about ipv4 header:- a) The IPv4 header has two length fields. Which are they and

ID: 643668 • Letter: A

Question

about ipv4 header:-

a) The IPv4 header has two length fields. Which are they and what are the purposes and limitations of each? (2p)

b) What is the purpose of the TTL (time-to-live) field? How do routers handle the TTL field when forwarding a datagram? (2p)

c) Is the IPv4 header checksum end-to-end or hop-by-hop? Why? (2p)

d)Why usually is the IP header presented as rows of 4 bytes and why is the header length always given in multiple of 32 bytes? (2p)

e) Which fields of the IP header change from router to router? (2p)

a) Header Length field:4 bits expressing number of 32 bit words.limit is 60 bytes.

Total length field: 16 bits expressing byte length of whole datagram.limit is 64k.

b)The purpose of TTL field limit is to limit the lifetime of a datagram inorder to avoid loops.

Routers receiving a datagram with TTL>1 decrements the TTL and forwards the datagram.If TTL<=1 ,the router does not forward the datagram.

c)hop-by-hop - the header may change on a hop- by-hop basis.For example,the TTL field decrements,and some options may be changed by a router.

e)The TTL will be reduced by one every time it passes a router.Because this changes the IP header,the checksum also has to be recalculated.

d) Each row is represented by 4 bytes in IP header.IP header length is a unit of 32 bits which is always represented in multiple of 32 bytes.

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