TCP flow control — sliding window, receive buffer

4.3.21 · Coding › Computer Networks

Flow control exist kyun karta hai?

Receive buffer kya hai?

Sender ka constraint first principles se derive karna

Chalo rule build karte hain, memorize nahi karte.

Step 1 — sender ke bytes ko naam do.

LastByteSent = highest byte number jo sender ne network mein push kiya hai.
LastByteAcked = highest byte number jo receiver ne acknowledge kiya hai.

Yeh do kyun? In-flight (unacknowledged) bytes exactly woh hain jo bheje gaye hain lekin abhi tak acked nahi hue: $in-flight = LastByteSent - LastByteAcked$

Step 2 — receiver ka promise. Jab receiver ne rwnd advertise kiya, usne guarantee di ki uske paas utna free room hai. Toh sender ko yeh maintain karna chahiye: $LastByteSent - LastByteAcked \leq rwnd$

Yeh exactly sahi kyun hai: har in-flight byte eventually receive buffer mein land karegi. Agar in-flight bytes ≤ free space hai, toh buffer unhe sab absorb kar sakta hai bina overflow ke. Q.E.D. — sliding-window rule bas yeh hai ki "in-flight ≤ advertised free space."

Isse sliding window kyun kehte hain

Figure — TCP flow control — sliding window, receive buffer

Worked Example 1 — basic accounting

Worked Example 2 — window ZERO ho jaata hai

Worked example

Receiver overwhelmed hai: app reading band kar deti hai. Bytes pile up hote hain jab tak occupancy = RcvBuffer = 4000 na ho jaaye. $rwnd = 4000 - 4000 = 0$ Receiver Window = 0 advertise karta hai. Sender ko rukna padega.

Deadlock ka khatara (aur uska fix): Jab app finally read kare, buffer free ho jaata hai aur receiver chahega ki kahe "rwnd ab 2000 hai". Lekin receiver tab hi segments bhejta hai jab uske paas data ya ACK hota hai bhejne ke liye — agar koi data flow nahi ho raha, toh woh update kabhi nahi jaayega, aur sender hamesha ke liye wait karta rahega.

Fix — Zero-Window Probe: Sender periodically tiny 1-byte probe segments bhejta rehta hai. Receiver ka reply current (ab non-zero) window carry karta hai, connection ko unstick karta hai. Yeh kyun kaam karta hai: yeh receiver ko ek fresh window advertisement emit karne ke liye force karta hai.

Worked Example 3 — window scaling (16-bit limit)

Recall Feynman: 12-saal ke bacche ko explain karo

Imagine karo tum apne dost ke cup mein hose se paani daal rahe ho. Tumhara dost dheere dheere peeta hai. Har moment tumhara dost chillata hai "mere cup mein ITNI jagah baaki hai!" Tum utna hi daalte ho, zyada nahi — taaki cup kabhi overflow na ho. Jab cup bhar jaata hai, tumhara dost chillata hai "ZERO jagah!" aur tum daalana band kar dete ho. Tum thodi thodi der mein hose ko thapthapate rehte ho poochne ke liye "koi jagah abhi?" Jab tumhara dost kuch peeta hai aur chillata hai "okay, jagah hai!", tum phir daalana shuru karte ho. Woh cheekha hua number hai window, cup hai receive buffer, aur thapthapana hai zero-window probe.

Flashcards

TCP flow control kaunsi problem solve karta hai?

Ek fast sender ka slow receiver ke finite receive buffer ko overflow kar dena (yeh ek endpoint problem hai, network problem nahi).

Advertised receive window rwnd ka formula

rwnd = RcvBuffer − (LastByteRcvd − LastByteRead).

Core sliding-window inequality jo sender ko maanni chahiye

LastByteSent − LastByteAcked ≤ rwnd (in-flight bytes ≤ advertised free space).

Sender abhi kitne MORE bytes transmit kar sakta hai?

rwnd − (LastByteSent − LastByteAcked).

rwnd wire pe kahan carry hota hai?

Receiver ke bheje TCP segments ke 16-bit Window field mein.

rwnd ke 0 hone pe deadlock kya hai, aur uska fix?

Agar koi data flow nahi hota toh receiver window update nahi bhej paata; fix = sender periodic 1-byte zero-window probes bhejta hai taaki fresh advertisement mile.

rwnd aur cwnd mein farq?

rwnd receiver ke buffer ko protect karta hai (receiver set karta hai); cwnd network ko protect karta hai (sender estimate karta hai). Sender min(rwnd, cwnd) use karta hai.

Window scaling kyun zaroori hai aur yeh kaise kaam karta hai?

16-bit Window max 65535 bytes tak jaata hai, high bandwidth-delay links ke liye bahut chhota hai; Window Scale option field ko 2^s se multiply karta hai, handshake pe negotiate hota hai.

In-flight bytes ko rwnd se kyun subtract karte hain?

Woh bytes pehle se receive buffer mein reserved seats le chuke hain; woh advertised room ke against count hote hain.

Connections

TCP congestion control — cwnd, AIMD
TCP three-way handshake & options
Bandwidth-delay product
Stop-and-wait vs sliding window protocols
TCP segment header format
Buffer overflow & packet loss