CMPT 471 Networking II

1. CMPT 471Networking II OSPF

2. OSPF messages • Encapsulated in IP datagrams • 5 types of messages, all message types begin with a common header • Message types are • Hello • establish and test neighbor reachability (two OSPF router may be neighbors if they are on the same network segment) • elect designated routers on multi access networks • Database description (initial router network topology database)

3. OSPF messages • 5 types of messages, all message types begin with a common header including • Link status request used for forming adjacencies • Adjacencies are agreements to exchange LSAs, • adjacencies are formed with designated routers for multi access networks, and neighbors • Link status update (also used for forming adjacencies) • Link status acknowledgement (ACKs each link status update message)

4. link state advertisements (1) • Router link advertisement • Originated by all routers • Flooded throughout a single area only • Describes the states of the router’s interfaces to the area • Network link advertisement • Originated by broadcast networks • Flooded throughout a single area only • Contains a list of routers connected to the network

5. link state advertisements (2) • Summary link advertisement • Originated by border area routers • Flooded throughout a area and backbone • Describes a route outside the local area but within the AS • AS external link advertisement • Originated by AS boundary area routers • Flooded throughout the AS • Contains a route to a destination outside the AS in another AS

6. Maintaining databases • Each router originates some LSAs, routes to neighbor networks, and for designated routers routes to each router the is part of its group. • The LSAs originating at a router are flooded through the local area each time they are updated, an periodically (every 30 minutes) • Summary LSA’s are sent by border area routers to provide summary information on their areas through the backbone to other areas

7. Flooding protocol: conditions • A message(LSA) contains a database record. A database record contains information about one link between two routers in the graph discussed earlier. (one link is in one direction) • Each message contains a time stamp or message number • These message numbers are used by the receiving node to determine age of the record • Send means transmit through all attached interfaces except the one on which the incoming message arrived

8. Flooding protocol • Receive message: Find the corresponding record in the local database if it exists • If the record is not yet in the local database add the record. Send the message • If the record’s message number is larger than the message number in the data base, replace the message in the database with the new record. Send the message. • If the records message number is the same as the message number in the database do nothing • If the records message number is smaller than the message number in the database, send the record in the database back through the interface on which the message arrived

9. OSPF common message header Comer 2000: figure 16.7

10. OSPF header fields • Version: specifies version being used, currently 2 • Type: see slide 23 for details • Message length: length of the packet in octets • Source router IP: IP address of the router wishing to send the message • Area ID: identify the area the router is a part of (backbone routers have an Area ID of 0) • Checksum: computed for the whole OSPF packet (excluding the authentication field) • Authentication type: 0 for none 1 for 8 character password 2 for cryptographic authentication

11. The Hello protocol • Hello is used • To check that links are operational. • To initiate and maintain neighbor relationships. Two routers on the same network segment may become neighbors • To maintain the connectivity • To elect the designated router • To elect the backup designated router

12. Hello message format Comer 2000: figure 16.8

13. Hello message fields (1) • Network mask: the network/subnetwork mask of the network to which the interface connects • Dead timer: If an update LSA is not received within the dead time the neighbor relationship is broken (dead time > hello interval) • Hello interval: interval between sending of successive Hello packets

14. Hello message fields (2) • Gway priority: priority of the router, each router is configured with a set priority. These priorities are used in the election of designated routers • Designated Router: Address of the designated router or 0 if no router has yet been designated. • Backup Designated Router: Address of the backup designated router or 0 if no router has yet been designated.

15. Hello message fields (3) • Neighbor IP addresses • When a router receives a hello message from another router it adds the IP address of the router that sent that hello message to it’s list of neighbors • For any subsequent Hello messages sent by the receiving router the IP address of the sending router is included in the list of neighbors of the sending router (in the hello message)

16. Becoming a neighbor (hello) NO Hello received before end of deadtime interval DOWN INIT Hello received Does not contain my address Hello received Does contain my address ExSTART DR and BDR elected 2WAY

17. States: Interface state machine • Down: • The initial state of a neighbor conversation. • Indicates that no recent information (hello’s) have been received from the other router • Init: • A Hello packet has just been seen from the other router. Bidirectional communication has not yet been established ( the receiving router’s IP did not appear in the Hello packet received from the other router). • The IP of the other router will be listed in the subsequent Hello packets sent • 2-way:

18. States: Interface state machine • Down: • Init: • 2-way: • Bidirectional communication has established • A Hello packet containing this routers address has been received from the other router. The two routers are now neighbors • The (Backup) Designated Router is selected from the set of neighbors in state 2-Way or greater.

