Link State & OSPF. Spring 2013 CE Advanced Networks 1

Transcription

1 Link State & OSPF Spring 2013 CE Advanced Networks 1

2 AdministraBvia StaBc RouBng lab Wording problems VM stability issues How are the labs going? I will start upgrades of VMs today will send e- mail w/ new password when done OpportuniBes Cruzio I m waibng to hear back NMO SoTware Development for Cisco Advanced Services waibng for applicabons Expect more from campus network operabons group Next week Link Layer lab due Wednesday, 4/24 Link- State RouBng quiz Thursday, 4/25 Project proposal due Tuesday 4/30 Topic DraT outline What you need to invesbgate Spring 2013 CE Advanced Networks 2

3 Routing Introduction Remember delivery of IP packets implemented by two processes Forwarding Routing Dynamic routing process is a distributed computation Triggered by topology changes Processing defined by a routing protocol Output of the computation is forwarding state Goal of computation is convergence After finite sequence of topology changes Process should terminate updates to forwarding state Forwarding state should be correct Loop-free (after convergence vs. instantaneous) With desired characteristics: performance, possibly conform to policy 3 Spring 2013 CE Advanced Networks

4 Fate-sharing Principle The fate-sharing model suggests that it is acceptable to lose the state information associated with an entity if, at the same time, the entity itself is lost. Dave Clark, Design Philosophy of the DARPA Internet Protocols, SIGCOMM 88. Benefits Ensures the failure of any single component of an internet does not invalidate state located elsewhere in the internet Localizing the effects of any failures More robust system Internet routing architecture co-locates Forwarding state Routing process that computes the state Compared with virtual-circuit routing? 4 Spring 2013 CE Advanced Networks

5 Routing Protocol A set of algorithms and messages that are used to exchange topology information and populate the forwarding table with the routing protocol s choice of best paths. Purpose: Discover remote subnets Maintain up-to-date forwarding tables Choose the best path to destination subnets Components of a routing protocol: Algorithm: Procedures for Processing routing information Selecting best-paths The data structures needed for these steps. Routing protocol messages: Discover neighboring routers Exchange topology information 5 Spring 2013 CE Advanced Networks

6 Classifying Routing Protocols Function: Intra-domain/Interior Gateway Protocol (IGP) Inter-domain/Exterior Gateway Protocol (EGP) Algorithm distinguished by information exchanged: Distance-Vector Link-State Path-Vector 6 Spring 2013 CE Advanced Networks

7 Functional Classification An autonomous system (AS) or routing domain is a region of the Internet that is administered by a single entity UCSC s network IBM s corporate network AT&T s ISP network Ethernet Router Ethernet Ethernet Router Autonomous System 1 Routing inside an AS Focus is on performance Popular protocols: RIP, OSPF, IS-IS Ethernet Called intra-domain or internal gateway (IGP) routing Routing between ASs Focus is on policy Popular protocol: BGP Called inter-domain or external gateway (EGP) routing Router Router Router Autonomous System 2 Ethernet Router Ethernet 7 Spring 2013 CE Advanced Networks

8 How ensure correct routes? Recall requirement for correctness of routing protocol Loop-free Desired path characteristics Two strategies for ensuring correctness Use identical algorithm for selecting paths Share minimal topology information Use identical path selection algorithm at all nodes Used for IGP/Intra-domain routing Use link-state or distance vector protocol Use custom (private) algorithm for selecting paths Share full path information Use policy-specific path selection algorithm at each node Used for EGP/Inter-domain routing Use path-vector protocol Spring 2013 CE Advanced Networks 8

9 Algorithm Classification Distance-Vector Vectors of destination and distance sent to neighbors Tell your neighbors about the rest of the network Destination in terms of a network prefix Distance in terms of a metric: hop count, delay, bandwidth Use Distributed Bellman-Ford path selection algorithm Popular protocol: Routing Information Protocol (RIP) Link-State Flood description of your links (link state) Tell the rest of the network about your neighbors Links described by End-point routers of subnet in internet Cost of subnet: delay, bandwidth Use Dijkstra path selection algorithm Popular protocol: Open Shortest Path First (OSPF) Path-Vector Routes advertised as full-paths Paths described by sequence of ASs Popular protocol is Border Gateway Routing Protocol (BGP) 9 Spring 2013 CE Advanced Networks

10 Destination-Based Forwarding Internet roubng uses a single path per desbnabon Des-na-on- based forwarding is a restricted version of single- path A path through a node to a desbnabon must be an extension of the path from the node to the des3na3on. This is coming back to haunt us Spring 2013 CE Advanced Networks 10

