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SBU CSE 590 - Network Routing

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Network RoutingAcknowledgement: Slides borrowed from Richard Y. Yang @ YaleRecap: Network Layer Services• Transport packets from source to dest• Network layer protocol in every host, routerMajor components:BAS1ED2JCGM LMajor components:• Control plane– addressing scheme is crucial for usability, mobility– compute routing from sources to destinations• Data plane: forwarding– move packets from input interface to appropriate output interface(s)2AS2D1GIKNRecap: Key Problems√ Location managementRouting with lossyand dynamic wireless linksAEDCBFand dynamic wireless links• Broadcast wireless channels3EDRouting Overview• The problem of routing is to find a good path for each source destination pairCB• A typical measure for a good path is that it is the shortest path according to some metric4AEDFLink Metric• One possibility is to assign each link a metric of 1 (hop-count based routing)AEDCBF– maximizes the distance traveled by each hop• low signal strength -> high loss ratio• uses a higher TxPower -> interference– different links have different qualities5Performance of Shortest Hop Count6AEDCBFExample Metric: ETX• ETX: The predicted number of data transmissions required to successfully transmit a packet over a link–the ETX of a path is the sum of the ETX values of the links over that paththe links over that path• Examples:– ETX of a 3-hop route with perfect links is 3– ETX of a 1-hop route with 50% loss is 27“A High-Throughput Path Metric for Multi-Hop Wireless Routing” by D. De Couto, D. Aguayo, J. Bicket, R. Morris. Mibicom 2003.http://meraki.com/about/Acquiring ETX• Measured by broadcasting dedicated link probe packets with an average period τ (jittered by ±0.1τ)• Delivery ratio:– count(t-w,t) is the # of probes received during window w– w/τ is the # of probes that should have been received8ETX: Example9ETX: Advantage• Tends to minimize spectrum use, which can maximize overall system capacity (reduce power too)–each node spends less time retransmitting data• ETX has problems and is not the only link metric. • We will revisit link metrics next class.10Outline• Routing– overview– computing shortest path routing11AEDCBF2213112535Link-State Routing Algorithms• Separation of topology distribution from route computation• Used in OSPF, the dominant intradomain routing protocol used in the Internet• Net topology, link costs are distributed to all nodes• Link state distribution accomplished via “link state broadcast” • Each node (locally) computes its paths to all destinations12BASEFJCGM LLink State Broadcast13represents linkrepresents a node that has received updateAH DGIKNBASEFJCGM LLink State Broadcast14AH DGIKNLink State BroadcastBASEFJCGM L15AH DGIKNTo avoid forwarding the same update multiple times, each update has a sequence number. If an arrived update does not have a higher seq, discard!- The packet received by E from C is discarded- The packet received by C from E is discarded as well - Node H receives packet from two neighbors, and will discard one of themSummary of Link State Routing• Separation of topology distribution from route computation•Whenever a link metric changes, the node •Whenever a link metric changes, the node broadcasts new value• Q: Does link state routing work well in a network with dynamic link status?16Distance Vector Routing Algorithm• Based on the Bellman-Ford algorithm– at node X, the distance (or any additive link quality metric) to Y is updated by))(),((min)()(YdZXdYdZXNZX+=∈where dX(Y) is the current distance at node X from X to Y, N(X) is the set of the neighbors of X, and d(X, Z) is the distance of the direct link from X to Z• Implemented in the RIP routing protocol and some wireless mesh networks17Distance Table: ExampleAEDCB781212d ()AA0B7D∞Edistance tables from neighborsA1B15D∞computationE’sdistance, forwarding table1, Adistance table E sends to its neighborsA: 1Below is just one step! The algorithm repeats for ever!18ABCD07∞∞1d(E,A)701∞8d(E,B)∞∞202d(E,D)18∞∞1589∞∞∞421, A8, B4, D2, DA: 1B: 8C: 4D: 2E: 0Distance Vector in the Presence of Topology Dynamics• Good news propagate fastLink AB is up19Link AB is upDistance Vector in the Presence of Topology Dynamics• Bad news propagate slowly: the count-to-infinity problem20Link AB is downor cost increasessubstantiallyThe Reverse-Poison (Split-horizon) HackAEDCB781212d ()AA0B7D∞Edistance tables from neighborsA1B15D∞computationE’sdistance, forwarding table1, Adistance table E sends to its neighborsTo ATo BTo DIf the path to a dest is through neighbor h, report ∞ to neighbor h for dest.21ABCD07∞∞1d(E,A)701∞8d(E,B)∞∞202d(E,D)18∞∞1589∞∞∞421, A8, B4, D2, Ddistancethrough neighborA: ∞B: 8C: 4D: 2E: 0A: 1B: ∞C: 4D: 2E: 0A: 1B: 8C: ∞D: ∞E: 0An Example Where Split-Horizon Fails111122 When the link between C and D fails, C will set its distance to D as ∞ However, unfortunate timing can cause problem- A receives a new update (∞) from C, then a previous update from B (when B thought C was good) arrives; then A will use B to go to D- A sends the good news to C- C sends the good news to BDestination-sequenced distance vector protocol (DSDV)• There are optimizations but we present the base protocol– Only handle the case when link is broken• Let’s assume the destination node is D•Basic idea:•Basic idea:– DSDV tags each route with a sequence number– Each destination node D periodically advertisesmonotonically increasing even sequence numbers– When a node realizes that the link that it uses to reach destination D is broken, the node increases the sequence number for D to be one greater than the previous one (odd number).23DSDV: Details• Periodical and triggered updates– periodically D increases its seq# SDby 2 and broadcasts with (SD, 0)– if A is using B as next hop to D and A discovers that the link AB is brokenthat the link AB is broken• A increases its sequence number SA by 1 (odd)• sets dAto ∞, and• sends (SA, dA) to all neighbors24DSDV: Details• Update after receiving a message– Assume B sends to A the information (SB, dB), where SBis the sequence number at B for destination D and dBis the distance from B to D; when A receives (SB, dB)when A receives (S, d)– if SB > SA, then // higher seq#, always update» SA= SB» if (dB== ∞) dA = ∞; else dA= dB+ d(A,B)– else if SA == SB, then // conditional


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SBU CSE 590 - Network Routing

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