GATE CS 2013 Q14 — Concept of Layering: OSI and TCP/IP Protocol Stacks

To find the number of times a packet visits the network layer and the data link layer, let us trace the path of the packet from the source $S$ to the destination $D$ through two intermediate routers $R_1$ and $R_2$:

Let us analyze the layer transitions at each node:

1. At Source ($S$):
   • The packet is created and passed down to the Network layer (1st visit).
   • It is then passed down to the Data Link layer (1st visit) for framing before transmission.
2. At Router ($R_1$):
   • The frame is received and processed up to the Data Link layer (2nd visit).
   • The packet is passed up to the Network layer (2nd visit) to determine the next hop.
   • It is then sent back down to the Data Link layer (3rd visit) to be encapsulated into a new frame.
3. At Router ($R_2$):
   • The frame is received and processed up to the Data Link layer (4th visit).
   • The packet is passed up to the Network layer (3rd visit) for routing.
   • It is then sent back down to the Data Link layer (5th visit) for the final hop.
4. At Destination ($D$):
   • The frame is received and processed up to the Data Link layer (6th visit).
   • The packet is passed up to the Network layer (4th visit) to be delivered to the higher layers.

Summary of Visits:

• Network layer: $1 \text{ (at } S) + 1 \text{ (at } R_1) + 1 \text{ (at } R_2) + 1 \text{ (at } D) = 4$ times
• Data link layer: $1 \text{ (at } S) + 2 \text{ (at } R_1) + 2 \text{ (at } R_2) + 1 \text{ (at } D) = 6$ times

Thus, the network layer is visited 4 times and the data link layer is visited 6 times.