11 Review The Internet implements a distributed routing architecture that is triggered by topology change events. Convergence of routing in the Internet depends on the stability of the network topology for a sufficient period of time. The Fate-Sharing Principle It is acceptable to lose the state information associated with an entity if, at the same time, the entity itself is lost. Ensures the failure of any single component of an internet does not invalidate state located elsewhere in the internet Localizing the effects of any failures Results in a more robust system Achieved in Internet by co-locating Forwarding state Routing process that computes the state 11 Spring 2013 CE Advanced Networks

12 Review An Autonomous System (AS) is a region of the Internet that is administered by a single entity and follows a single routing policy. Functional classification of routing protocols IGP - routing inside an AS Share minimal topology information Use identical path selection algorithm at all nodes Optimize performance EGP - routing between ASs Share full path information Use custom path selection algorithm at each node to implement desired policies Enforce policies 12 Spring 2013 CE Advanced Networks

13 Review Algorithmic classification of routing protocols Distance-vector Send vectors of distances to destinations to neighbors Tell your neighbors about the rest of the network Distributed computation Link-state Flood description of your links to all routers Tell the rest of the network about your neighbors Distributed database Path-vector Distance-vector with full paths Spring 2013 CE Advanced Networks 13

14 Review Destination-based forwarding A path through a node to a destination must be an extension of the path from the node to the destination Tends concentrates traffic on a subset of the network topology. Spring 2013 CE Advanced Networks 14

15 Link- State Spring 2013 CE Advanced Networks 15

16 RouBng Algorithms Distance- Vector Vectors of desbnabon and distance sent to neighbors Tell your neighbors about the rest of the network DesBnaBon in terms of a network prefix Distance in terms of a metric: hop count, delay, bandwidth Use Distributed Bellman- Ford path selecbon algorithm Popular protocol: RouBng InformaBon Protocol (RIP) Link- State Flood descripbon of your links (link state) Tell the rest of the network about your neighbors Links described by End- point routers of subnet in internet Cost of subnet: delay, bandwidth Use Dijkstra path selecbon algorithm Popular protocol: Open Shortest Path First (OSPF) Path- Vector Routes adverbsed as full- paths Paths described by sequence of ASs Popular protocol is Border Gateway RouBng Protocol (BGP) Spring 2013 CE Advanced Networks 16

17 RouBng Algorithms Distance- Vector Vectors of desbnabon and distance sent to neighbors Tell your neighbors about the rest of the network DesBnaBon in terms of a network prefix Distance in terms of a metric: hop count, delay, bandwidth Use Distributed Bellman- Ford path selecbon algorithm Popular protocol: RouBng InformaBon Protocol (RIP) Link- State Flood descripbon of your links (link state) Tell the rest of the network about your neighbors Links described by End- point routers of subnet in internet Cost of subnet: delay, bandwidth Use Dijkstra path selecbon algorithm Popular protocol: Open Shortest Path First (OSPF) Path- Vector Routes adverbsed as full- paths Paths described by sequence of ASs Popular protocol is Border Gateway RouBng Protocol (BGP) Spring 2013 CE Advanced Networks 17

18 How ensure correct routes? Recall requirement for correctness of roubng protocol Loop- free Desired path characterisbcs Two strategies for ensuring correctness Use idenbcal algorithm for selecbng paths Share minimal topology informabon Use idenbcal path selecbon algorithm at all nodes Used for IGP/Intra- domain roubng Use link- state or distance vector protocol Use custom (private) algorithm for selecbng paths Share full path informabon Use policy- specific path selecbon algorithm at each node Used for EGP/Inter- domain roubng Use path- vector protocol Spring 2013 CE Advanced Networks 18

19 How ensure correct routes? Recall requirement for correctness of roubng protocol Loop- free Desired path characterisbcs Two strategies for ensuring correctness Use idenbcal algorithm for selecbng paths Share minimal topology informabon Use idenbcal path selecbon algorithm at all nodes Used for IGP/Intra- domain roubng Use link- state or distance vector protocol Use custom (private) algorithm for selecbng paths Share full path informabon Use policy- specific path selecbon algorithm at each node Used for EGP/Inter- domain roubng Use path- vector protocol Spring 2013 CE Advanced Networks 19

20 Link- State Protocols Are Interior- Gateway Protocols (IGPs) Exchange link- state informabon Pair of routers connected by a subnet Cost of subnet (hop count, delay, etc.) Conceptually, very simple Spring 2013 CE Advanced Networks 20

