Question

Transport for routing protocols :

(a) Why is OSPF carried over “raw” IP? Explain why it does not need a transport layer protocol.

(b) Without a transport layer protocol, how does OSPF deal with reliability?

(c) Why is BGP carried over TCP?

Answer 1

OSPF - Open Shortest Path First is a routing protocol and it uses Dijkstra's Algorithm.

a) OSPF carried over “raw” IP because it forms IP datagrams directly, and for packaging it uses protocol 89 for IP Protocol field. Means it doesn't uses transport protocols such as User Datagram Protocol (UDP) or Transmission Layer Protocol (TCP). This is the reason behind why it does not need a transport layer protocol.
Reference: https://en.wikipedia.org/wiki/Open_Shortest_Path_First#OSPF_messages

b) LSAck ( Link State Acknowledgment ) is one of the five message types of OSPF protocol. These message provides reliability to link-state exchange process by acknowleding Link State Update message explicitly.

c) BGP (Border Gateway Protocol) is designed to exchange routing information among autonomous systems on the internet. BGP is an another application layer protocol. At the time of creating BGP , TCP was already there and they just wanted to take advantage of TCP. BGP uses TCP as its transport protocol to eliminate the need of implimenting update fragmentation, retransmission, acknowledgement, and sequencing explicitly.