The KingTut Protocol

The sender sends all file contents in packets. Every packet is UDP. Each packet will have the following format

Name	Meaning	Starting Offset
Version	A signed int, normally the number '4'	4 bytes
Offset	A signed int, the position of the NEXT byte to come AFTER this packet.	4 bytes
CRC	A crc calculated by running ComputeChecksum over the whole packet including header. When computing the CRC, it assumes there are zeros in this field.	4 bytes
Len	A signed int, the length of the data portion of this packet	4 bytes
Data	The actual data	'Len' bytes

Acks have the following format

Name Meaning Starting Offset

Version A signed int, normally '4' 0

nextPos A signed int, the next byte to send 4

The sender will begin by sending data. The first packet will consist of :

Name Start Position Value

Version 0 4

Offset 4 'len'

CRC 8 1212121212 (or some such thing)

Len 12 length of the data to follow

Data 16 "King Tut was a young boy ..." (or some such thing)

You have 0.5 seconds to reply. If you fail to reply a new packet will be sent.
You should

Check to see that the version number is corrent. It should be '4'.
Check to see that the Offset makes sense. You should only accept data that contigiously extends the data already received.
Check to see that the CRC makes sense. To do this

Recover the CRC. Call this CRC1
Put zeros into the CRC field.
Compute the CRC of the whole packet Call this CRC2
If CRC1 == CRC2 then OK, else bad

If the packet is OK, you should

Accept the data, displaying it on the screen
Compute the new nextPos
Send an ack with this nextPos

The only legal values of nextPos are

0 (first time only)
The immediately previous value (the one in the last packet received)
The old nextPos + the length of data received

The maximum packet length including header is 1024 bytes. The minimum packet length is 16 bytes. Every ack packet is 8 bytes long.

Name	Start Position	Value
Version	0	4
Offset	4	'len'
CRC	8	1212121212 (or some such thing)
Len	12	length of the data to follow
Data	16	"King Tut was a young boy ..." (or some such thing)