Official PYQ · 2014Set 2 · Q25Data Link Layer: Framing, Error Detection, MAC, Ethernet BridgingNAT1 mark

GATE CS 2014 Set 2 Q25 — Data Link Layer: Framing, Error Detection, MAC, Ethernet Bridging

Router 1: Connects Ethernet LAN L 1 L1 L 1 to ISP 1.
Router 2 &amp; Router 3: Internal/boundary routers within ISP 1.
Router 4 &amp; Router 5: Internal/boundary routers within ISP 2.
Router 6: Connects ISP 2 to Token-Ring LAN L 2 L2 L 2 .

In the diagram shown below, $L1$ is an Ethernet LAN and $L2$ is a Token-Ring LAN. An $IP$ packet originates from sender $S$ and traverses to $R$ , as shown. The links within each $\text{ISP}$ and across the two $\text{ISP}$ s, are all point-to-point optical links. The initial value of the $\text{TTL}$ field is $32$ . The maximum possible value of the $\text{TTL}$ field when $R$ receives the datagram is _______.

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Answer: 26

To find the maximum possible value of the Time-to-Live (TTL) field when the receiver $R$ receives the datagram, we need to understand how TTL is processed across different network layers:

Layer 3 (Network Layer) Processing: The TTL field is part of the IP header. Every router (Layer 3 device) that forwards the IP packet decrements the TTL value by at least $1$ .
Layer 2 (Data Link Layer) Processing: LAN devices such as switches, bridges, or hubs operating at the Data Link Layer (within Ethernet $L1$ and Token-Ring $L2$ ) do not modify or decrement the TTL field.