Question

2) Assume a new RFC for TFTP was written for TFTP-16. The only change made was that the max block size for a data packet was changed to 16 bytes instead of 512 bytes. Host A wants to read a text file, 'read_me.txt", with the 35 bytes of content, "For sale: baby shoes... never worn. from Host B in "octet" mode using TFTP-16. Draw the complete sequence diagram for this communication in the space below. For each packet sent across the network, please identify the contents of EVERY TFTP FIELD INCLUDING OPCODE and the UDP fields source port, destination port, and segment length (no need to find the checksum, but it does use 2 bytes). Additionally, assume no errors in the communication and no retransmissions.

Answer 1

TFTP-16 :

TFTP with 16 bit packet (octet mode):

1. As in this example host A want to read a file from host B, so Host A issue a request packet RRQ to host B at port 69. The packet contains the file name and mode of transfer.
2. Host B will reply (Ack) with data packet with block no. (starting 1) and data. The port used in this case is any random port.
3. The last packet if contains data less than maximum tftp size that means end of file. If size of packet is exact multiple of max size of tftp then last packet is sent with 0 size data to indicate end of data.
4. Host A will send the ack packet with block No for which Ack is sent. As we are not calculating check sum no error messages will be sent and no retransmission.

Block # is starting 0 to datazise/max_size+1

Opcode File Name "Read me.txt Mode 1 Byte octet Data Packet: Oncode Block# 1,2.3 Data Data Opcnde 4 Block# 1,2,3 Host B Host A RRQ Data-1 Ack-1 Data-n Ack-n

RRQ Packet:

Op code: as pet above diagrams ;Source port= Any random port (a’s TID ), Destination Port=69 Segment length= UDP Headers+tftp header= 8 Bytes [UDP Header) +(2+1+12+6+1)[ tftp header]+2 (check sum)= 32 Bytes

Since here max segment length 16 and 2 bytes are used as checksum so actual data to be sent 14 Bytes.

So data Block number=35/14=2+1=3 Block No. (0,1,2)

Data Packet-n

Op code= As per diagram of packet, source port=B’s TID, destination port=port from RRQ packet (A’s TID)

Block # n-1

Segment length= UDP Headers+tftp header+data= 8 Bytes [UDP Header) +(4 Byes)[ tftp header]+ 16 Bytes[Data]2 (check sum)= 30 Bytes

Ack packet-n

Op code= As per diagram of packet, source port=B’s TID, destination port=port from RRQ packet (A’s TID)

Block # n-1

Segment length= UDP Headers+tftp header= 8 Bytes [UDP Header) +(4 Byes)[ tftp header]+ 2 Bytes check sum=16 Bytes