Fragmentation Suppose that host A is connected to a router R

(Fragmentation)

Suppose that host A is connected to a router R1, R1 is connected to another router R2, and R2 is connected to host B. Suppose that a TCP message that contains 900 bytes of data and 20 bytes of TCP header is passed to the IP (IPv4) code at host A for delivery to B. Assume that link A-R1 can support a maximum frame size of 1024 bytes including a 14-byte frame header, link R1-R2 can support a maximum frame size of 512 bytes, including an 8 byte-frame header, and link R2-B can support a maximum frame size of 512 bytes, including a 12-byte frame header. Assume a 20 byte IPv4 header.

Up to how many bytes of IP packets can each of the three links carry?

Solution

Link A-R1:

Having Length = 940

ID = x

DF = 0

MF = 0

Offset = 0

Link R1-R2:

(a)

Length = 500

ID = x

DF = 0

MF = 1

Offset = 0

(b)

Length = 460;

ID = x

DF = 0

MF = 0

Offset = 60

Link R2-B:

(a)

Length = 500

ID = x

DF = 0

MF = 1

Offset = 0

(b)

Length = 460

ID = x

DF = 0

MF = 0

Offset = 60