21 Link- State Protocols Maintains a topology database of all the links it has heard of IniBalize with the subnets it is connected to. Floods link- state updates describing its directly connected subnets, including any changes to these links. Tell the rest of the network about your neighbors ParBcipates in the flooding of link- state updates from other routers. On update of its topology database Runs a shortest- path algorithm on the database to compute routes Dijkstra is most efficient Updates its forwarding table with any changes. Spring 2013 CE Advanced Networks 21

22 Characterizing Link State Link- State # updates per link change? One. How far propagate updates? Flooded to all nodes. One update, global distribubon. Scaling problems due to flooding As we ll see next lecture, the characteris3cs of distance vector are very different hint at a much beeer solu3on Spring 2013 CE Advanced Networks 22

23 Dijstra Shortest- Path Algorithm Breadth- first search of paths, by increasing path cost, for best paths to all desbnabons. Terminate when path has been found for all desbnabons. Maintain two sets DesBnaBons for which shortest paths have been found. Permanently labeled desbnabons P IniBalize with self DesBnaBons for which candidate shortest paths have been found. Iterate Temporarily labeled desbnabons T IniBalize with my neighbors. Move shortest path in T, say for desbnabon D, to P Add routes for D s neighbors, that are extensions of the path to T, to T if they are shorter than the current path in T for each neighbor. The relaxa3on step. Repeat unbl a route has been added to P for all desbnabons Spring 2013 CE Advanced Networks 23

24 More formally E is the set of edges. w ij is the weight of the link between nodes i and j. P and T see previous slide. P and T entries are triples, <d, p, w>: d is the desbnabon p is the predecessor w is the link weight algorithm Dijkstra begin 1 Push(<i,i,0>, P ); 2 for each {(i, j) E} 3 Insert(<j,i, ij >, T ); 4 while ( T > 0) begin 5 <x,p x,d x > Min(T ); 6 DeleteMin(T ); 7 Push(<x,p x,d x >, P ); 8 for each {(x, j) E} 9 if (T j = ) 10 then Insert(<j,x,d x + xj >, T ) 11 else if (d x + xj < T j.d j ) 12 then DecreaseKey(<j,x,d x + xj >, T ); end end Figure 2. Traditional Dijkstra Shortest-Path Algorithm. Spring 2013 CE Advanced Networks 24

25 Dijkstra path cost B,2 7 C, A, E, F, 2 2 G, 4 H, B,2 C, A,0 B,2 G, B,2 E,4 F, C, H, C, D, A,0 E,4 F, D, A,0 E,4 F,6 D, G,5 H, B,2 C, G,5 H, B,2 C,9 A,0 E,4 F,6 D, A,0 E,4 F,6 D, G,5 H,8 B,2 G,5 H,8 C,9 A,0 E,4 F,6 D,10 G,5 H,8 Spring 2013 CE Advanced Networks 25

26 TranslaBng to a Protocol Dijkstra requires a centralized implementabon Maintain a full graph of the network, on an event- driven basis Re- compute routes as the graph changes Flood changes to your links Brute- force protocol. Straight- forward, easy to understand Inefficient lots of overhead Spring 2013 CE Advanced Networks 26

27 Review Dijkstra Iterates on next shortest path Requires centralized computabon LS protocols Is an IGP Implements a centralized roubng model Floods link- state updates describing current state of its links Tell the rest of the network about your neighbors Use Dijkstra algorithm because it is most efficient shortest- path algorithm Spring 2013 CE Advanced Networks 27

28 The Challenge of Internet RouBng Independent roubng computabons at each router Need to compute paths that Support desbnabon- based forwarding Are shortest Are loop- free This is trickier than it looks Spring 2013 CE Advanced Networks 28

29 Examples illustrabng challenges of distributed roubng Spring 2013 CE Advanced Networks 29

30 Correct solubon depends on combinabon of roubng algorithm and algebra used for metrics. Spring 2013 CE Advanced Networks 30

31 OSPF Spring 2013 CE Advanced Networks 31

32 OSPF OSPF = Open Shortest Path First The most widely used roubng protocol The complexity of OSPF is significant History: 1989: RFC 1131 OSPF Version : RFC1247 OSPF Version : RFC 1583 OSPF Version 2 (revised) 1997: RFC 2178 OSPF Version 2 (revised) 1998: RFC 2328 OSPF Version 2 (current version) Spring 2013 CE Advanced Networks 32

