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Master the fundamental concepts of tcp/ip from scratch through this focused micro-challenge.
You have read the whole brief, and the concepts above stay free on every task. Writing and running the code needs a plan.
Three hints are available for this task, revealed one at a time inside the code workspace so you can struggle productively before seeing them.
Every task includes starter code, theory, and hidden tests so you can implement and verify locally in the browser.
How it worksTCP's 16-bit window field tells the sender how many unacknowledged bytes it may have in flight. For example, with window 4000 the sender can transmit bytes 0 through 3999 before waiting for an ACK.
When ACK=1000 arrives, bytes 0-999 are acknowledged and the window slides:
1000-3999 (3000 bytes)4000-4999The sender tracks four regions: acknowledged, in-flight, sendable-within-window, and blocked-outside-window.
If the receiver buffer fills up, it advertises window 0. The sender probes with 1-byte segments until space opens. Throughput is bounded by window / RTT: a 65535-byte window on a 100ms RTT link caps around 5 Mbps without window scaling (RFC 7323). The receiver advertises available buffer space in every ACK; as the application reads data, the window grows and the sender can push more bytes in flight toward destinations like 10.0.0.5:8080.
This task asks you to simulate sliding window flow control. The Linux kernel auto-tunes receive buffers per socket (net.ipv4.tcp_rmem) based on exactly this mechanism. RFC 7323's window scaling option exists because the 16-bit window field caps throughput at roughly 5 Mbps on a 100ms-RTT link without scaling beyond 65535 bytes.
Given a receive window in bytes, an MSS, and a sequence of cumulative ACK advances, simulate the send window. The sender transmits only full MSS segments and can never have more than window bytes in flight. After each ACK the window slides: bytes in flight drop by the acknowledged amount and the sender immediately fills the freed space, rounded down to whole MSS segments.
Input (stdin): one line: window mss K. Then K ACK advances (bytes newly acknowledged by each ACK, in order).
Output (stdout):