11/13/2017 Network Layer (SSL) Network-layer functions. Recall the two network-layer functions:

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1 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet 5.4 inter-as routing: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL) 5- Network-laer functions Recall the two network-laer functions: forwarding: move packets from device inputs to device outputs data routing: determine route taken b each packet from its source to control destination Two approaches to structure network control : per-router control (routers echange messages) logicall centralied control (SDN) Network Laer (SSL) 5-2

2 Per-router control Individual processes in routers interact with each other b message echange and compute forwarding tables Routing Algorithm control data Network Laer (SSL) 5-3 Graph abstraction 5 Graph: G = (N,E) N = set of routers = { u, v, w,,, } 2 u v w 5 2 E = set of links ={ (u,v), (u,), (v,), (v,w), (,w), (,), (w,), (w,), (,) } Remark: Graph abstraction is also useful in other network contets Eample: P2P, where N is set of peers and E is set of TCP connections Network Laer (SSL) 5-4 2

3 Graph abstraction: link costs u 2 5 c(, ) = cost of link (, ) v 3 w cost could be, or inversel 5 proportional to bandwidth, etc Cost of path (, 2, 3,, p ) = c(, 2 ) + c( 2, 3 ) + + c( p-, p ) Routing protocol tries to find least-cost paths cost of path computation is ad hoc if the link cost metric is not additive what if links have asmmetric costs for opposite directions? eample if queueing delas are included, then use a directed graph as model Network Laer (SSL) 5-5 Routing Algorithm classification Global or decentralied information? Global info: all routers have complete topolog, link costs link state protocols Static or dnamic? Static update onl after topolog change Decentralied info: router knows phsicallconnected neighbors, link costs to neighbors distance vector protocols Dnamic periodic update in response to link cost changes ma result in route flaps Network Laer (SSL) 5-6 3

4 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet 5.4 routing among the ISPs: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL) 5-7 A Link-State Routing protocol net topolog, link costs known to ever node accomplished via link state broadcast all nodes have same info Dijkstra s algorithm (ou should have learned it) computes least cost paths from one node ( source ) to all other nodes in a graph iterative: after k iterations, source knows least-cost paths to k destinations ields forwarding table for source node Network Laer (SSL) 5-8 4

5 Link State Broadcast Flooding Source node of link state t sends packets B C to all neighbors Intermediate node A resends to neighbors ecept where packet arrived Man duplicates D E which must be recognied b nodes F Network Laer (SSL) 5-9 Distance Vector Algorithm basis Bellman-Ford Equation (dnamic programming) Define d () := cost of least-cost path from to Then d () = min {c(,v) + d v () v } where min is taken over all neighbors v of Network Laer (SSL) 5-0 5

6 Bellman-Ford eample u 2 Clearl, d v () = 5, d () = 3, d w () = 3 B-F equation sas: 5 d u () = min { c(u,v) + d v (), v 3 w c(u,) + d (), 5 c(u,w) + d w () } 2 = min {2 + 5, 3 + 3, } = 4 The node that achieves minimum is net hop in shortest path put it in forwarding table Network Laer (SSL) 5- Distance Vectors Protocol () Node knows each neighbor v: c(,v) sends its own distance vector (DV) estimate [D X (): є N ] to its neighbors periodicall where D () denotes estimate of least cost from to From each neighbor v, receives [D v (): є N ] Network Laer (SSL) 5-2 6

7 Distance Vector Protocol (2) When a node receives a new DV estimate from a neighbor, it updates its own DV estimate using B-F equation: D () min v {c(,v) + D v ()} for each node N If the v that achieves least is new, node updates its forwarding table and DV Eventuall, assuming that link costs and topolog do not change, the estimate D () converges to the actual least cost d () for all, Network Laer (SSL) 5-3 Distance Vector Protocol - summar Distributed, iterative, asnchronous Initiall, D () = c(,) if and are direct neighbors; otherwise, D () = Each node: waits for a change in local link cost or a msg from a neighbor recomputes estimates if DV estimate for an dest has changed, updates its own state and notifies its neighbors Network Laer (SSL) 5-4 7

8 =min{2+0,7+} =2 node table node table fr rom from from D () = min{c(,) + D (), c(,) + D ()} 2 0 node table 7 0 fr rom D () = min{c(,) + D (), c(,) + D ()} =min{2+,7+0} =3 update outgoing interface for dest o Each row in a table is a distance vector o Assume snchronous operations for this eample and also received their DV updates 2 7 time Network Laer (SSL) 5-5 node table node table fr rom from 2 0 node table 7 0 from fr rom from from fro om from No more change time Network Laer (SSL) 5-6 from 2 7 8

9 Distance Vector: good news travels fast detects a lower link, updates its DV, and sends new DV to node. receives s s updated DV, updates its own DV, and sends new DV to its neighbors. 4 later, receives s updated DV. s least costs do not change. 50 A similar interaction between nodes and. The DV protocol converges quickl for good news Network Laer (SSL) 5-7 Distance Vector: count to infinit problem Link cost increase: Y still has stale information saing that it can go to X via Z in 6 44 messages echanged between and before protocol stabilies Poisoned reverse: 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) will this completel solve count to infinit problem? bad news travels slowl! Network Laer (SSL) 5-8 9

