Network Layer: Control Plane 5-2

Transcription

1 Network Laer: Control Plane EECS Chapter 5: network laer control plane chapter goals: understand principles behind network control plane traditional routing algorithms SDN controlllers Internet Control Message Protocol network management and their instantiation, implementation in the Internet: OSPF, BGP, OpenFlow, ODL and ONOS controllers, ICMP, SNMP Network Laer: Control Plane 5-

2 Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane 5-3 Network-laer functions Recall: two network-laer functions: forwarding: move packets from router s input to appropriate router s output routing: determine route taken b packets from source to destination data plane control plane Two approaches to structuring network control plane: per-router control (traditional) logicall centralied control (software defined networking) Network Laer: Control Plane 5-4

3 Per-router control plane Individual routing algorithm components in each and ever router interact with each other in control plane to compute forwarding tables Routing Algorithm Local forwarding table header output control plane data plane Network Laer: Control Plane 5-5 Logicall centralied control plane A distinct (tpicall remote) controller interacts with local control agents (CAs) in routers to compute forwarding tables Remote Controller CA CA CA CA CA control plane data plane Network Laer: Control Plane 5-6 3

4 Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane 5-7 Routing protocols Routing protocol goal: determine good paths (equivalentl, routes), from sending hosts to receiving host, through network of routers path: sequence of routers packets will traverse in going from given initial source host to given final destination host good : least cost, fastest, least congested routing: a top-0 networking challenge! Network Laer: Control Plane 5-8 4

5 Graph abstraction of the network 5 graph: G = ( V, E ) u v 3 3 w 5 V = set of routers = { u, v, w,,, } E = set of links ={ (u,v), (u,), (v,), (v,w), (,w), (,), (w,), (w,), (,) } aside: graph abstraction is useful in other network contets, e.g., PP, where V is set of peers and E is set of TCP connections Network Laer: Control Plane 5-9 Graph abstraction: costs u 5 v 3 3 w 5 c(, ) = cost of link (, ) e.g., c(w,) = 5 cost could alwas be one (hop), 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 Network Laer: Control Plane 5-0 5

6 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 updates in response to link cost changes more responsive but more route oscillation; routing loops Network Laer: Control Plane 5- Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane 5-6

7 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 from one node ( source ) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost paths to k destinations Notation: c(,): link cost from node to ; = if not direct neighbors D(v): current value of cost of path from source to destination v p(v): predecessor node along path from source to destination v N': set of nodes whose least cost paths definitivel known Network Laer: Control Plane 5-3 Dijsktra 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 cost to v is either old cost to v or known 4 shortest path cost to w plus cost from w to v */ 5 until all nodes in N' Network Laer: Control Plane 5-4 7

8 Dijkstra s algorithm: eample D(v) D(w) D() D() D() Step N' p(v) p(w) p() p() p() 0 u 7,u 3,u 5,u uw 6,w 5,u,w uw 6,w,w 4, 3 uwv 0,v 4, 4 5 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 Network Laer: Control Plane 5-5 Dijkstra s algorithm: another eample 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, 5 u v 3 3 w 5 * Check out the online interactive eercises for more eamples: Network Laer: Control Plane 5-6 8

9 Dijkstra s algorithm: eample () resulting shortest-path tree from u: u v w resulting forwarding table in u: destination v w link (u,v) (u,) (u,) (u,) (u,) Network Laer: Control Plane 5-7 Dijkstra s algorithm, discussion algorithm compleit: n nodes each iteration: need to check all nodes, w, not in V n(n+)/ comparisons: O(n ) more efficient implementation using a heap: O(nlogn) oscillations possible: e.g., support link cost equals amount of carried traffic D A +e e B C e initiall D A +e 0 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 D A +e 0 0 +e C B given these costs, find new routing. resulting in new costs Network Laer: Control Plane

10 Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane 5-9 Distance vector algorithm Bellman-Ford equation (dnamic programming) Let d () := cost of least-cost path from to then d () = min {c(,v) + d v () } v cost from neighbor v to destination cost to neighbor v min taken over all neighbors v of Network Laer: Control Plane 5-0 0

