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
      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



 

                                                          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 <- source|destination|protocol|identification;
        (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
                             for this BUFID; DONE.
        (17)         TIMER <- MAX(TIMER,TTL);
        (18)         give up until next fragment or timer expires;
        (19) timer expires: flush all reassembly with this BUFID; DONE.

      In the case that two or more fragments contain the same data



 

September 1981                                                          
                                                       Internet Protocol
                                                           Specification



      either identically or through a partial overlap, this procedure
      will use the more recently arrived copy in the data buffer and
      datagram delivered.

  Identification

    The choice of the Identifier for a datagram is based on the need to
    provide a way to uniquely identify the fragments of a particular
    datagram.  The protocol module assembling fragments judges fragments