Host A is sending alarge file to Host B over TCP. The MTU (maximum transmission

Question

Host A is sending alarge file to Host B over TCP. The MTU (maximum transmission unit) for the local link layer is 200 bytes. What is the MSS (maximum segment size) assuming the usual header sizes? Host A has LastByteSent = 567 and LastByteAcked = 352, and in the last ACK, the rwnd (received window) field was 412. How many new pipelined segments can Host A send (ignoring cwnd)? What are their sizes?

What is cwnd? Suppose that cwend is currently 609. Does this change your answer? Explain.

Thank you very much

Explanation / Answer

MSS calculation

First we will see definitions of MTU and MSS

MTU (Maximum Transfer Unit) : It is used to measure the maximum amount of data in bytes transferred over layer 1, which is 1500

MSS (Maximum Segment Size) : It specifies the maximum amount of data specified over a single TCP segment.

We know the formula for MSS calculation

MSS = MTU (Maximum Transfer Unit) - 20 bytes (TCP Header) - 20 bytes (IP Header)

Here, given MTU size is 200 bytes.

Therefore, MSS = 200 - 20 - 20 = 160 bytes

MSS = 160 bytes

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Given information is,

LastByteSent = 567, LastByteAcked = 352 and rwnd in last ACK = 412.

Here, rwnd means the receiver host B advertises to the sender host A that how much data it can receive and buffer. According to value received in last ACK, rwnd = 412, receiver host B indicates that his window size is 412 bytes and he is able to receive and buffer the data up to 412 bytes.

Hence, new pipelined segments can Host A send (ignoring cwnd) are : rwnd / MSS = 412 / 160 = 2.5

Therefore, new pipelined segments can Host A send (ignoring cwnd) are approximately 2 each having size 160 bytes. Because MSS (Maximum Segment Size) is 160.

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cwnd : It is congestion window acts as sender's flow control which totally depends on capacity and conditions of underlying network.

cwnd is usually referred to in multiples of maximum segment size (MSS). This means that , an MSS of 160 and cwnd of 609 results in 160*609 = 97440 BYTES.

This will not change our answer because, the amount of data we can send is the minimum of the amount the receiver says it can receive and the amount the sender thinks it can send.

Therefore, the amount the receiver says it can receive = rwnd = 412 bytes

the amount the sender thinks it can send = 97440 bytes

so, Minimum (97440, 412) = 412

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