10 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet inter-as routing: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL) 5-9 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 EIGRP (Cisco) distance vector with loopfreedom Network Laer (SSL)

11 RIP ( Routing Information Protocol) distance vector algorithm included in BSD-UNIX Distribution in 982 distance metric: # of hops (ma = 5 hops) From router A to subnets: u v destination hops u A B w v 2 w 2 3 C D 3 2 Network Laer (SSL) 5-2 RIP advertisements distance vectors: echanged with neighbors ever 30 sec via Response Message (also called advertisement) each advertisement: list of up to 25 destination subnets within AS Network Laer (SSL) 5-22

12 RIP Table processing RIP routing tables managed b application-level process called routed (daemon) advertisements sent in UDP packets, periodicall sent routed routed Transprt (UDP) network (IP) link phsical forwarding table forwarding table Transprt (UDP) network (IP) link phsical Network Laer (SSL) 5-23 OSPF (Open Shortest Path First) open : publicl available uses Link State algorithm OSPF advertisement carries one entr per neighbor router advertisements disseminated to entire AS (via flooding) carried in OSPF messages directl over IP (rather than TCP or UDP) securit: all OSPF messages authenticated Note: IS-IS routing protocol: nearl identical to OSPF Network Laer (SSL)

13 Hierarchical OSPF Network Laer (SSL) 5-25 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet 5.4 inter-as routing: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL)

14 Routing among ISPs scale: hundreds of millions destination i subnets: forwarding tables still too large after aggregation of prefies Link State and Distance Vector do not scale administrative autonom internet is a network of networks each network admin wants to control routing in its own network Network Laer (SSL) 5-27 Hierarchical Routing autonomous sstems (ASes) stub vs. transit ASes transit AS has an AS number from ICANN routers in an AS run the same intra-as routing protocol different ASes can run different intra-as routing protocols Gatewa router has direct link to a gatewa router in another AS for inter-as routing Network Laer (SSL)

15 Internet inter-as routing basic ideas Gatewa routers run BGP (Border Gatewa Protocol): the de facto standard an AS advertises its eistence using BGP to rest of Internet: I am here and. gatewa routers echange reachabilit information with neighboring ASes (using eternal BGP on TCP connections) and 2. the propagate p reachabilit information to all internal routers of the AS (using internal BGP on TCP connections); 3. good routes to other ASes selected based on reachabilit information and also polic Network Laer (SSL) 5-29 Forwarding table entries X 3b 3c 3a AS3 a c d b Intra-AS Routing AS Forwarding table Inter-AS Routing 2a 2c 2b AS2 intra-as protocol sets entries for internal destinations inter-as & intra-as protocols set entries for eternal destinations Network Laer (SSL)

16 Eample: Setting forwarding table in router d AS (using ebgp) learns that subnet is reachable via AS3 (gatewa c) but not via AS2 gatewa c (using ibgp) propagates this reachabilit info to all other routers in AS for subnet, router d determines from intra-as routing info that its interface I is on the least cost path to the link (identified b subnet prefi u) between c and 3a installs forwarding table entr (,I) 3c 3a 3b AS3 u a c d b AS 2a 2c 2b AS2 Note: both inter-as and intra-as protocols are used Network Laer (SSL) 5-3 BGP - advertiing paths ebgp session: two BGP routers ( peers ) echange messages advertising paths to various destination network prefies ( path vector protocol) when AS3 advertises a prefi to AS, AS3 promises it will forward datagrams towards that prefi AS3 can aggregate prefies in its advertisement other networks 3c 3a 3b AS3 BGP message c a AS d b 2a 2c 2b AS2 other networks Network Laer (SSL)

17 BGP : distributing path information using ebgp session between 3a and c, AS3 sends prefi reachabilit info to AS. c can then use ibgp to distribute new prefi info to all routers in AS when a router learns of a new prefi, it creates entr for prefi in its forwarding table. in this eample, gatewa router b can then re-advertise such new reachabilit info to AS2 over b-to-2a ebgp session other networks 3b 3a AS3 a AS c d ebgp session ibgp session 2a b 2c 2b AS2 other networks Network Laer (SSL) 5-33 Path attributes & BGP routes advertised prefi includes BGP attributes. prefi + attributes = route two important attributes: AS-PATH: contains ASes through which prefi advertisement has passed: e.g, AS 67, AS 7 NEXT-HOP: the router interface (its subnet IP address) that begins the AS path there ma be multiple links from current AS to net-hop AS when a gatewa router receives route advertisement, it checks for loop uses the AS s import polic to accept or reject route Network Laer (SSL)

