Question 14

The transmission path connects $S \to R_1 \to R_2 \to D$ across 3 physical links.

1. At Source Host $S$:

   • The packet is constructed and passes down through the Network Layer (1 visit).
   • It passes through the Data Link Layer to be framed and transmitted on link 1 (1 visit).

2. At Intermediate Router $R_1$ (a Layer-3 device):

   • Frame arrives from link 1 and is processed by the Data Link Layer (1 visit).
   • Packet moves up to the Network Layer for routing lookup (1 visit).
   • Packet moves down to the Data Link Layer for transmission on link 2 (1 visit).
     Total at $R_1$: Network Layer = 1, Data Link Layer = 2.

3. At Intermediate Router $R_2$:

   • Frame arrives from link 2 into Data Link Layer (1 visit).
   • Packet moves up to Network Layer for routing lookup (1 visit).
   • Packet moves down to Data Link Layer for transmission on link 3 (1 visit).
     Total at $R_2$: Network Layer = 1, Data Link Layer = 2.

4. At Destination Host $D$:

   • Frame arrives from link 3 into Data Link Layer (1 visit).
   • Packet moves up to Network Layer for delivery to transport layer (1 visit).
     Total at $D$: Network Layer = 1, Data Link Layer = 1.

Summing across all entities:

• Network Layer visits: $1 (S) + 1 (R_1) + 1 (R_2) + 1 (D) = 4$ times.
• Data Link Layer visits: $1 (S) + 2 (R_1) + 2 (R_2) + 1 (D) = 6$ times.

This matches option C.