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Dynamic Routing Protocols II OSPF Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol. 1 Distance Vector vs. Link State Routing • With distance vector routing, each node has information only about the next hop: • • • • Node A: to reach F go to B Node B: to reach F go to D Node D: to reach F go to E Node E: go directly to F • Distance vector routing makes poor routing decisions if directions are not completely correct (e.g., because a node is down). A B C D E F • If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged. 2 Distance Vector vs. Link State Routing • In link state routing, each node has a complete map of the topology A • If a node fails, each node can calculate the new route B C D E A F A • Difficulty: All nodes need to have a consistent view of the network B C D E A F B C D E C D E B C D E A A B B F C A D F F E B C D E F F 3 Link State Routing: Properties • Each node requires complete topology information • Link state information must be flooded to all nodes • Guaranteed to converge 4 Link State Routing: Basic princples 1. Each router establishes a relationship (“adjacency”) with its neighbors 2.Each router generates link state advertisements (LSAs) which are distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link) 3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network as a graph with weighted edges 4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network 5 Operation of a Link State Routing protocol Received LSAs Link State Database Dijkstra’s Algorithm IP Routing Table LSAs are flooded to other interfaces 6 Dijkstra’s Shortest Path Algorithm for a Graph Input: Graph (N,E) with N the set of nodes and E the set of edges dvw link cost (dvw = infinity if (v,w) E, dvv = 0) s source node. Output: Dn cost of the least-cost path from node s to node n M = {s}; for each n M Dn = dsn; while (M all nodes) do Find w M for which Dw = min{Dj ; j M}; Add w to M; for each n M Dn = minw [ Dn, Dw + dwn ]; Update route; enddo 7 OSPF • OSPF = Open Shortest Path First • The OSPF routing protocol is the most important link state routing protocol on the Internet • The complexity of OSPF is significant • History: – – – – – 1989: RFC 1131 1991: RFC 1247 1994: RFC 1583 1997: RFC 2178 1998: RFC 2328 OSPF Version 1 OSPF Version 2 OSPF Version 2 (revised) OSPF Version 2 (revised) OSPF Version 2 (current version) 8 Features of OSPF • Provides authentication of routing messages • Enables load balancing by allowing traffic to be split evenly across routes with equal cost • Type-of-Service routing allows to setup different routes dependent on the TOS field • Supports subnetting • Supports multicasting • Allows hierarchical routing 9 Example Network 10.10.10.2 .2 10.1.1.0 / 24 .4 2 .0 /2 4 .3 •Link costs are called Metric /2 .5 5 .5 .5 10.1.5.0/24 10.10.10.2 .0 .1 . .3 1 .8 3 .6 .1 2 .6 .4 10.1.3.0 / 24 10 3 1 10.1.7.0 / 24 .2 .3 • Metric is in the range [0 , .4 10.1.4.0 / 24 .1 Router IDs are selected independent of interface addresses 2 4 .2 10.10.10.6 10 4 .1 10.10.10.4 10.1.6.0 / 24 10.10.10.1 10.10.10.5 216] • Metric can be asymmetric 10 Link State Advertisement (LSA) 10.10.10.1 4 .1 .2 .2 10.1.1.0 / 24 10.1.4.0 / 24 3 2 .1 . 2 .0 /2 10.1.3.0 / 24 .2 10 4 • The LSA of router 10.10.10.1 is as follows: .1 • Link State ID: 10.10.10.1 = Router ID • Advertising Router: 10.10.10.1 = Router ID • Number of links: 3 = 2 links plus router itself 10.10.10.2 .3 .3 .3 • Description of Link 1: Link ID = 10.1.1.1, Metric = 4 • Description of Link 2: Link ID = 10.1.2.1, Metric = 3 • Description of Link 3: Link ID = 10.10.10.1, Metric = 0 10.1.5.0/24 10.10.10.3 Each router sends its LSA to all routers in the network (using a method called reliable flooding) 11 Network and Link State Database 10.10.10.2 .2 .2 2 .0 /2 4 .3 4 .5 /2 .3 .6 .0 .1 . .4 .8 10 .2 .3 .6 10.1.7.0 / 24 10.1.6.0 / 24 10.1.4.0 / 24 .1 Each router has a database which contains the LSAs from all other routers .4 10.1.3.0 / 24 10.1.1.0 / 24 .4 10.10.10.6 .1 .1 10.10.10.4 10 10.10.10.1 .5 .5 10.1.5.0/24 10.10.10.2 10.10.10.5 LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age Router-LSA 10.1.10.1 10.1.10.1 0x9b47 0x80000006 0 Router-LSA 10.1.10.2 10.1.10.2 0x219e 0x80000007 1618 Router-LSA 10.1.10.3 10.1.10.3 0x6b53 0x80000003 1712 Router-LSA 10.1.10.4 10.1.10.4 0xe39a 0x8000003a 20 Router-LSA 10.1.10.5 10.1.10.5 0xd2a6 0x80000038 18 Router-LSA 10.1.10.6 10.1.10.6 0x05c3 0x80000005 1680 12 Link State Database • The collection of all LSAs is called the link-state database • Each router has an identical link-state database – Useful for debugging: Each router has a complete description of the network • If neighboring routers discover each other for the first time, they will exchange their link-state databases • The link-state databases are synchronized using reliable flooding 13 OSPF Packet Format OSPF Message IP header OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89 OSPF Message Header Body of OSPF Message Message Type Specific Data LSA LSA ... ... LSA TTL: set to 1 (in most cases) LSA Header LSA Data Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters) 14 OSPF Packet Format OSPF Message Header 2: current version is OSPF V2 version Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement Standard IP checksum taken over entire packet Authentication passwd = 1: Authentication passwd = 2: Body of OSPF Message type message length source router IP address ID of the Area from which the packet originated Area ID checksum authentication type authentication authentication 32 bits 64 cleartext password 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Prevents replay attacks 15 OSPF LSA Format LSA Link Age LSA Header LSA Header LSA Data Link Type Link State ID advertising router link sequence number checksum length Link ID Link 1 Link Data Link Type #TOS metrics Metric Link ID Link 2 Link Data Link Type #TOS metrics Metric 16 Discovery of Neighbors • Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces. • If two routers share a link, they can become neighbors, and establish an adjacency 10.1.10.1 10.1.10.2 Scenario: Router 10.1.10.2 restarts OSPF Hello OSPF Hello: I heard 10.1.10.2 • After becoming a neighbor, routers exchange their link state databases 17 Neighbor discovery and database synchronization 10.1.10.1 Discovery of adjacency Scenario: Router 10.1.10.2 restarts 10.1.10.2 OSPF Hello OSPF Hello: I heard 10.1.10.2 After neighbors are discovered the nodes exchange their databases Database Description: Sequence = X Sends database description. (description only contains LSA headers) Acknowledges receipt of description Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005 Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005 Sends empty database description Database description of 10.1.10.2 Database Description: Sequence = X+1 18 Regular LSA exchanges 10.1.10.1 Link State Request packets, LSAs = Router-LSA, 10.1.10.1, Router-LSA, 10.1.10.2, Router-LSA, 10.1.10.3, Router-LSA, 10.1.10.4, Router-LSA, 10.1.10.5, Router-LSA, 10.1.10.6, 10.1.10.1 sends requested LSAs Link State Update Packet, LSAs = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005 10.1.10.2 10.1.10.2 explicitly requests each LSA from 10.1.10.1 10.1.10.2 has more recent value for 10.0.1.6 and sends it to 10.1.10.1 (with higher sequence number) Link State Update Packet, LSA = Router-LSA, 10.1.1.6, 0x80000006 19 Routing Data Distribution • LSA-Updates are distributed to all other routers via Reliable Flooding • Example: Flooding of LSA from 10.10.10.1 10.10.10.1 10.10.10.2 LSA ACK 10.10.10.4 LSA Update database Update database 10.10.10.2 Update database LSA 10.10.10.6 LSA ACK Update database Update database 10.10.10.5 20 Dissemination of LSA-Update • A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet) • Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes. • Acknowledgements of LSA-updates: • explicit ACK, or • implicit via reception of an LSA-Update • Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play? 21 Dynamic Routing Protocols III More OSPF Relates to Lab 4. This module covers additional details on the Open Shortest Path First (OSPF) routing protocol. Functional Requirements of OSPF • Fast convergence and low consumption of network resources • A descriptive routing metric – Configurable – Value ranges between 1 and 65,535 – No restriction on network diameters (RIP has a limit of 15) • Equal-cost multipath – A way to do load balancing Functional Requirements of OSPF • Routing Hierarchy – Support large routing domains • Separate internal and external routes • Support of flexible subnetting schemes – Route to arbitrary [address,mask] combinations using variable length subnet masks (VLSMs) • Security • Type of Service Routing OSPF Basics (The Essence) • Distributed, replicated database model – Describes complete routing topology • Link State Advertisements (LSAs, sometimes called Link State Announcements) – Carry local piece of routing topology • Distribution of LSAs using reliable flooding • Link state database – Identical for all the routers OSPF Packet Format OSPF Message IP header OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89 OSPF