feat: complete 1.2

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@@ -39,7 +39,35 @@ TCP establishes connections through a three-way handshake and terminates them gr
 
 ### 1.2 Go-Back-N Protocol (5%)
 
-[To be completed]
+While Reliable Data Transfer (RDT) protocols are essentially stop-and-wait protocols that send one packet and wait for acknowledgment before sending the next, Go-Back-N (GBN) achieves higher throughput by allowing multiple packets to be transmitted without waiting for individual acknowledgments. GBN accomplishes this through several key mechanisms:
+
+#### 1. Sliding Window Mechanism
+GBN uses a sliding window of size N that allows the sender to transmit up to N unacknowledged packets. The window "slides" forward as acknowledgments are received, enabling continuous transmission without waiting for each individual ACK.
+
+#### 2. Sequence Number Space
+GBN uses a finite sequence number space (typically modulo 2^k) where packets are numbered sequentially. The sender maintains:
+- **base**: oldest unacknowledged packet sequence number
+- **nextseqnum**: next sequence number to be used
+- **Window boundary**: base ≤ sequence numbers < base + N
+
+#### 3. Cumulative Acknowledgments
+The receiver sends cumulative ACKs, where ACK(n) acknowledges all packets up to and including sequence number n. This allows a single ACK to acknowledge multiple packets, reducing ACK traffic and simplifying the protocol.
+
+#### 4. Selective Retransmission Strategy
+When a timeout occurs or a NAK is received, GBN retransmits all unacknowledged packets starting from the oldest unacknowledged packet (base). This "go-back-N" behavior ensures reliable delivery while maintaining simplicity.
+
+#### 5. Receiver Window Size of 1
+The receiver maintains a window of size 1, accepting only the next expected packet in sequence. Out-of-order packets are discarded and the receiver sends a duplicate ACK for the last correctly received packet. This maintains simplicity at the receiver.
+
+#### 6. Timer Management
+GBN uses a single timer for the oldest unacknowledged packet. When the timer expires, all unacknowledged packets are retransmitted, and the timer is restarted.
+
+#### Performance Benefits
+- **Pipeline Utilization**: Multiple packets can be "in flight" simultaneously, utilizing available bandwidth more efficiently than stop-and-wait
+- **Higher Throughput**: Transmission rate approaches channel capacity when the window size is appropriately chosen relative to the bandwidth-delay product
+- **Reduced Idle Time**: Sender doesn't wait for individual ACKs before continuing transmission
+
+**Key Insight**: GBN trades some efficiency (retransmitting correctly received but out-of-order packets) for simplicity and improved throughput compared to stop-and-wait protocols.
 
 ### 1.3 IPv6 Transition (5%)