TFTP packet size and MTU

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Logically, the higher the packet size the less number of packets required to send a file. This is true when I try 128, 512, 1024 and 1428 bytes options. However, when I go for 2048, 4096, till 32768 bytes options, this logic is no longer applicable and I observe the following in WireShark:

TFTP runs over UDP, and a single UDP packet is sent as a single IP packet.

If the IP packet is too big to be sent in a single link-layer packet, the sending IP layer "fragments" it into smaller packets, each of which can be sent in a single link-layer packet, and the receiving IP layer must reassemble all the fragments of the original IP packet and provide that to the receiving UDP layer. UDP has no mechanism to handle this itself, unlike TCP, which divides the stream of data provided to it into "segments", each of which can be made small enough to fit in a single link-layer packet.

The maximum size of a raw Ethernet packet, without counting the FCS (which is usually not captured by Wireshark, so it won't show up in the packet or in the frame length) is 1514 bytes; that's 14 bytes of Ethernet header (destination address, source address, type/length), and 1500 bytes of payload.

A TFTP data packet sending 1024 bytes of data, sent as an IPv4 packet over Ethernet, and with no IP options, will have 14 bytes of Ethernet header, 20 bytes of IPv4 header, 8 bytes of UDP header, 4 bytes of TFTP header, and 1024 bytes of data, for a total of 1070 bytes, which is <= 1514, so that can be sent in an un-fragmented IPv4 packet.

A TFTP data packet sending 1428 bytes of data, sent as an IPv4 packet over Ethernet, and with no IP options, will have 14 bytes of Ethernet header, 20 bytes of IPv4 header, 8 bytes of UDP header, 4 bytes of TFTP header, and 1428 bytes of data, for a total of 1474 bytes, which is <= 1514, so that can be sent in an un-fragmented IPv4 packet.

A TFTP data packet containing 2048 bytes of data, sent as an IPv4 packet over Ethernet, and with no IP options, will have 14 bytes of Ethernet header, 20 bytes of IPv4 header, 8 bytes of UDP header, 4 bytes of TFTP header, and 2048 bytes of data, for a total of 2094 bytes, which is > 1514, and which thus would have to be sent as more than one fragment of a fragmented IPv4 packet.

Packets size is not equal to the specified option. for example: for 2048 bytes option, packet size= 614 bytes !!

The first fragment will probably be the maximum size, with 14 bytes of Ethernet header, 20 bytes of IPv4 header, 8 bytes of UDP header, 4 bytes of TFTP header, and 1468 bytes of data and the second fragment will contain the remaining 2048 - 1468 = 580 bytes of data; it does not ... (more)