11 Bellman-Ford eample 5 Compute cost from u to : v 3 w Clearl, d v () = 5, d () = 3, d w () = 3 5 u B-F equation sas: 3 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 Network Laer: Control Plane 5- Distance vector algorithm: variables D () = estimate of least cost from to maintains distance vector D = [D (): є N ] node : knows cost to each neighbor v: c(,v) maintains its neighbors distance vectors. For each neighbor v, maintains D v = [D v (): є N ] Network Laer: Control Plane 5-

12 Distance vector algorithm: ke idea ke idea: from time-to-time, each node sends its own distance vector estimate to neighbors when receives new DV estimate from 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 () converge to the actual least cost d () Network Laer: Control Plane 5-3 Distance vector algorithm: properties iterative, asnchronous: each local iteration caused b: local link cost change DV update message from 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 from neighbor) recompute estimates if DV to an dest has changed, notif neighbors Network Laer: Control Plane 5-4

13 node table from cost to 0 7 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = from cost to D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 3 node table from cost to 0 7 node table from cost to 7 0 time Network Laer: Control Plane 5-5 node table from node table from cost to 0 7 cost to D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = 0 from from cost to cost to from from cost to cost to D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 3 7 node table from cost to 7 0 from cost to from cost to time Network Laer: Control Plane 5-6 3

14 Distance vector: link cost changes link cost changes: v node detects local link cost change v updates routing info, recalculates distance vector v 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 from, updates its table, computes new least cost to, 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. * Check out the online interactive eercises for more eamples: Network Laer: Control Plane 5-7 Distance vector: link cost changes () link cost changes: v node detects local link cost change v bad news travels slow - count to infinit problem! v 44 iterations before algorithm stabilies: see tet 4 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? Network Laer: Control Plane 5-8 4

15 Comparison of LS and DV algorithms message compleit LS: with n nodes, E links, O(nE) messages sent DV: echange between neighbors onl convergence time varies speed of convergence LS: O(n ) algorithm requires O(nE) messages ma have oscillations DV: convergence time varies ma cause 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 errors propagate through network Network Laer: Control Plane 5-9 Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane

16 Making routing scalable our routing stud thus far - idealied all routers identical network flat not true in practice scale: with billions of destinations: can t store all destinations in routing tables! routing table echange would swamp links! administrative autonom internet = network of networks each network administrator ma want to control routing in their own network Network Laer: Control Plane 5-3 Internet approach to scalable routing Aggregate routers into regions known as autonomous sstems (AS) (a.k.a. domains ) intra-as routing routing among hosts, routers in same AS ( network ) all routers in AS must run same intra-domain protocol routers in different AS can run different intra-domain routing protocols gatewa router: at edge of its own AS, has link(s) to router(s) in other AS es inter-as routing routing among AS es gatewas perform interdomain routing (as well as intra-domain routing) Network Laer: Control Plane 5-3 6

17 Interconnected ASes 3c 3a 3b AS3 a c d b Intra-AS Routing algorithm AS Forwarding table Inter-AS Routing algorithm a c b AS forwarding table configured b both intraand inter-as routing algorithms intra-as routing determine entries for destinations within AS inter-as and intra-as determine entries for eternal destinations Network Laer: Control Plane 5-33 Inter-AS tasks suppose router in AS receives datagram destined outside of AS: router should forward packet to gatewa router, but which one? AS must:. learn which destinations are reachable through AS, which through AS3. propagate this reachabilit info to all routers in AS job of inter-as routing! other networks 3c 3a 3b AS3 a AS c d b a AS c b other networks Network Laer: Control Plane