Message Header Body of OSPF Message Message Type Specific Data LSA LSA TTL: set to 1 (in most cases) LSA Header Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters) LSA Data ... ... LSA OSPF Packet Format OSPF Message Header 2: current version is OSPF V2 version Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement Standard IP checksum taken over entire packet Authentication passwd = 1: Authentication passwd = 2: Body of OSPF Message type message length source router IP address ID of the Area from which the packet originated Area ID checksum authentication type authentication authentication 32 bits 64 cleartext password 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Prevents replay attacks OSPF LSA Format LSA Link Age LSA Header LSA Header LSA Data Link Type Link State ID advertising router link sequence number checksum length Link ID Link 1 Link Data Link Type #TOS metrics Metric Link ID Link 2 Link Data Link Type #TOS metrics Metric LSAs (1) • Identifying LSAs – LS type field – Link State ID field • Mostly carries addressing information • E.g. IP address of externally reachable network – Advertising Router field • Originating router’s OSPF router ID LSAs (2) • Identifying LSA instances – Needed to update self-originated LSAs – LS Sequence Number field • 32 bit values • Monotonically increasing until some max value • 600 years to roll over! • LSA checksum and LS Age guard against potential problems LSAs (3) • Verifying LSA contents – LS Checksum field • Computed by the originating router and left unchanged thereafter • LS age field not included in checksum • Removing LSAs from databases – LS Age field • Ranges from 0 to 30 min. • Max Age LSAs used to delete outdated LSAs LSAs (4) • Other LSA Header fields – Options field • Sometimes used to give special treatment during flooding or routing calculations – Length field • Includes LSA header and contents • Ranges from 20-65535 bytes Link State Database • • • • • Collection of all OSPF LSAs Databases exchanged between neighbors Synchronization thru reliable flooding Gives the complete routing topology Each OSPF router has identical link-state database Reliable Flooding • Robustness – Updates flooded over all the links , so failure of any link does not affect database synchronization – LSAs refreshed every 30 minutes – LSA checksum field detects corruption – Flooding loops avoided by LS Age field – MinLSInterval limits rate of LSA origination – Receivers can refuse to accept LSA updates if they received an update less than a second ago Routing Calculations • • • • • Link costs configurable by administrator Smaller values for more preferred links Must make sense to add link costs Different costs for each link direction possible Dijkstra’s shortest path algorithm – Incrementally calculates tree of shortest paths – Each link in the network examined once – Computes multiple shortest paths (equal-cost multipath) IP Multicast to Send/Receive Changes • Multi-Access networks – All routers must accept packets sent to the AllSPFRouters (224.0.0.5) address – All Designated Router (DR) and Backup Designated Router (BDR) must accept packets sent to the AllDRouters (224.0.0.6) address • Hello packets are sent to the AllSPFRouters address (Unicast for point-to-point and virtual links) Hierarchical Routing • Technique used to build large networks • Minimizes consumption of network resources: – Router memory – Router computing resources – Link bandwidth • Flat Routing: linear increase in routing table size • Hierarchical Routing: size increases logarithmically An Example of Hierarchical Routing (1) 10.0.3 10.3.0.0/16 10.3.1 10.0.0.0/8 10.3.2 10.1.3 10.2.3 10.1.0.0/16 10.2.0.0/16 10.1.1 10.1.2 10.2.1 10.2.2 An Example of Hierarchical Routing (2) • • • • Consider a router in 10.1.1 Assume 16 entries in each of the first level partitions With flat routing, 9*16 = 144 entries/router With 3 level hierarchy, the router has 16 entries within 10.1.1.0/24 + entries for 10.1.2.0/24, 10.1.3.0/24,10.2.0.0/16 and 10.3.0.0/16 for a total of 20 entries. • Significant reduction in routing table size • But might lead to suboptimal routing OSPF Areas • Two-level hierarchical routing scheme through the use of areas • Areas identified by 32-bit id • Each area has its own link state database which is a collection of network-LSAs and router-LSAs • Area’s topology hidden from all other areas • Interconnection of areas through area border routers (ABRs) • ABR leaks IP addressing information to other areas through summary LSAs OSPF Areas • Reduction in link state databases of an area • Reduction in amount of flooding