18 Eamples of NEXT-HOP for dest X For routers in AS, net-hop is u, path vector is AS3, For routers in AS2, net-hop is v, path vector is AS, AS3, dest X other networks 3b 3a u AS3 a AS c d ebgp session ibgp session 2a b v 2c 2b AS2 other networks Network Laer (SSL) 5-35 BGP route selection router ma learn man routes to the same prefi. Router must select one route Criteria. local preference value attribute (polic decision) Import rule: customer routes are preferred over peer routes, which are preferred over provider routes 2. shortest AS-PATH 6. closest NEXT-HOP router: hot potato routing (additional criteria) Network Laer (SSL)

19 BGP route eport polic eample () W A B C X legend: provider network customer network: Y A,B,C are provider networks X,W,Y are customers (of provider networks) Xi is dual-homed: d attached to two networks X does not want to route from B via itself to C.. so X will not advertise to B that it has a route to C Network Laer (SSL) 5-37 BGP route eport polic eample (2) W A B C A advertises path AW to B B advertises path BAW to X X legend: Should B advertise path BAW to C? Y peer provider networks customer networks: No! B gets no revenue for routing CBAW since neither C nor A nor W is a customer of B B wants to route onl to/from its customers Eport rule: peer/provider routes advertised to customers onl; customer routes advertised to all neighbor ASes Network Laer (SSL)

20 Wh different Intra- and Inter-AS routing? Scale: hierarchical routing reduces table sie, also update traffic Polic (including financial consideration) Intra-AS: single admin, so no polic decisions needed Inter-AS: admin wants control over how its own traffic is routed (b import rule), who routes through its network (b eport rule). ) Performance: Intra-AS: can focus on performance Inter-AS: polic dominates performance Network Laer (SSL) 5-39 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet 5.4 inter-as routing: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL)

21 Software defined networking (SDN) Internet network laer historicall has been implemented via distributed, per-router approach monolithic routers run proprietar implementations of Internet protocol standards (IP, RIP, IS-IS, OSPF, BGP) in proprietar router OS (e.g., Cisco IOS), forwarding packets Other devices in data different middle boes for other network functions: firewalls, load balancers, NAT boes,.. switches for laer-2 forwarding ~2005: rethinking separation of network control from data Network Laer (SSL) 5-4 Software defined networking (SDN) 4. programmable control routing applications access control Remote Controller load balance 3. control functions eternal to data- switches control data CA 2: generalied flowbased forwarding (e.g., OpenFlow) CA CA CA CA. control, data separation Network Laer (SSL)

22 ISP Traffic engineering 5 u 3 v w Q: what if network operator wants to split u-to- traffic along uvw and u (load balancing)? A: can t do it using current protocols based upon shortest-path computation Network Laer (SSL) 5-43 Laers in SDN architecture Network-control apps use network state information provided b SDN controller SDN controller (net OS) maintains network state information, statistics, and flow tables ma be implemented as a distributed sstem for performance and fault-tolerance Data switches fast, simple, commodit switches communicate with controller to provide state information and receive flow tables routing network-control applications access control load balance northbound API SDN Controller (network operating sstem) southbound API control data Network Laer (SSL) 5-44 SDN-controlled switches 22

23 Software defined networking (SDN) Logicall centralied controller controller provides accurate network state information to network-control applications centralied computation of flow tables is easier than distributed computation using protocol messages greater fleibilit and better control of traffic flows Open standards allow unbundling of network functionalit data boes, SDN controllers can be provided d b different vendors Observation: SDN is (mainl) for networks under the same administrative control e.g., Google SDN uses both inter-as and intra-as routing protocols: BGP between datacenters and IS-IS for intra-datacenter Network Laer (SSL) 5-45 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet 5.4 inter-as routing: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL)

24 ICMP: Internet Control Message Protocol above IP in network laer ICMP msgs carried in IP datagrams error reporting: unreachable network, host, port, protocol echo request/repl (used b ping) Tpe Code description 0 0 echo repl (to ping) 3 0 dest. network unreachable 3 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) ICMP message tpe, code plus first 8 btes of IP datagram causing error 9 0 route advertisement t 0 0 router discover 0 TTL epired 2 0 bad IP header Network Laer (SSL) 5-47 Traceroute uses ICMP messages Source sends series of UDP segments to dest First has TTL = Second has TTL=2,, each with unlikel port number When nth datagram arrives to nth router: Router discards datagram and sends to source a TTL epired message with name of router & IP address When TTL epired message arrives, source calculates RTT Traceroute does this 3 times for each TTL value Stopping criterion for source Such a UDP segment arrives at destination host Destination returns msg dest port unreachable packet Upon receipt of this msg, source stops. Network Laer (SSL)

25 Chapter 5: outline 5. introduction 5.2 routing protocols link state distance vector 5.3 intra-as routing in the Internet 5.4 inter-as routing: BGP 5.5 The SDN control 5.6 ICMP: The Internet Control Message Protocol 5.7 Network management and SNMP Network Laer (SSL) 5-49 End of Chapter 5 Network Laer (SSL)