Question

1. What is the difference between packet fragmentation (i.e., at network layer) and frame fragmentation (i.e., at link layer) in terms of purpose? 2. 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 800 bytes of data and 20 bytes of TCP header is passed to the IP function at host A for delivery to B. Show the Total length, DF, MF, and Fragment offset fields of the IP header in each packet transmitted over the three links. (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 432 bytes including a 12-byte frame header.) (*hint: the Fragment offset field is denominated by 8-bytes, not bytes) 3. What is the purpose of the path MTU discovery process (see textbook Figure 5-42) and why does it need to set the DF bit in the IP headers?

Answer 1

Packet fragmentation breaks large IP packets, while frame fragmentation divides large frames for transmission on a specific link. The IP header fields for transmitting a TCP message over three links are explained, and the purpose of the path MTU discovery process is discussed.

Step-by-step explanation:

Difference between Packet Fragmentation and Frame Fragmentation

The purpose of packet fragmentation at the network layer is to break a large IP packet into smaller fragments to fit the maximum transmission unit (MTU) size of the network. This is necessary when the packet size exceeds the MTU of a particular link in the network.

On the other hand, frame fragmentation at the link layer is used to divide a large frame into smaller frames that can be transmitted over a particular link with a smaller frame size.

IP Header Fields for Packet Transmission

The total length of the original TCP message is 820 bytes (800 bytes of data + 20 bytes of TCP header). When transmitting over the three links, the IP header fields will be:

1. Link A-R1: Total length: 1024 bytes, DF: 0, MF: 1, Fragment offset: 0
2. Link R1-R2: Total length: 512 bytes, DF: 1, MF: 0, Fragment offset: 0
3. Link R2-B: Total length: 432 bytes, DF: 1, MF: 0, Fragment offset: 0

Purpose of Path MTU Discovery Process

The purpose of the Path MTU Discovery process is to determine the maximum transmission unit (MTU) size that can be used to transmit packets without fragmentation along the entire path from source to destination. It needs to set the DF (Don't Fragment) bit in the IP headers to ensure that routers along the path do not fragment the packets.