traffic needed for synchronization • Reduction in the cost of the shortest path calculations • Increased robustness Area Organization • All the areas are connected to area 0.0.0.0 also called the Backbone Area • Need not have a direct physical connection though – Virtual links provide logical link to backbone – Summary LSAs tunneled across non backbone areas • Exchange of routing information between areas using Distance Vector Protocol – Absence of redundant paths between areas – Not subject to convergence problems OSPF Areas • Group of nodes/networks • Per area topology DB – Invisible outside the area – Reduces routing traffic • Backbone Area is contiguous Area 0 Area 2 Area 3 Backbone Area – All others areas must connect to the backbone • Virtual Links Area 4 Area 1 Router Classification IR Area 3 Area 2 ABR/BR Area 0 ASBR To another AS Area 1 IR/BR • Internal Router (IR) • Area Border Router (ABR) • Backbone Router (BR) • Autonomous System Border Router (ASBR) OSPF Route Types Area 2 Area 0 ABR Area 3 Intra-Area Route – All routes within an area ASBR To Another AS Inter-Area Route – Routes announced from area to another by an ABR External Route – Routes imported into OSPF from another protocol or Static routes Inter-Area Route Summarization • Prefix or all subnets • Prefix or all networks R2 FDDI With Summarization Dual Ring Network Next Hop Summarization Area 0 R1 (ABR) 1 R1 Area 1 Network Next Hop Without Backbone 1.A R1 1.B R1 1.C R1 1.A 1.B 1.C External Routes • Redistributed into OSPF • Flooded without changes throughout the AS • OSPF supports two type of external metrics – Type 1 – Type 2 (Default) RIP IGRP OSPF Redistribute EIGRP BGP etc. Topology/Links-State DB • A router has a separate DB for each area it belongs to • All routers within an area have an identical DB • SPF calculation is done separately for each area • LSA flooding is limited to the particular area Protocol Functionality • Bringing up adjacencies • LSA Types • Area Classification The Hello Protocol • Responsible to establish and maintain neighbor relationships • Elects designated router in multi-access networks Hello FDDI Dual Ring Hello Hello Designated Router (DR) One per multi-access network Generates network links advertisements Assists in DB synchronization Designated Backup Router Designated Router Designated Router by Priority • Configured priority (per interface) • Otherwise determined by the highest router ID 131.108.3.2 131.108.3.3 DR R1 Router ID = 144.254.3.5 144.254.3.5 R2 Router ID = 131.108.3.3 Neighbor States • 2-way – The router sees itself in other Hello packets – DR is selected from neighbors in state 2-way or greater 2-way DR BDR Neighbor States • Full – Routers are fully adjacent – DB is synchronized – Relationship to the DR and BDR Full DR BDR When to Become Adjacent • • • • • • Underlying network is point-to-point Underlying network type is virtual link The router itself is the DR The router itself is the BDR The neighboring router is the DR The neighboring router is the BDR LSAs Propagate Along Adjacencies DR BDR • LSAs acknowledged along adjacencies Different Types of LSAs • Five LSA types – Type 1 : – Type 2 : – Type 3 y 4: – Type 5 y 7: Router LSA Network LSA Summary LSA External LSA Router LSA (Type 1) • Describes the state and cost of the router’s link to the area • All the router’s links in an area must be described in a single LSA • Flooded throughout the particular area and not beyond • Router indicates whether it is an ASBR, ABR, or the end point of a virtual link Network LSA (Type 2) • Generated for every transit broadcast or NBMA network • Describes all the routers attached to the network • Only the DR originates this type of LSA • Flooded throughout the area and not beyond Summary LSA (Type 3 y 4) • Describes a destination outside the area but still within the AS • Flooded throughout a single area • Originated by an ABR • Only intra-area routes are advertised into the backbone (Area 0) • Type 4 is the information about the ASBR External LSA (Type 5) • Defines routes to destinations outside the AS • Default route is also sent as external • Two Types of external LSA: • E1: Considers the total cost of to the external destination • E2: Considers only the cost of the outgoing interface to the external destination Issues not covered • OSPF Network Types – Broadcast subnets – NBMA Subnets • • • • OSPF Extensions Multicast Routing using OSPF (MOSPF) OSPF Management and a whole lot of others! Further Reading • John T. Moy, OSPF - An Anatomy of an Internet Routing Protocol • Christian Huitema, Routing in the Internet • RFC 2178