Question

Could i please have assistance with the following questions.

1) Assume that there is no option field in the header of an IPv4 datagram. Which field(s) of the IPv4 header change from router to router?

2) Assume a machine has an IP address IP1 and it wants to communicate with another machine with IP address IP2. When does it need to use a gateway/router to communicate with IP2?

3) why do we say that IP provides “best-effort” service?

Answer 1

1)

Following Fields of IPv4 changes from router to router

TTL: Value of Time-To-Leave is decreased by 1 at reach router.

Header Checksum, Flag bits, Total Length, Offset : If DF = 0, Then if size of the packet is > Maximum transfer Unit (MTU) then router fragments the packet as a result packet total length, Flag bits changes and Header Checks needs to be computed again.

2)

Machine which has address IP1 checks whether IP2 is in the same network or not. It computes it by IP and Subnet Mask. If IP2 is in the same network according to IP1 then it sends it directly else it will communicate with default gateway or router.

3)

Because,

• IP puts maximum efforts to deliver the packets to destination. But it doesn't guarantee the delivery of packet. That it is IP is non reliable.
• It puts maximum efforts to deliver the packet but IP doesn't worry about recovery of lost packets.