OSPF (Open Shortest Path First) Link-state based routing protocol: an Interior Gateway Protocol (IGP) – for inside ASs

What is OSPF? Open SPF Runs over IP directly, protocol number 89 Developed by IETF IGP working group, RFC2328 SPF Each router floods link-state information through its neighbors to other routers Based on the flooded link-state information, each router maintains a complete link-state database Based on the link-state database, a routing table is constructed using SPF (e.g., Dijkstra’s) algorithm Runs over IP directly, protocol number 89 Haobo Wang (modified by M. Veeraraghavan)

Features of OSPF Use flexible metrics instead of only hop count Supports variable-length subnetting Allows load balancing among equal-cost paths Supports multiple routes; one for each IP type of service (ToS) Authenticates route exchanges Quick convergence Uses multicast rather than broadcast of its messages to reduce network load Haobo Wang (modified by M. Veeraraghavan)

Hierarchical OSPF AS is organized as two-level hierarchy AS is partitioned into self-contained areas Areas are interconnected by a backbone area Areas are identified by a 32-bit area ID 0.0.0.0 is reserved for the backbone area Four types of routers Internal router, area border router, backbone router, autonomous system boundary router (ASBR) Haobo Wang (modified by M. Veeraraghavan)

OSPF AS organized into a 2-level hierarchy (ASBR) Within each area, border router responsible for routing outside the area Exactly one area is backbone area Backbone area contains all area border routers and possibly others Haobo Wang (modified by M. Veeraraghavan)

OSPF packets Five types of OSPF packets OSPF common header Hello(1) Database description(2) Link-State Request(3)/Update(4)/Acknowledgement(5) OSPF common header Version Type (1-5) Packet Length Router ID Area ID Checksum Authentication Type Authentication Haobo Wang (modified by M. Veeraraghavan)

OSPF common header fields Version number: 2 Type: Type of OSPF packet Packet length: in bytes, includes OSPF header Router ID: 32-bit number assigned to each OSPF running router – uniquely identifies router within AS Area ID: any four-byte number (0.0.0.0 reserved for backbone area) Checksum: error detection Three Authentication related fields: to authenticate OSPF packets Haobo Wang (modified by M. Veeraraghavan)

Backup designated router Hello packet Network mask Hello interval Options Priority Dead interval Designated router Backup designated router Neighbor 1 … Neighbor n Haobo Wang (modified by M. Veeraraghavan)

Hello packet fields Network mask: Subnet mask of the interface the packet is sent on Hello interval: Number of seconds between Hello packets Options: optional capabilities supported by the router Priority: of the router – used in election of designated router Dead interval: Number of seconds before declaring a nonresponding neighbor down Designated router/Backup: Every broadcast network with at least two routers has a designated router. This sends network LSAs for the broadcast network. This field is 0.0.0.0 if there is no designated router Neighbors: Router ID of each neighbor from whom Hello packets have recently been received Haobo Wang (modified by M. Veeraraghavan)

Database description packet Interface MTU Options Zero I M S Database description sequence number LSA header(s) LSA header LSA: Link State Advertisement Link-state age Options Link-state type Link-state ID Advertising router Link-state sequence number Link-state checksum Length Haobo Wang (modified by M. Veeraraghavan)

Database description packet fields Interface MTU: MTU of the associated interface Options: optional capabilities supported by the router I bit: Init bit set to 1 if this packet is the first in the sequence of database description packets M bit: More bit is set to 1 if more database description packets are to follow MS bit: Master/Slave bit Database description seq. no.: identifies the packet number sequentially so that a receiver can detect a missing packet Haobo Wang (modified by M. Veeraraghavan)

LSA header fields Link-state age: time since LSA generation Options: optional capabilities supported by the router Link-state type: router LSA, network LSA, summary LSA for IP networks, summary LSA for ASB routers, AS-external LSAs Link-state ID: describes routing domain for the LSA; depends on LSA type Advertising router: router ID of the router that generated the LSA Link-state sequence number: Numbers LSAs sequentially to identify old and duplicate LSAs Link-state checksum: entire contents of LSA except link-state age Length: in bytes of LSA including LSA header Haobo Wang (modified by M. Veeraraghavan)

Link-state Request/Update/ Acknowledgement Link-state type Link-state ID Advertising router … (above 3 fields repeated for each link) Link-state Update Number of LSAs LSA 1 … LSA n Link-state Acknowledgement Haobo Wang (modified by M. Veeraraghavan)

Router LSA 32 bits LSA header V E B #links Type #TOS Metric per-link V E B #links Type #TOS Metric per-link fields Link ID Link Data Repeat per-link fields for each link Haobo Wang (modified by M. Veeraraghavan)

