GATE CS 2005 Q25 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

In the Selective Reject (Selective Repeat) sliding window protocol, to avoid any ambiguity between new frames and retransmitted frames, the sum of the sender's window size ($W_T$) and the receiver's window size ($W_R$) must not exceed the total number of available sequence numbers ($N$).

For Selective Repeat, the sender and receiver window sizes are typically equal:

Thus, the condition is:

Given that the sequence numbers are represented using $n$ bits, the total number of available sequence numbers is:

Substituting $N = 2^n$ into the inequality:

Therefore, the maximum window size is $2^{n-1}$, which corresponds to Option B.