Put a Packet Message Back in Order

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A message can be divided into smaller pieces. This paper model labels each piece with a message and position, and states the total separately. Use position rather than arrival order. Real network protocols have different fields and guarantees; this is not a literal packet format.

Worked example

Pieces 2, 1 and 3 arrive in that order. Reconstruct positions 1, 2, 3. A second copy of piece 2 does not replace missing piece 3.

Arrival: M/3 LIBRARY, M/1 MEET, M/2 AT THE. Reconstructed: M/1 MEET, M/2 AT THE, M/3 LIBRARY. — Message labels and positions matter; arrival order can differ.

Message M: Three Pieces

Message M has three numbered pieces: M/1 says MEET, M/2 says AT THE, and M/3 says LIBRARY. They arrive M/3, M/1, M/2. Each label identifies both message and position. A card from another message must not be inserted just because its number fits.

Question 1 Which piece belongs first? M/1 M/3 Whichever arrives first

Question 2 What is the reconstructed message? LIBRARY MEET AT THE MEET AT THE LIBRARY AT THE LIBRARY MEET

Question 3 How many different positions are needed? Answer

Question 4 If the received labels are M/1, M/2, M/2, which position is missing? Answer

Question 5 Can N/3 replace the missing M/3? Yes; only the number matters No; it belongs to another message Yes; all messages are identical