33 What We Cover Messages Router IDs Flooding process Metrics Designated routers Areas Spring 2013 CE Advanced Networks 33

34 OSPF Messages An OSPF message can contain one of five packet types. OSPF is embedded directly in an IP frame (doesn t use UDP) Protocol field is set to 89 (OSPF) DesBnaBon address is typically set to one of two mulbcast addresses: ( AllSPFRouters Hello messages) or ( AllDRouters roubng info to Designated Routers ). If the OSPF packet is encapsulated in an Ethernet frame, the desbnabon MAC address is also a mulbcast address: E or E ! Spring 2013 CE Advanced Networks 34

35 OSPF Packet Types Hello: Used to establish and maintain adjacency with other OSPF routers. DBD: The database descripbon (DBD) packet contains an abbreviated list of the sending router s link- state database and is used by receiving routers to check against the local link- state database. LSR: Receiving routers can then request more informabon about any entry in the DBD by sending a link- state request (LSR). LSU: Link- state update (LSU) packets are used to reply to LSRs and to announce new informabon. LSUs contain seven different types of link- state adverbsements (LSA). LSAck: When an LSU is received, the router sends a link- state acknowledgment (LSAck) to confirm receipt of the LSU. Spring 2013 CE Advanced Networks 35

36 OSPF Message Header 2: current version is OSPF V2 OSPF Message Header 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 Body of OSPF Message version type message length source router IP address Area ID checksum authentication type authentication authentication 32 bits ID of the Area from which the packet originated 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks Spring 2013 CE Advanced Networks 36

37 RouterIDs Router ID plays an important role in OSPF Uniquely idenbfies each router in a roubng domain Used in Designated Router elecbon process (explained later) Router ID is an IP address of a router Router ID Router ID Router ID Cisco routers use following algorithm to determine Router ID IP address configured with OSPF router-id command If not configured, use highest IP address of a loopback interface If no loopback interfaces, use highest acbve IP address of physical interface Router ID Advantage of loopback interface is it cannot fail Router ID Router ID Spring 2013 CE Advanced Networks 37

38 OSPF Hello Packets Used to Discover OSPF neighbors and establish neighbor adjacencies Elect the Designated Router and Backup Designated Router on multiaccess networks such as Ethernet and Frame Relay Negotiate Hello interval (e.g. 10 sec on Ethernet segments) Dead interval: time to declare neighbor down (4x Hello interval) Network type Five network types Point-to-point Point-to-multipoint Broadcast multiaccess (Ethernet) Nonbroadcast multiaccess (Frame Relay) Virtual links Spring 2013 CE Advanced Networks 38

39 OSPF LSU Packets Link State Update (LSU) packets Used for OSPF routing updates Contain one or more LSAs Link State Advertisements (LSAs) Contain route information for destination networks There are 11 types of LSAs Spring 2013 CE Advanced Networks 39

40 Link State AdverBsement (LSA) The LSA of router is as follows: Link State ID: = can be Router ID AdverBsing Router: = Router ID Number of links: 3 = 2 links plus router itself DescripBon of Link 1: Link ID = , Metric = 4 DescripBon of Link 2: Link ID = , Metric = 3 DescripBon of Link 3: Link ID = , Metric = / / / / / Each router sends its LSA to all routers in the network (using a method called reliable flooding) Spring 2013 CE Advanced Networks 40

41 LSA Format LSA Link Age Link Type LSA Header LSA Data LSA Header Link State ID advertising router link sequence number checksum length Link ID Link 1 Link 2 Link Type Link Type #TOS metrics #TOS metrics Link Data Link ID Link Data Metric Metric Spring 2013 CE Advanced Networks 41

42 OSPF Metrics The OSPF metric is called cost. The following passage is from RFC 2328: A cost is associated with the output side of each router interface. This cost is configurable RFC 2328 does not specify which values should be used to determine the cost. In Cisco IOS Link cost is 10 8 link bandwidth. Reference bandwidth (10 8 ) can be changed with auto-cost referencebandwidth! Modify link bandwidth value with bandwidth! Spring 2013 CE Advanced Networks 42

43 Link State Database The collecbon of all LSAs is called the link- state database Each router has and idenbcal link- state database Useful for debugging: Each router has a complete descripbon of the network If neighboring routers discover each other for the first Bme, they will exchange their link- state databases The link- state databases are synchronized using reliable flooding Spring 2013 CE Advanced Networks 43