LSA fields bit V (Virtual): 1 when router is a virtual link endpoint bit E (External): 1 when router is an ASBR bit B (Border): 1 when router is an area border router # links: # of links described in this LSA Per link: Type: Point-to-point connection to another router: 1 Connection to a transit network: 2 Connection to a stub network: 3 Virtual link: 4 Haobo Wang (modified by M. Veeraraghavan)

LSA fields contd. Per-link: #TOS: number of TOS metrics other than the required link metric given to this link; if none, 0 Link ID: Identifies the type of node connected to this link If type 1, Neighboring router's Router ID If type 2, IP address of Designated Router If type 3, IP network/subnet number If type 4, Neighboring router's Router ID Link Data: depends upon type of link For links to stub networks: Network's IP address mask For point-to-point links: the interface's MIB-II ifIndex value. For the other link types: the router interface's IP address. Metric: cost of using this link Haobo Wang (modified by M. Veeraraghavan)

Link state updates Each Link State Update packet carries a set of new link state advertisements (LSAs) one hop further away from their point of origination. A single Link State Update packet may contain the LSAs of several routers. Each LSA is tagged with the ID of the originating router and a checksum of its link state contents. Haobo Wang (modified by M. Veeraraghavan)

OSPF operations Hello protocol Database synchronization Propagation of link-state information Building of routing table Haobo Wang (modified by M. Veeraraghavan)

Hello Protocol Hello packets are transmitted to all interfaces periodically Discover neighbors, establish and maintain neighbor adjacency relationships Elect Designated Router (DR) if there are multiple routers in a broadcast network Haobo Wang (modified by M. Veeraraghavan)

Database synchronization Two neighboring routers exchange database description packets to synchronize their link-state databases. Database description includes only a list of LSA headers. New or more up-to-date LSAs will be requested later Packets sent by master are acknowledged by slave Haobo Wang (modified by M. Veeraraghavan)

Propagation of link-state information Link-state request sent When a router wants to update parts of its link-state database Link-state update sent When a link state is requested, or When a link-state changes, or Periodically Link-state acknowledgement sent in response to a link-state update Link-state updates retransmitted periodically until acknowledged Haobo Wang (modified by M. Veeraraghavan)

Flooding LSAs A node receiving a link-state update selectively first installs each LSA in the update into its LSA database Then it decides on which of its other links to flood the LSA it may decide not to flood an LSA out a particular interface if there is a high probability that the attached neighbors have already received the LSA. Haobo Wang (modified by M. Veeraraghavan)

Building of routing table Router S has knowledge of the entire area topology (complete link-state database) Some algorithm such as Dijkstra’s is used to generate shortest path tree, rooted at router S Only the next hop will be used in the routing table Haobo Wang (modified by M. Veeraraghavan)

Example Assume that all RT3 interfaces have 3 as their host ID AREA 1 Backbone area 192.1.2 3 RT1 N1 1 N3 192.1.1 RT4 3 RT2 18.10.0.6 N2 1 8 6 RT3 RT6 This is the link metric - note NOT hostID 192.1.3 2 N4 192.1.4 Construct Router LSA for RT3 Haobo Wang (modified by M. Veeraraghavan)

RT3’s router-LSA for Area 1 Link to N4: [COMMENT] Link ID = 192.1.4.0 ; IP Network number Link Data = 0xffffff00; Network mask Type = 3; connects to stub network # TOS metrics = 0 metric = 2 LS age = 0; true on origination Options =; LS type = 1; indicates router-LSA Link State ID = 192.1.1.3; RT3's Router ID Advertising Router = 192.1.1.3; RT3's Router ID bit E = 0; not an AS boundary router bit B = 1; area border router #links = 2 Link to N3: [COMMENT] Link ID = 192.1.1.4 ; IP address of Desig. Rtr. Link Data = 192.1.1.3; RT3's IP interface to net Type = 2; connects to transit network # TOS metrics = 0 metric = 1 Audio asks you to consult slides 10, 12, 14, 24: if new slides are added, these numbers will change, but here are the slide titles slide 10: DDP+ LSA header slide 12: LSA header fields slide 14: Router LSA slide 24: Example Haobo Wang (modified by M. Veeraraghavan)

RT3's router-LSA for the backbone LS age = 0; always true on origination Options = ; LS type = 1; indicates router-LSA Link State ID = 192.1.1.3; RT3's router ID Advertising Router = 192.1.1.3; RT3's router ID bit E = 0; not an AS boundary router bit B = 1; area border router #links = 1 Link to RT6 Link ID = 18.10.0.6; Neighbor's Router ID Link Data = 0.0.0.3; MIB-II ifIndex of P-P link Type = 1; connects to router # TOS metrics = 0 metric = 8 Haobo Wang (modified by M. Veeraraghavan)

References Section 8.7.2 of Communication Networks by A. Leon Garcia and I. Widjaja RFC 2328 (can be obtained from www.ietf.org) Haobo Wang (modified by M. Veeraraghavan)