18 Intra-AS Routing also known as interior gatewa protocols (IGP) most common intra-as routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First (IS-IS protocol essentiall same as OSPF) IS: Intermediate Sstem IGRP: Interior Gatewa Routing Protocol (Cisco proprietar for decades, until 06) Network Laer: Control Plane 5-35 OSPF (Open Shortest Path First) open : publicl available uses link-state algorithm link state packet dissemination topolog map at each node route computation using Dijkstra s algorithm router floods OSPF link-state advertisements to all other routers in entire AS link state: for each attached link carried in OSPF messages directl over IP (rather than TCP or UDP) IS-IS routing protocol: nearl identical to OSPF Network Laer: Control Plane

19 OSPF: More Details Carries OSPF messages directl over IP protocol 89 supports reliable message transfer supports link state broadcast Checks if links are operational sending HELLO messages Obtains link-state database from neighbor routers OSPF messages are sent if link states change periodicall (30 minutes) Network Laer 4-37 OSPF advanced features securit: all OSPF messages authenticated (to prevent malicious intrusion) multiple same-cost paths allowed (onl one path allowed in RIP) integrated unicast and multicast support: Multicast OSPF (MOSPF, RFC 584) uses same topolog database as OSPF hierarchical OSPF in large domains Network Laer: Control Plane

20 Hierarchical OSPF boundar router backbone router area border routers backbone area 3 area area internal routers Network Laer: Control Plane 5-39 Hierarchical OSPF two-level hierarch: local area, backbone. link-state advertisements onl in area each nodes has detailed area topolog, but onl knows direction (shortest path) to networks in other areas. area border routers: summarie distances to networks in its own area; advertise this info to other area border routers. backbone routers: run OSPF routing within the backbone. boundar routers: connect to other AS es. Network Laer: Control Plane

21 Hierarchical OSPF Routing Packet routing within a domain: local router > area border router (intra-area) > backbone router(s) > area border router of destination network > destination local router Inter-AS routing: local router > area border router (intra-area) > backbone router(s) > boundar router > another AS Network Laer 4-4 Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane 5-4

22 Internet inter-as routing: BGP BGP (Border Gatewa Protocol)(RFC 47): the de facto inter-domain routing protocol glue that holds the Internet together BGP provides each AS a means to: ebgp: obtain subnet reachabilit information from neighboring AS es (gatewa routers) ibgp: propagate reachabilit information to all ASinternal routers. determine good routes to other networks based on reachabilit information and polic allows subnet to advertise its eistence to the rest of Internet: I am here Network Laer: Control Plane 5-43 BGP Forwarding Tables Packets routed using CIDRied prefies, e.g., / Forwarding table entries have the form (prefi, interface) Network Laer 4-44

23 ebgp, ibgp connections b b a c 3b a c d 3a 3c d AS 3d AS ebgp connection ibgp connection AS 3 c gatewa routers run both ebgp and ibgp protocols Other routers: internal routers Network Laer: Control Plane 5-45 BGP basics BGP connection: two BGP routers ( peers ) echange BGP messages over a semi-permanent TCP connection (port 79): advertising paths to different destination network prefies (BGP is a path vector protocol) when AS3 gatewa router 3a advertises path AS3,X to AS gatewa router c: AS3 promises to AS it will forward datagrams towards X AS b AS 3 3b a d c AS a b c 3a 3d 3c BGP advertisement: AS3, X X d Network Laer: Control Plane

24 BGP path advertisement AS b AS3 3b a c d AS,AS3,X AS a b c 3a AS3,X 3d 3c X d AS router c receives path advertisement AS3, X (via ebgp) from AS3 router 3a Based on AS polic, AS router c accepts path AS3,X, propagates (via ibgp) to all AS routers Based on AS polic, AS router a advertises (via ebgp) path AS, AS3, X to AS router c Network Laer: Control Plane 5-47 BGP path advertisement () AS b AS3,X AS3 3b a c d AS,AS3,X AS a b c 3a AS3,X 3d 3c X d gatewa router ma learn about multiple paths to destination: AS gatewa router c learns path AS,AS3,X from a AS gatewa router c learns path AS3,X from 3a Based on polic, AS gatewa router c chooses path AS3,X, and advertises path within AS via ibgp Network Laer: Control Plane

