on.x.x.x or 9.68.x.x! Moility Wnt to move round nd not lose onnetions. Wnt ess to (seure?) work network from home. Wnt to keep sme ell phone # in different ity.! Possile solutions? VLNs, VPNs, roming... now sunet/re ode isn t geogrphil.! Future: omplex routing/swithing for omplex world 4 IP ddress -> onnetion/virtul iruit identifier. Metro Enet.