19. Uses for HELLO messages • The hello protocol • establishes neighbor relationships • Maintains neighbor relationships, checks that links are operating • elects designated (DR) • elects backup designated routers (BDR)

20. Maintaining Neighbor relationships • Each router will send Hello messages to its neighbors once each Hello interval. These messages will contain the addresses of all neighbors • Messages are encapsulated in UDP packets so may not arrive (best effort delivery) • The ‘dead interval’ is usually ≥3X the Hello interval • Each time a hello message containing router A’s address is received the ‘dead timer’ for the neighbor relation with A is reset to the ‘dead interval’ • The neighbor relationship is broken when no ‘hello’ is received from neighbor A before the dead timer expires

21. DR election and booting • After a router is booted OSPF will be in a “waiting” state for up to a “waiting interval” equal to the dead time. During this interval • Hello packets are sent, but the router will not propose itself for election as DR or BDR • Information in received Hello packets will be saved • At the end of the “waiting” period the BDR and DR are elected based on accumulated information from the received hello packets. • The addresses of the elected routers are included in subsequent Hello messages

22. DR election and booting • After a router is booted OSPF will be in a “waiting” state for up to a “waiting interval” equal to the dead time. During this interval • When leaving the waiting state the router may • Accept the election of other routers as both DR and BDR and form adjacencies with both the BDR and the DR • Become DR (will not replace a previously elected DR) • Become BDR (will not replace a previously elected BDR)

23. Electing DR router: state DOWN Interface turned on Waiting DR DR Other BDR

24. Backup designated router • A router cannot declare itself both BDR and DR • If no router has declared itself a BDR in its hello packet, (by inserting their own address in the BDR field) then • Routers that are declared to be DR by having their own address in the designated router field of some LSA's from other routers are ignored • The router with the highest priority (that is not declared as DR) will be declared BDR

25. Backup designated router • If some routers have a declared themselves backup designated routers in their Hello packets then the declared designated backup router with the highest priority is elected backup designated router. • If no routers have declared themselves designated routers in their hello packets, the backup designated router is promoted to be designated router. Then a new backup designated router is elected

26. Electing designated routers • If some routers have a declared themselves designated router in their Hello packets then the declared designated router with the highest priority is elected designated router. • If a new DR or BDR has been declared repeat the procedure for determining BDR and DR. This assures that no router is both DR and BDR • See the protocol for more details on this process

27. Adjacency • Routers establish an adjacency if they will be exchanging LSAs. • The DR for a network segment will have adjacency relationships with • Neighbors on that network segment • DR for each attached network or network segment • A router that attaches to n networks and is not the DR for any of those networks will be adjacent to the DR for each network it attaches to. • LSAs are exchanged between routers with adjacency relationships (established and maintained by the HELLO protocol)

28. Establishing adjacency ExStart Negotiation done EXCHANGE Exchange done FULL Transfer done LOADING

29. Synchronizing of databases • Uses an exchange protocol • When an adjacency relationship is established between two routers the databases of those two routers need to be synchronized • Initial synchronization is done using the exchange protocol • Subsequently synchronization is maintained through the other protocols within OSPF • If a adjacency is broken an then reestablished the synchronization using the exchange protocol will be repeated.

30. Database description message Comer 2000: figure 16.9

31. Fields: Database Description • I,M,S: Bits used to indicate master/slave status and router initiating the exchange • Database sequence number: Unique number labeling a particular database synchronization procedure • Database record description: the remaining information in the DD message (link type, link id, advertising router … )

32. Synchronizing databases : 1 • One of the adjacent routers is declared the master the other is declared the slave. The master is the router that initiates the exchange. The master is responsible for retransmission • The master sends an initialization packet (setting sequence number) with I=M=S=1 • The slave acks with the same sequence number and I=M=1 S=0 • Negotiation complete, the router moves into EXCHANGE state

33. Synchronizing databases : 2 • The master then sends a database description (DD) packets describing the next link in its database. I=0 M=S=1 • The slave sends a DD packet describing the next link in its own database in response. I=S=0 M=1. This packet is treated as an ACK of the masters packet. If the ACK is not received the master will retransmit its packet • This procedure repeats until all database description information has been exchanged. If the master runs out of records before the slave it will continue sending empty DD packets until it receives an ACK with M set to 0 indicating the slave has completed sending its database information

34. Synchronizing databases: 3 • When the master or slave receives a DD message it will compare the record to its own database. • It will check to see if there is a record with the same description • If the record is not in their database they will place the description in the ‘records to request’ list • It will check the sequence number (age) to see if the received record is newer • If the received record is newer they will place the description in the ‘records to request’ list

35. Synchronizing databases: 4 • When the records to request list on both the master and slave are complete • All database records in the master’s database have been compared to the slave’s database • All database records in the slave’s database have been compared to the master’s database • The routers move into LOADING state

36. Processing the records to request list • Each record on the records to request list will be requested using a link state request message. One link state request message may request several record updates • Each record update requested is described by three pieces of data, link type, link id, and advertising router • In response to a received link state request the router will build a link state update message containing the data in the requested records • Each time an update message is received in response the request message the corresponding entry in the ‘records to request’ list is removed.

37. Link state request message Comer 2000: figure 16.10

38. Link status update message Comer 2000: figure 16.11