Question

3) [10 marks] Refer to following Figure 2.14 and Figure 16.3. This question is about encapsulation, decapsulation, and protocols. Figure 2.14 illustrates that a TFTP Trivial File Transfer Protocol) packet is encapsulated in a UDP (User Datagram Protocol) segment that in turn is encapsulated in an IP (Internet Protocol) datagram. In Figure 2.14, the length of each row is 32 bits (4 bytes). For example, the length of row "Source Address" is 32 bits long. Figure 16.3 gives the format of IEEE 802.3 frame (used in a LAN), which carries an IP datagram. The "Pad" field in Figure 16.3 is to make sure that the total length of the "LLC Data" and "Pad" is always greater or equal to 46, which means that, when the length of LLC Data is smaller than 46, some Pad bytes will be added. The TFTP data is "3185", which consists of 4 characters. Each character is represented by 8 bits. a) What will be the length of the frame assembled at the sender side (from the beginning of "Preamble" to the end of "FCS")? b) What will be the length of the frame received at the receiver side (from the beginning of "Preamble" to the end of "FCS")? c) What will be the "Total Length" of IP Datagram? d) What will be the "Segment Length" of UDP Segment? e) What will be the total length of TFTP Packet? What will be received by the Transport Layer (UDP) at the receiver side?

Answer 1

Answer:

a) IP datagram has 9 fields (rows) of 4 bytes each. Hence,

length of IP datagram = 9*4 = 36 bytes

This datagram will be stored in the LLC data field of IEEE 802.3 frame. As the length of datagram is 36 bytes, padding will done and 10 padding bytes will be added to fulfill the minimum requirement of 46 bytes.

Hence, length of 802.3 frame = 7 + 1 + 6 + 6 + 2 + 46 + 4 = 72 bytes

b) The length of frame received at the receiver end will be same as sent from sender end i.e. 72 bytes. FCS (Frame Check Sequence) will be used to detect corrupted data.

c) As said above, length of IP datagram will be 36 bytes.

d) Segment length of UDP segment will be 2 bytes as clear from figure 2.14.

e) Total length of TFTP packet = 2 * 4 = 8 bytes

f) At transport layer, UDP segment will be received.