44 Link State Database / / / 24 Each router has a database which contains the LSAs from all other routers / / / / / LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age Router-LSA x9b47 0x Router-LSA x219e 0x Router-LSA x6b53 0x Router-LSA xe39a 0x a 20 Router-LSA xd2a6 0x Router-LSA x05c3 0x Spring 2013 CE Advanced Networks 44

45 Neighbor Discovery Router mulbcasts OSPF Hello packets on all OSPF- enabled interfaces. If two routers share a link, they can become neighbors, and establish an adjacency OSPF Hello Scenario: Router restarts OSPF Hello: I heard ATer becoming a neighbor, routers exchange their link state databases Spring 2013 CE Advanced Networks 45

46 Neighbor discovery and database synchronizabon Scenario: Router restarts Discovery of adjacency OSPF Hello OSPF Hello: I heard After neighbors are discovered the nodes exchange their databases Sends database description. (description only contains LSA headers) Acknowledges receipt of description Database Description: Sequence = X Database Description: Sequence = X, 5 LSA headers = Router-LSA, , 0x Router-LSA, , 0x Router-LSA, , 0x Router-LSA, , 0x a Router-LSA, , 0x Router-LSA, , 0x Database Description: Sequence = X+1, 1 LSA header= Router-LSA, , 0x Database Description: Sequence = X+1 Sends empty database description Database description of Spring 2013 CE Advanced Networks 46

47 Regular LSA exchanges sends requested LSAs Link State Request packets, LSAs = Router-LSA, , Router-LSA, , Router-LSA, , Router-LSA, , Router-LSA, , Router-LSA, , Link State Update Packet, LSAs = Router-LSA, , 0x Router-LSA, , 0x Router-LSA, , 0x Router-LSA, , 0x a Router-LSA, , 0x Router-LSA, , 0x Link State Update Packet, LSA = Router-LSA, , 0x explicitly requests each LSA from has more recent value for and sends it to (with higher sequence number) Spring 2013 CE Advanced Networks 47

48 DisseminaBon 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 informabon, the router will not flood the packet) ExcepBon: Infrequently (every 30 minutes), a router will flood LSAs even if there are no new changes. Acknowledgements of LSA- updates: explicit ACK, or implicit via recepbon of an LSA- Update Spring 2013 CE Advanced Networks 48

49 Why Designated Routers? Large number of adjacencies Full mesh of adjacencies n(n - 1)/2 adjacencies Excessive load from flooding LSAs Spring 2013 CE Advanced Networks 49

50 ElecBng Designated Router OSPF elects a Designated Router (DR) on mulbaccess networks DR is collecbon and distribubon point for LSAs on network Backup Designated Router (BDR) also elected for case where DR fails Spring 2013 CE Advanced Networks 50

51 ElecBng Designated Router ElecBon held through exchange of Hello messages DR/BDR elecbon criteria DR is router with highest interface priority BDR is router with second- highest interface priority If interface prioribes are equal, use Router ID Spring 2013 CE Advanced Networks 51

52 ElecBng Designated Router Further elecbons only occur on failure of DR/BDR On DR failure BDR promoted to DR New BDR elected On BDR failure New BDR elected Live (B)DR not replaced by elecbon. If DR dies BDR replaces DR ElecBon held to replace BDR If BDR dies ElecBon held to replace BDR Spring 2013 CE Advanced Networks 52

53 OSPF Areas An OSPF Area is a set of routers that share link state informabon Goal is to address scalability problem with OSPF flooding Area 0 is the backbone area All areas must be connected to he backbone area Best pracbce to use area 0 in single- area OSPF Eases conversion to mulb- area Spring 2013 CE Advanced Networks 53

54 Review OSPF message types Hello - establish and maintain adjacency with OSPF routers DataBase DescripBon (DBD) - summary of my database Link- State Request (LSR) - request for detailed entry Link- State Update (LSU) - response to LSR Link- State Ack (LSAck) - acknowledge receipt of LSU Router IDs Uniquely idenbfies each router in a roubng domain Used in Designated Router elecbon process Best pracbce is to use loopback interface... it can t fail Spring 2013 CE Advanced Networks 54

55 Review OSPF metrics Cost is associated with the output side of each router interface. In IOS is 10 8 / link bandwidth (inversely proporbonal to bandwidth) Designated routers MiBgate impact of full- mesh on # of adjacencies in topology (n(n- 1)/2) DR is collecbon and distribubon point for LSAs on network OSPF deals with scaling problem by using areas, area 0 is the backbone. Spring 2013 CE Advanced Networks 55