TCP Sliding Window — Before and After the ACK

Given: previous ACK number (left edge of window) = 22,001, window size = 10,000 bytes. A segment arrives with ACK = 24,001 and window advertisement = 12,000.

Before

• Left edge (SND.UNA) = 22,001 → bytes < 22,001 are already acknowledged.
• Right edge = 22,001 + 10,000 − 1 = 32,000 → sender may transmit bytes 22,001 … 32,000.

 byte:  ... 21999 | 22001 22002 ............ 32000 | 32001 32002 ...
                  |<-------- window = 10,000 bytes --------->|  cannot
        ACKed      ^ SND.UNA                         right edge   send
                   (next byte expected by receiver = 32,001)

After (ACK = 24,001, new window = 12,000)

• Bytes 22,001 – 24,000 (2,000 bytes) are now acknowledged → they leave the window.
• New left edge = 24,001.
• New right edge = 24,001 + 12,000 − 1 = 36,000.
• The window has effectively slid forward by 2,000 bytes and grown by 2,000 bytes, opening 4,000 bytes of new sendable space at the right edge.

 byte:  ... 24000 | 24001 24002 ............ 36000 | 36001 36002 ...
                  |<-------- window = 12,000 bytes --------->|  cannot
        ACKed      ^ SND.UNA (new)                right edge     send
                   (next byte expected = 36,001)

Summary

Item	Before	After
ACK / left edge	22,001	24,001
Window size	10,000	12,000
Last byte in window	32,000	36,000
Bytes newly acknowledged	—	22,001 – 24,000 (2,000 B)
Newly sendable bytes	—	32,001 – 36,000 (4,000 B)

The sender may now transmit bytes 24,001 through 36,000 (12,000 bytes total, including any already sent but unacknowledged), and it must keep SND.NXT ≤ 36,001.