1. Nov 11, 2004CS573: Network Protocols and Standards1 IP Routing: OSPF Network Protocols and Standards Autumn 2004-2005

2. Nov 11, 2004CS573: Network Protocols and Standards2 Issues Design of OSPF Separating hosts and routers Broadcast networks (Ethernet, FDDI, …) Non-broadcast networks (ATM, X.25, …) Splitting very large networks into areas

3. Nov 11, 2004CS573: Network Protocols and Standards3 Separating Hosts and Routers Instead of link-state records for hosts: R-H1 R-H2 R-H3 Use simplification based on subnet model - One link between the router and the subnet: “link to stub network” - Identified by its subnet number RH1 H2 H3

4. Nov 11, 2004CS573: Network Protocols and Standards4 Stub Networks Multiaccess/broadcast stub network Single router attached to the network N is network IP address and associated mask N RT N Graph Representation

5. Nov 11, 2004CS573: Network Protocols and Standards5 Point to Point Networks Unnumbered point-to-point network Interfaces to point-to-point network not assigned IP addresses RT1 RT2 Graph Representation RT2

6. Nov 11, 2004CS573: Network Protocols and Standards6 Point to Point Networks Numbered point-to-point network Ia and Ib are interface IP addresses RT1 RT2 Graph Representation RT2 IaIb IaIb

7. Nov 11, 2004CS573: Network Protocols and Standards7 Stub Networks Host directly attached to a router Host route Mask is 0xFFFF FFFF (255.255.255.255) Host RT Host Graph Representation

8. Nov 11, 2004CS573: Network Protocols and Standards8 Broadcast Networks Such networks characterized by: Full connectivity Broadcast capability Issues (when N routers coexist on a broadcast network): # Adjacencies: N(N-1)/2 Each router would advertise: N-1 links to other routers One link to the subnetwork Solution: reduce number of adjacencies to N One router is designated through election Election of the designated router is through “Hello Protocol”

9. Nov 11, 2004CS573: Network Protocols and Standards9 Broadcast Networks Reducing the number of link state records using designated router Database will include two links per router One link from router to virtual node Called router link type 2 (transit network) Advertised by the router itself Appropriate metric Link from virtual node to router Called network link Advertised by designated router Network links have a null metric

10. Nov 11, 2004CS573: Network Protocols and Standards10 Broadcast Networks RT1 RT2 RT3 RT4 N RT1RT2 RT4RT3 Multiaccess/broadcast transit network - 3 or more routers attached to the network

11. Nov 11, 2004CS573: Network Protocols and Standards11 Broadcast Networks Simplifying flooding A router sends a link state advertisement to the designated router only using 224.0.0.6 “all designated routers” multicast address If advertisement is new, designated router floods the link state on all its interfaces (including the network on which it is received!) using 224.0.0.5 “all OSPF routers” multicast address

12. Nov 11, 2004CS573: Network Protocols and Standards12 Broadcast Networks Designated router forms adjacencies with all routers in the broadcast network For reliability, there is one backup designated router Backup designated router also forms adjacencies with all routers in the broadcast network Listens to 224.0.0.6 but remains silent in terms of flooding, etc. Failure of designated router is discovered by means of Hello protocol Smoother transition occurs since the backup router has already formed all the adjacencies

13. Nov 11, 2004CS573: Network Protocols and Standards13 Non-broadcast Networks IP over X.25 networks Popular in Europe in 1980’s IP over ATM Static configurations Avoid N(N-1)/2 overhead Use designated router On-demand circuits Permanent circuits are for links between routers and the “designated router”

14. Nov 11, 2004CS573: Network Protocols and Standards14 Link State DB records Five types of link state records 1. Router link 2. Network link 3. Summary link (IP network) 4. Summary link (to a border router) 5. External link Types 3 and 4 are used when OSPF areas are used

15. Nov 11, 2004CS573: Network Protocols and Standards15 OSPF Router ID Each OSPF router is assigned an OSPF router ID 32 bit number uniquely identifying the router within the OSPF domain When the router interfaces have IP addresses assigned to them, then the OSPF router ID is one of the router’s IP address

