OTL - Old Total Length NFB - Number of Fragment Blocks MTU - Maximum Transmission Unit
Procedure:
IF TL =< MTU THEN Submit this datagram to the next step in datagram processing ELSE IF DF = 1 THEN discard the datagram ELSE To produce the first fragment: (1) Copy the original internet header; (2) OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF; (3) NFB <- (MTU-IHL*4)/8; (4) Attach the first NFB*8 data octets; (5) Correct the header: MF <- 1; TL <- (IHL*4)+(NFB*8); Recompute Checksum; (6) Submit this fragment to the next step in datagram processing; To produce the second fragment: (7) Selectively copy the internet header (some options are not copied, see option definitions); (8) Append the remaining data; (9) Correct the header: IHL <- (((OIHL*4)-(length of options not copied))+3)/4;
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September 1981 Internet Protocol Specification
TL <- OTL - NFB*8 - (OIHL-IHL)*4); FO <- OFO + NFB; MF <- OMF; Recompute Checksum; (10) Submit this fragment to the fragmentation test; DONE.
In the above procedure each fragment (except the last) was made the maximum allowable size. An alternative might produce less than the maximum size datagrams. For example, one could implement a fragmentation procedure that repeatly divided large datagrams in half until the resulting fragments were less than the maximum transmission unit size.
An Example Reassembly Procedure
For each datagram the buffer identifier is computed as the concatenation of the source, destination, protocol, and identification fields. If this is a whole datagram (that is both the fragment offset and the more fragments fields are zero), then any reassembly resources associated with this buffer identifier are released and the datagram is forwarded to the next step in datagram processing.
If no other fragment with this buffer identifier is on hand then reassembly resources are allocated. The reassembly resources consist of a data buffer, a header buffer, a fragment block bit table, a total data length field, and a timer. The data from the fragment is placed in the data buffer according to its fragment offset and length, and bits are set in the fragment block bit table corresponding to the fragment blocks received.
If this is the first fragment (that is the fragment offset is zero) this header is placed in the header buffer. If this is the
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