GATE CSE 2015 Set 2 | Question: 34

Question

Assume that the bandwidth for a $\text{TCP}$ connection  is $1048560$ bits/sec. Let $\alpha$ be the value of RTT in milliseconds (rounded off to the nearest integer) after which the $\text{TCP}$ window scale option is needed. Let $\beta$ be the maximum possible window size with window scale option. Then the values of $\alpha$ and $\beta$ are 

• A. $63$ milliseconds, $65535$ $\times $2$^{14}$
• B. $63$ milliseconds, $65535$ $\times $2$^{16}$
• C. $500$ milliseconds, $65535$ $\times $2$^{14}$
• D. $500$ milliseconds, $65535$ $\times $2$^{16}$

Comments

it is not sequence number....it is maximum window size

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Awesome question 😍

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An interesting read: https://lwn.net/Articles/92727/

Answer 1

In TCP when the bandwidth-delay product increases beyond $64\;\textsf{K}$ receiver window scaling is needed. 

The bandwidth-delay product is the maximum amount of data on the network circuit at any time and is measured as RTT * Bandwidth. This is not the time for sending data rather just the time for sending data without acknowledgement. 

So, here, we have bandwidth delay product $= (1048560 / 8) B \ast \alpha  = 64\;\textsf{K}$

$\alpha = (64\;\textsf{K} \ast 8 ) / 1048560 = 0.5\;\text{s} = 500$ milliseconds. 

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When window scaling happens, a $14$ bit shift count is used in $\text{TCP}$ header. So, the maximum possible window size gets increased from $2^{16}-1$ to $(2^{16}-1) \ast 2^{14}$ or from $65535$ to $65535 \ast 2^{14}$

http://en.wikipedia.org/wiki/TCP_window_scale_option

Comments

Why did you equate (1048560/8) × alpha= 64k???

Is 64k the bandwidth delay product?

If yes why? I'm unable to get the logic.please help sir.

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@Vgone  if you look in TCP header fomat  ...window size field has  16 bits hence maximum number represented  by it is 2^16-1=65535 …..maximum buffer size(maximum amount of data on network i.e. bandwidth delay product) = 2^16 bytes=64KB

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@Digvijay Pandey  Section 2.3 in https://tools.ietf.org/html/rfc1323#page-10 clearly explains why 14 bits shift count is used in TCP header.

TCP determines if a data segment is "old" or "new" by testing whether its sequence number is within 2**31 bytes of the left edge of the window, and if it is not, discarding the data as "old". To insure that new data is never mistakenly considered old and vice- versa, the left edge of the sender's window has to be at most 2**31 away from the right edge of the receiver's window. Similarly with the sender's right edge and receiver's left edge. Since the right and left edges of either the sender's or receiver's window differ by the window size, and since the sender and receiver windows can be out of phase by at most the window size, the above constraints imply that 2 * the max window size must be less than 2**31, or max window. 

 

Since the max window is 2**S (where S is the scaling shift count) times at most 2**16 - 1 (the maximum unscaled window), the maximum window is guaranteed to be < 2*30 if S <= 14. Thus, the shift count must be limited to 14 (which allows windows of 2**30 = 1 Gbyte). If a Window Scale option is received with a shift.cnt value exceeding 14, the TCP should log the error but use 14 instead of the specified value.

Answer 2

answer is C.
because TCP window scale option is needed when size increases more than 65535 B. it means alpha (RTT) should be the time taken to send 65535 B to the receiver. 

Time to send 65535 B = 65535 * 8/1048560 *1000 = 500 ms. 

so alpha will be 500.

maximum window size with window scale option is possible in TCP is 1073725440 B which is 65535*2^14 .

http://en.wikipedia.org/wiki/TCP_window_scale_option

Comments

PLEASE CHECK CALCULATION ONCE:
because TCP window scale option is needed when size increases more than 65535 B. it means alpha (RTT) should be the time taken to send 65535 B to the receiver. 

Time to send 65535 B = 65535 * 8/1048560 *1000 = 500 ms. 

IT SHOULD BE: // Time to send 65535 B = 65535 * 8*1000/1048560 = 500 ms. 

so alpha will be 500.

maximum window size with window scale option is possible in TCP is 1073725440 B which is 65535*2^14 .

http://en.wikipedia.org/wiki/TCP_window_scale_option

Answer 3

The TCP window scale option is an option to increase the receiver window size allowed in Transmission Control Protocol above its former maximum value of 65,535 bytes.

65,535 bytes = 64KB = $2^{16}$ B

The scaling option allows us to increase the window size from 64KB to 1 GB!

 

• When is scaling used?

When the bandwidth-delay product exceeds the value of 64K or $2^{16}$, we use scaling.
Bandwidth must be in bytes/sec and delay (RTT) must be in ms.

 

• What happens when we scale?

Window size increases from 64KB to 1GB, ie, from $2^{16}$B to $2^{30}$B

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Now, coming to the question:

Calculating α

131070 * RTT = $2^{16}$

=> 131070 * RTT = 65536

=> RTT = 500 ms

 

Calculating β

Window size would be increased from  $2^{16}$B to $2^{30}$B

i.e. from 65536 to 65536 * $2^{14}$B

 

Option C

Comments

@JashanArora

How did you get 64K as Bandwidth delay? If it is Standard value, then I think Bandwidth is a concern. 

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I think, the maximum window size of receiver is $2^{16}$, so if the bandwidth is maximum $2^{16}$ then receiver will be filled in one go/ one second. If bandwidth is more than $2^{16}$ we can transfer more than the size that a receiver can advertise. So we need to add extra bits so that sender can show more buffer size that it can receive. And the extra bits are available in the option field of the tcp header.

Please correct me if I'm wrong somewhere.

Answer 4

Lets talk about concept first..

Basically We are not utilizing the given bandwidth at its fullest. Sticking to the question, it causes due to the delay caused in sending the data.

(You can consider the example of satellite link , It causes much delay for acknowledgement to arrived after sending data equal to current window size)

One of the solution is we can consider upgrading window size that will pack the data , utilizing maximum of the given bandwidth.

The units we consider for data is in Bytes and delay in "ms"

Lets have a look at our Question.

Given,

B = (1048560/8) B/s

RTT ( alpha ) = X ms ( say)

Current ( default) window size, RWIN = 65535 B

( RWIN = Reciver window )

Using Bandwidth * Delay product ,

B * X = 65535 ,

X = 500 ms.

To use the given Bandwidth  to ita fullest , we can scale our window upto 1GB ( fixed standard)

i.e with scaling factor of 2^14 B

(i.e Shifting 14 bits to left )

So, scaled window size becomes,

65535 * 2^ 14.

(2^16 * 2^ 14 = 2^30 , i.e 1GB )

 

 

https://www.speedguide.net/faq/what-is-the-bandwidth-delay-product-185

 

https://networklessons.com/cisco/ccnp-route/bandwidth-delay-product/