25 Path attributes and BGP routes advertised prefi includes BGP attributes prefi + attributes = route two important attributes: AS-PATH: list of ASes through which prefi advertisement has passed NEXT-HOP: IP addresses of the router interface that begins the AS-PATH Network Laer: Control Plane 5-49 BGP, OSPF, forwarding table entries Q: how does router set forwarding table entr to distant prefi? AS a local link interfaces at a, d b AS3,X c d AS,AS3,X AS a b d AS3,X c AS3 3a AS3,X 3b 3d 3c phsical link X dest interface X recall: a, b, c learn about dest X via ibgp from c: path to X goes through c d: OSPF intra-domain routing: to get to c, forward over outgoing local interface Network Laer: Control Plane

26 BGP, OSPF, forwarding table entries () Q: how does router set forwarding table entr to distant prefi? AS a b d c AS a b c AS3 3a 3b 3d 3c X d dest interface X recall: a, b, c learn about dest X via ibgp from c: path to X goes through c d: OSPF intra-domain routing: to get to c, forward over outgoing local interface a: OSPF intra-domain routing: to get to c, forward over outgoing local interface Network Laer: Control Plane 5-5 BGP route selection router ma learn about more than one route to destination AS, selects route based on the following elimination rules:. local preference value attribute: polic decision. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing 4. additional criteria Polic-based routing: gatewa receiving route advertisement uses import polic to accept/decline path (e.g., never route through AS Y). AS polic also determines whether to advertise path to other other neighboring ASes Network Laer: Control Plane 5-5 6

27 BGP: achieving polic via advertisements W A B C X legend: provider network customer network: Y Suppose an ISP onl wants to route traffic to/from its customer networks (does not want to carr transit traffic between other ISPs) A advertises path Aw to B and to C B chooses not to advertise BAw to C: B gets no revenue for routing CBAw, since none of C, A, w are B s customers C does not learn about CBAw path C will route CAw (not using B) to get to w Network Laer: Control Plane 5-53 BGP: achieving polic via advertisements () W A B C X legend: provider network customer network: Y Suppose an ISP onl wants to route traffic to/from its customer networks (does not want to carr transit traffic between other ISPs) A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks polic to enforce: X does not want to route from B to C via X.. so X will not advertise to B a route to C Network Laer: Control Plane

28 Shortest AS-PATH AS b AS3,X AS3 3b a c d AS,AS3,X AS a b c 3a AS3,X 3d 3c X d gatewa router ma learn about multiple paths to destination: AS gatewa router c learns path AS,AS3,X from a AS gatewa router c learns path AS3,X from 3a Shortest AS-PATH is AS3,X Network Laer: Control Plane 5-55 Hot Potato Routing AS b AS3 3b a d c AS,AS3,X AS b 5 a c 0 63 d 3a AS3,X 3c 3d X OSPF link weights d learns (via ibgp) it can route to X via a or c hot potato routing: choose local gatewa that has least intradomain cost (e.g., d chooses a, even though more AS hops to X); does not consider inter-domain cost! selfish; ma lead to longer end-to-end dela > appl shortest AS-PATH before hot potato. Network Laer: Control Plane

29 BGP messages BGP messages echanged between peers over TCP connections BGP messages: OPEN: opens TCP connection to remote BGP peer and authenticates sending BGP peer UPDATE: advertises new path (or withdraws old path) KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous message; also used to close connection Network Laer: Control Plane 5-57 Wh different Intra-, Inter-AS routing? polic: inter-as: admin wants control over how its traffic routed, who routes through its net. intra-as: single admin, so no polic decisions needed scale: hierarchical routing saves table sie, reduced update traffic performance: intra-as: can focus on performance inter-as: polic ma dominate over performance Network Laer: Control Plane

30 Chapter 5: outline 5. introduction 5. routing protocols link state distance vector 5.3 intra-as routing in the Internet: OSPF 5.4 routing among the ISPs: BGP 5.5 The SDN control plane 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer: Control Plane