16. Nov 11, 2004CS573: Network Protocols and Standards16 LSA Header LS AGELS TYPE LINK STATE ID OPTIONS ADVERTISING ROUTER LS SEQUENCE NUMBER LS CHECKSUMLENGTH ADVERTISEMENT DATA (LINKS) OSPF HEADER TYPE=4 #ADVERTISEMENTS ADVERTISEMENT 1 ADVERTISEMENT N … 4 BYTES Advertisement Header

17. Nov 11, 2004CS573: Network Protocols and Standards17 LSA Header Fields Link state Type Type of LS record (1, 2, 3, 4, or 5) Link state ID Chosen by the advertising router Generally an IP address Sequence Number Identifies one particular advertisement Checksum Protects header as well as content Length Total length of the record (including the 20-byte header)

18. Nov 11, 2004CS573: Network Protocols and Standards18 LSA Header Fields Advertising router The OSPF ID of the sender Age 16-bit unsigned integer indicating the time in seconds since the link state record was first advertised Options: E: used in Hello protocol T: Set when router supports nonzero TOS Removed from the latest version of the standard (RFC2328) ET RFC 1583 Definition Type of Service External Links

19. Nov 11, 2004CS573: Network Protocols and Standards19 Multiple Areas Hierarchical routing Decreased routing overhead Size of link state DB Duration of route computation Volume of messages exchanged Split the network into set of independent parts by a backbone Each area operates like an independent network Database includes only the state of the area’s links Flooding stops at the boundaries Routers compute routes within the area Cost of routing proportional to the size of the area

20. Nov 11, 2004CS573: Network Protocols and Standards20 Multiple Areas How to glue the network together? Some routers belong to several areas Typically to the backbone and to one lower-level area At least one area border router in each area Area border routers Maintain several link state databases (one for each area to which they belong) Emit special link state records (summaries) to signal reachability of networks in each area

21. Nov 11, 2004CS573: Network Protocols and Standards21 Stub Areas Areas where there is only one exit point, or the exit point is not a function of the external destination Stub area does not need to know the topology of the rest of the AS All external traffic goes to the exit point Obviously, no AS boundary router can be internal to the stub areas R

22. Nov 11, 2004CS573: Network Protocols and Standards22 OSPF Packet Formats OSPF directly over IP, using protocol number 89 OSPF does not explicitly support fragmentation, but protocol messages can generally be split This should be used rather than IP fragmentation OSPF packets are sent with an IP TOS of 0 OSPF packets are sent with IP precedence set to Internetwork control All OSPF packets use the same OSPF header OSPF Multicast addresses (sent with TTL=1) 224.0.0.5: All OSPF routers 224.0.0.6: OSPF designated and backup routers

23. Nov 11, 2004CS573: Network Protocols and Standards23 The Common OSPF Header VERSION #TYPEPACKET LENGTH ROUTER ID AREA ID AUTHENTICATION CHECKSUMAUTYPE

24. Nov 11, 2004CS573: Network Protocols and Standards24 OSPF Header Fields Version #: set to 2 (current version) Type: The OSPF packet type 1. Hello 2. Database description 3. Link state request 4. Link state update 5. Link state acknowledgement Packet length: Number of bytes in the header including the header Router ID: The IP selected for identifying the router Area ID: The value 0 is reserved for backbone area. Commonly, an IP address is used to identify the areas Checksum: Computed on the whole OSPF packet, excluding the 8-octet authentication field

25. Nov 11, 2004CS573: Network Protocols and Standards25 OSPF Header Fields AUTYPE: Identifies the authentication algorithm. Only three values are identified in the standard itself: 0: No authentication Exchanges not authenticated Authentication field ignored; can be set to anything 1: Simple authentication “Clear password” type of authentication; all packets must contain the right value, pre-configured for that area Used to prevent unconfigured routers from joining in 2: Cryptographic authentication Secret key is used to generate a digest of the packet Digest is added at the end of the packet; size not included in the header 64-bit field is restructured to contain digest size, key ID, and sequence number (to protect against replay attacks)