last fragment ( that is the more fragments field is zero) the total data length is computed. If this fragment completes the datagram (tested by checking the bits set in the fragment block table), then the datagram is sent to the next step in datagram processing; otherwise the timer is set to the maximum of the current timer value and the value of the time to live field from this fragment; and the reassembly routine gives up control.
If the timer runs out, the all reassembly resources for this buffer identifier are released. The initial setting of the timer is a lower bound on the reassembly waiting time. This is because the waiting time will be increased if the Time to Live in the arriving fragment is greater than the current timer value but will not be decreased if it is less. The maximum this timer value could reach is the maximum time to live (approximately 4.25 minutes). The current recommendation for the initial timer setting is 15 seconds. This may be changed as experience with
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September 1981 Internet Protocol Specification
this protocol accumulates. Note that the choice of this parameter value is related to the buffer capacity available and the data rate of the transmission medium; that is, data rate times timer value equals buffer size (e.g., 10Kb/s X 15s = 150Kb).
Notation:
FO - Fragment Offset IHL - Internet Header Length MF - More Fragments flag TTL - Time To Live NFB - Number of Fragment Blocks TL - Total Length TDL - Total Data Length BUFID - Buffer Identifier RCVBT - Fragment Received Bit Table TLB - Timer Lower Bound
Procedure:
(1) BUFID <- sourcedestinationprotocolidentification; (2) IF FO = 0 AND MF = 0 (3) THEN IF buffer with BUFID is allocated (4) THEN flush all reassembly for this BUFID; (5) Submit datagram to next step; DONE. (6) ELSE IF no buffer with BUFID is allocated (7) THEN allocate reassembly resources with BUFID; TIMER <- TLB; TDL <- 0; (8) put data from fragment into data buffer with BUFID from octet FO*8 to octet (TL-(IHL*4))+FO*8; (9) set RCVBT bits from FO to FO+((TL-(IHL*4)+7)/8); (10) IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8) (11) IF FO = 0 THEN put header in header buffer (12) IF TDL # 0 (13) AND all RCVBT bits from 0 to (TDL+7)/8 are set (14) THEN TL <- TDL+(IHL*4) (15) Submit datagram to next step; (16) free all reassembly resources
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