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19 Cards in this Set
- Front
- Back
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Connection-Oriented Services |
The network layer establishes a connection between source and destination Packets are sent along this connection The decision about the route is made once at connection establishment Routers/Switches are stateful |
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Connectionless Services |
The network layer treats each packet independently Route lookup for each packet (routing table) IP is connectionless IP routers are stateless |
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Network vs transport layer connection service |
– Network: between hosts through routers – Transport: between two processes (routers don’t care) |
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How do you hold information about route from A to all otherhosts? |
Table of host/network address and next-hop in every node. Dvs routing table |
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Routing tables |
Aggregate addresses Longest prefix match |
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The 3 layers of routing |
Topmost - AS Intermediate - Areas Low - Basic routing |
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Topmost: Autonomoussystems (AS) |
– An independent administrative domain – Routing between AS:s is called inter-domain routing / Externalrouting – Based on commercial agreements – Policies, Service-levelagreements |
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Intermediate: Areas |
– Routing inside an AS: Intra-domain routing / Internal routing – Best path based on hop/bw metrics |
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Low: Basic routing |
– Next-hop routing – Static routes |
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Dynamic routing protocols |
– Distance-Vector (Bellman-Ford) – Link-state (Dijkstra) |
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Popular routing protocols |
Exterior - BGP Interior - RIP, OSPF |
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RIP - Routing Information Protocol |
Metric is Hop Counts and uses UDP – RIP messages contain a vector of hop counts. – Every node sends its routes to its neighbors – Route information gradually spreads through the network – Every node selects the route with smallest metric. |
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Distance vector protocols |
• A node advertises its “distance vector” – A list (vector) of all nodes thatthe node knows about – The distance to each of them • Advertisements are sent toneighbours only • Each neighbour updates itsrouting table and sends the newdistance-vectors to itsneighbours• Bellman-Ford algorithm |
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Open Shortest Path First—OSPF |
• OSPF is a link-state protocol. – Builds Link State Advertisements (LSAs) – Distributes LSAs to all other routers – Computes delivery tree using the Dijkstra algorithm • OSPF uses IP directly (protocol field = 89) – Not UDP or TCP. • OSPF networks are partitioned into areas to minimizecross-area communication. |
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Link-State Protocols (SPF) |
• In SPF, every router does the following: 1. Actively test the status of all neighbours/links 2. Build a Link State Advertisement (LSA) from this information andpropagate it to all other routers within an area. 3. Using LSAs from all other routers, compute a shortest path delivery tree,typically using Dijkstra shortest path algorithm. • Advantages (over distance-vector): – More functionality due to computation on original data and no dependenceon intermediate routers – Fast Convergence • Disadvantage– uses more memory |
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Border Gateway Protocol—BGP |
• Inter-domain routing • Simple cases: use static routing • Main purpose: Network reach-ability between autonomous systems • BGP version 4 is the exterior routing protocol used in the Internet today. • BGP uses TCP – TCP is reliable: reduces the protocol complexity • BGP uses path-vector - enhancent of distance-vector. • BGP implements policies – chosen by the local administrator. |
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Path vector |
• Path-vector extends distance-vector– Instead of a simple cost, assign an AS-Path to every route – There may be many paths to the same destination (network prefix) – AS-Path used to implement policies and loop prevention |
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BGP Architecture |
• BGP interacts with the internal routing – Redistributes routes between the two domains • BGP really consists of two protocols: – E-BGP: coordinates between border routers between AS:s – I-BGP : coordinates between BGP peers within an AS |
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BGP Router Operation |
• A BGP router receives routes – BGP peers (E-BGP) – Redistribution: I-BGP/static routes • It aggregates routes • It filters and modifies routes – According to some policy • It advertizes routes to its EBGP neighbours in other AS:s |
