Find the size of Sender's window if:

Question

During a TCP connection, the size of the window advertised by the receiver is 20 KB. The last byte sent by the sender is 20480 and the last byte acknowledged by the receiver is 8384. If the current congestion window is 18 KB, then the current size of the sender's window is _______ (in KB).

Comments

yes , selected answer is not correct...

---

why you are giving this fact that the current effective size should be "Adv. Window - (Unacknowledged bytes)" 

One thing i need to tell you that effective size must me calculated as 

MaxWindow =  min (CongestionWindow  , AdvertisedWindow)

EffectiveWindow = MaxWindow – (LastByteSent -LastByteAcked)

So your effective window size must be Congestion Window-Unacknowledged bytes which is 6KB approx. 

Am i correct???please suggest me if i am wrong....

---

RcvBuffer = 20KB,

LastByteRcvd(20480)- LastByteRead(8384) $ \leq$ 20KB

rwnd = RcvBffer(20KB) - (12096B) = 8384B =8.18KB

Answer 1

in TCP congestion window changes dynamically depending on congestion window , advertised window by receiver , amount data unacknowledged .

Reason :

window size : amount of data we can transmit before acknowledgement received.

in sliding window protocol , until acknowledgment received,  copy of transmitted data kept in buffer. 

window size= min (congestion window , advertised window) = 18 KB

size of unacknowledged  data =20480-8384 =12096 bytes = 11.8 KB

as this 11.8 KB is not acknowledged till now, this data is present in buffer , so (effective window size ) the amount of data that is possible to transmit before acknowledgement received is 

current active window size = 18 KB - 11.8 KB = 6.2 KB 

Comments

What is an active sender window size?

Is it the size of data that is sent but not acknowledged or size of data not sent yet. Please clarify

---

but what if out of order packets are acknowledged as in SR protocol? just by seeing last byte acknowledged we cannot determine how many packets are left to acknowledge.

---

https://gateoverflow.in/3836/gate2005-it-73

from above question this question solution is totally wrong @srestha 

Answer 2

The Actual window size from the client side will not calculate the rwnd size on its own, it will just take the minimum of what is advertised to it, whatever data is there in the recvbuffer is not it’s concern. The min of (18,20) is 18, so 18 is the answer.