Mtu

Question

Consider a network having a host P connected to another host Q via two routers Rl and R2. P is connected to R1 through a link that can support a maximum frame length of 1024 bytes including header of size 12 bytes. R1 is connected to R2 through a link that supports a maximum frame length of 256 bytes including an 8bytes header. R2 is connected to the host Q through the link that can support a maximum frame length of 512 bytes including the frame header of 12bytes. Suppose a TCP message having 800bytes of data and 20 bytes of TCP header is passed to the IP at host P for delivery to Q. Find the total number of bytes that are sent from host P to Q (including all the headers).
https://gateoverflow.in/?qa=blob&qa_blobid=626145120644117638

Comments

is it 876?

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No, it is 2640

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How can header length of different networks be variable. According to TCP/IP protocol, header length has to be fixed throughout no matter how small or big the fragment is.

Also header length should not go below 20 bytes, but here it is going below this mark.

My best guess is that we have to take the minimum header length of 8 bytes and overall data size of 256 bytes and make packets according to this network and send.

In that case,

Total data to be sent = 800 + 20 = 820 bytes.

size of each fragment = 256 bytes including the header (248 bytes of data and 8 bytes of header)

So, total no. of packets that need to be sent = 820 / 248 = 3.306 => 4 packets, hence 4 IP headers will also be there in them = 8*4 = 32 bytes.

So, total bytes sent = 820 + 32 = 852 bytes.

@Arjun sir. Please explain.

Answer 1

is it 2528? if not plz explain..thank u

Answer 2

800(payload)+20 bytes of data is to be transmitted it will not be fragmented from p to r1

now from r1 to r2 it is to be fragmented 248b(payload)+8b(header)------now payload will be 248(it should be multiples of 8)*3+56+4*8(header)

now from r2 to q there is no problem as packet length is 256 only

 

total number of bytes that are sent from host P to Q (including all the headers) is 820+32=852