Algorithm

See also

This algorithm is an adaptation of the algorithm described in RFC 815.

DSN

Data Sequence Number

ACK

TCP Acknowledgement

SYN

TCP Synchronisation Flag

FIN

TCP Finish Flag

RST

TCP Reset Connection Flag

BUFID

Buffer Identifier

HDL

Hole Discriptor List

ISN

Initial Sequence Number

src

source IP

dst

destination IP

srcport

source TCP port

dstport

destination TCP port

DO {
   BUFID <- src|dst|srcport|dstport|ACK;
   IF (SYN is true) {
      IF (buffer with BUFID is allocated) {
         flush all reassembly for this BUFID;
         submit datagram to next step;
      }
   }

   IF (no buffer with BUFID is allocated) {
      allocate reassembly resources with BUFID;
      ISN <- DSN;
      put data from fragment into data buffer with BUFID
         [from octet fragment.first to octet fragment.last];
      update HDL;
   }

   IF (FIN is true or RST is true) {
      submit datagram to next step;
      free all reassembly resources for this BUFID;
      BREAK.
   }
} give up until (next fragment);

update HDL: {
   DO {
      select the next hole descriptor from HDL;

      IF (fragment.first >= hole.first) CONTINUE.
      IF (fragment.last <= hole.first) CONTINUE.

      delete the current entry from HDL;

      IF (fragment.first >= hole.first) {
         create new entry "new_hole" in HDL;
         new_hole.first <- hole.first;
         new_hole.last <- fragment.first - 1;
         BREAK.
      }

      IF (fragment.last <= hole.last) {
         create new entry "new_hole" in HDL;
         new_hole.first <- fragment.last + 1;
         new_hole.last <- hole.last;
         BREAK.
      }
   } give up until (no entry from HDL)
}

The following algorithm implement is based on IP Datagram Reassembly Algorithm introduced in RFC 815. It described an algorithm dealing with RCVBT (fragment received bit table) appeared in RFC 791. And here is the process:

  1. Select the next hole descriptor from the hole descriptor list. If there are no more entries, go to step eight.

  2. If fragment.first is greater than hole.last, go to step one.

  3. If fragment.last is less than hole.first, go to step one.

  4. Delete the current entry from the hole descriptor list.

  5. If fragment.first is greater than hole.first, then create a new hole descriptor new_hole with new_hole.first equal to hole.first, and new_hole.last equal to fragment.first minus one (-1).

  6. If fragment.last is less than hole.last and fragment.more_fragments is true, then create a new hole descriptor new_hole, with new_hole.first equal to fragment.last plus one (+1) and new_hole.last equal to hole.last.

  7. Go to step one.

  8. If the hole descriptor list is now empty, the datagram is now complete. Pass it on to the higher level protocol processor for further handling. Otherwise, return.