GATE CS 2020 Q15 — Routing Protocols: Shortest Path, Flooding, Distance Vector, Link State

To determine which statements are true, let us analyze each of them in detail:

• Statement I: "A router does not modify the $\textsf{IP}$ packets during forwarding."
  False. When a router forwards an $\textsf{IP}$ packet, it modifies several fields in the $\textsf{IP}$ header:
  • It decrements the Time to Live (TTL) field by at least $1$.
  • Since the TTL field changes, the router must recalculate the Header Checksum.
  • If the packet size exceeds the outgoing link's $\textsf{MTU}$, the router may fragment the packet, modifying the fragmentation flags and offset fields.
• Statement II: "It is not necessary for a router to implement any routing protocol."
  True. A router forwards packets based on its routing table. While dynamic routing protocols (like $\textsf{OSPF}$, $\textsf{BGP}$, or $\textsf{RIP}$) are commonly used to build this table automatically, they are not strictly mandatory. A router can function perfectly using manually configured static routes.
• Statement III: "A router should reassemble $\textsf{IP}$ fragments if the $\textsf{MTU}$ of the outgoing link is larger than the size of the incoming $\textsf{IP}$ packet."
  False. In the $\textsf{IPv4}$ protocol, reassembly of fragmented packets is performed exclusively at the destination host, not by intermediate routers. Reassembling fragments at intermediate routers would require significant memory buffers and processing power, and fragments of the same packet might travel along different paths.

Thus, only statement II is true.